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Backpatch FAQ's to 8.0.X for release.

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Bruce Momjian
2005-05-09 17:24:04 +00:00
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@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Tue Mar 15 17:38:18 EST 2005 Last updated: Mon May 9 13:15:04 EDT 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -26,6 +26,7 @@
1.10) How can I learn SQL? 1.10) How can I learn SQL?
1.11) How do I join the development team? 1.11) How do I join the development team?
1.12) How does PostgreSQL compare to other DBMSs? 1.12) How does PostgreSQL compare to other DBMSs?
1.13) Who controls PostgreSQL?
User Client Questions User Client Questions
@@ -67,35 +68,23 @@
4.11.3) Doesn't currval() lead to a race condition with other users? 4.11.3) Doesn't currval() lead to a race condition with other users?
4.11.4) Why aren't my sequence numbers reused on transaction abort? 4.11.4) Why aren't my sequence numbers reused on transaction abort?
Why are there gaps in the numbering of my sequence/SERIAL column? Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a TID? 4.12) What is an OID? What is a CTID?
4.13) Why do I get the error "ERROR: Memory exhausted in 4.13) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"? AllocSetAlloc()"?
4.14) How do I tell what PostgreSQL version I am running? 4.14) How do I tell what PostgreSQL version I am running?
4.15) Why does my large-object operations get "invalid large obj 4.15) How do I create a column that will default to the current time?
descriptor"? 4.16) How do I perform an outer join?
4.16) How do I create a column that will default to the current time? 4.17) How do I perform queries using multiple databases?
4.17) How do I perform an outer join? 4.18) How do I return multiple rows or columns from a function?
4.18) How do I perform queries using multiple databases? 4.19) Why do I get "relation with OID ##### does not exist" errors
4.19) How do I return multiple rows or columns from a function?
4.20) Why do I get "relation with OID ##### does not exist" errors
when accessing temporary tables in PL/PgSQL functions? when accessing temporary tables in PL/PgSQL functions?
4.21) What encryption options are available? 4.20) What replication solutions are available?
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does
it dump core?
5.2) How can I contribute some nifty new types and functions to
PostgreSQL?
5.3) How do I write a C function to return a tuple?
5.4) I have changed a source file. Why does the recompile not see the
change?
_________________________________________________________________ _________________________________________________________________
General Questions General Questions
1.1) What is PostgreSQL? How is it pronounced? 1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres. PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
PostgreSQL is an object-relational database system that has the PostgreSQL is an object-relational database system that has the
@@ -109,8 +98,8 @@
company. To get involved, see the developer's FAQ at company. To get involved, see the developer's FAQ at
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html
1.2) What is the copyright of PostgreSQL? 1.2) What is the copyright of PostgreSQL?
PostgreSQL is distributed under the classic BSD license. It has no PostgreSQL is distributed under the classic BSD license. It has no
restrictions on how the source code can be used. We like it and have restrictions on how the source code can be used. We like it and have
no intention of changing it. no intention of changing it.
@@ -142,31 +131,30 @@
CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
UPDATES, ENHANCEMENTS, OR MODIFICATIONS. UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
1.3) What platforms does PostgreSQL support? 1.3) What platforms does PostgreSQL support?
In general, any modern Unix-compatible platform should be able to run In general, any modern Unix-compatible platform should be able to run
PostgreSQL. The platforms that had received explicit testing at the PostgreSQL. The platforms that had received explicit testing at the
time of release are listed in the installation instructions. time of release are listed in the installation instructions.
Starting with version 8.0, PostgreSQL now runs natively on Microsoft PostgreSQL also runs natively on Microsoft Windows NT-based operating
Windows NT-based operating systems like Win2000, WinXP, and Win2003. A systems like Win2000, WinXP, and Win2003. A prepackaged installer is
prepackaged installer is available at available at http://pgfoundry.org/projects/pginstaller. MSDOS-based
http://pgfoundry.org/projects/pginstaller. MSDOS-based versions of versions of Windows (Win95, Win98, WinMe) can run PostgreSQL using
Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin. Cygwin.
There is also a Novell Netware 6 port at http://forge.novell.com, and There is also a Novell Netware 6 port at http://forge.novell.com, and
an OS/2 (eComStation) version at an OS/2 (eComStation) version at
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgre
SQL&stype=all&sort=type&dir=%2F. SQL&stype=all&sort=type&dir=%2F.
1.4) Where can I get PostgreSQL? 1.4) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use
ftp://ftp.PostgreSQL.org/pub/. For mirror sites, see our main web ftp://ftp.PostgreSQL.org/pub/.
site.
1.5) Where can I get support? 1.5) Where can I get support?
The PostgreSQL community provides assistance to many of its users via The PostgreSQL community provides assistance to many of its users via
email. The main web site to subscribe to the email lists is email. The main web site to subscribe to the email lists is
http://www.postgresql.org/community/lists/. The general or bugs lists http://www.postgresql.org/community/lists/. The general or bugs lists
@@ -174,38 +162,39 @@
The major IRC channel is #postgresql on Freenode (irc.freenode.net). The major IRC channel is #postgresql on Freenode (irc.freenode.net).
To connect you can use the Unix program irc -c '#postgresql' "$USER" To connect you can use the Unix program irc -c '#postgresql' "$USER"
irc.freenode.net or use any of the other popular IRC clients. A irc.freenode.net or use any other IRC clients. A Spanish one also
Spanish one also exists on the same network, (#postgresql-es), and a exists on the same network, (#postgresql-es), and a French one,
French one, (#postgresqlfr). There is also a PostgreSQL channel on (#postgresqlfr). There is also a PostgreSQL channel on EFNet.
EFNet.
A list of commercial support companies is available at A list of commercial support companies is available at
http://techdocs.postgresql.org/companies.php. http://techdocs.postgresql.org/companies.php.
1.6) How do I submit a bug report? 1.6) How do I submit a bug report?
Visit the PostgreSQL bug form at Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug. http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if Also check out our ftp site ftp://ftp.PostgreSQL.org/pub/ to see if
there is a more recent PostgreSQL version. there is a more recent PostgreSQL version.
1.7) What is the latest release? 1.7) What is the latest release?
The latest release of PostgreSQL is version 8.0.1. The latest release of PostgreSQL is version 8.0.2.
We plan to have major releases every ten to twelve months. We plan to have a major release every year, with minor releases every
few months.
1.8) What documentation is available? 1.8) What documentation is available?
PostgreSQL includes extensive documentation, including a large manual, PostgreSQL includes extensive documentation, including a large manual,
manual pages, and some test examples. See the /doc directory. You can manual pages, and some test examples. See the /doc directory. You can
also browse the manuals online at http://www.PostgreSQL.org/docs. also browse the manuals online at http://www.PostgreSQL.org/docs.
There are two PostgreSQL books available online at There are two PostgreSQL books available online at
http://www.postgresql.org/docs/books/awbook.html and http://www.postgresql.org/docs/books/awbook.html and
http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL http://www.commandprompt.com/ppbook/. There are a number of PostgreSQL
books available for purchase at books available for purchase. One of the most popular ones is by Korry
Douglas. A list of book reviews can be found at
http://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also http://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also
a collection of PostgreSQL technical articles at a collection of PostgreSQL technical articles at
http://techdocs.PostgreSQL.org/. http://techdocs.PostgreSQL.org/.
@@ -216,43 +205,31 @@
Our web site contains even more documentation. Our web site contains even more documentation.
1.9) How do I find out about known bugs or missing features? 1.9) How do I find out about known bugs or missing features?
PostgreSQL supports an extended subset of SQL-92. See our TODO list PostgreSQL supports an extended subset of SQL-92. See our TODO list
for known bugs, missing features, and future plans. for known bugs, missing features, and future plans.
1.10) How can I learn SQL? 1.10) How can I learn SQL?
The PostgreSQL book at First, consider the PostgreSQL-specific books mentioned above. Another
http://www.postgresql.org/docs/books/awbook.html teaches SQL. There is one is "Teach Yourself SQL in 21 Days, Second Edition" at
another PostgreSQL book at http://www.commandprompt.com/ppbook. There http://members.tripod.com/er4ebus/sql/index.htm. Many of our users
is a nice tutorial at like The Practical SQL Handbook, Bowman, Judith S., et al.,
Addison-Wesley. Others like The Complete Reference SQL, Groff et al.,
McGraw-Hill.
There is also a nice tutorial at
http://www.intermedia.net/support/sql/sqltut.shtm, at http://www.intermedia.net/support/sql/sqltut.shtm, at
http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM, http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM,
and at http://sqlcourse.com. and at http://sqlcourse.com.
Another one is "Teach Yourself SQL in 21 Days, Second Edition" at 1.11) How do I join the development team?
http://members.tripod.com/er4ebus/sql/index.htm
See the Developer's FAQ.
Many of our users like The Practical SQL Handbook, Bowman, Judith S., 1.12) How does PostgreSQL compare to other DBMSs?
et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
et al., McGraw-Hill.
1.11) How do I join the development team?
First, download the latest source and read the PostgreSQL Developers
FAQ and documentation on our web site, or in the distribution. Second,
subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
submit high quality patches to pgsql-patches.
There are about a dozen people who have commit privileges to the
PostgreSQL CVS archive. They each have submitted so many high-quality
patches that it was impossible for the existing committers to keep up,
and we had confidence that patches they committed were of high
quality.
1.12) How does PostgreSQL compare to other DBMSs?
There are several ways of measuring software: features, performance, There are several ways of measuring software: features, performance,
reliability, support, and price. reliability, support, and price.
@@ -267,13 +244,8 @@
Performance Performance
PostgreSQL's performance is comparable to other commercial and PostgreSQL's performance is comparable to other commercial and
open source databases. It is faster for some things, slower for open source databases. It is faster for some things, slower for
others. In comparison to MySQL or leaner database systems, we others. Our performance is usually +/-10% compared to other
are faster for multiple users, complex queries, and a databases.
read/write query load. MySQL is faster for simple SELECT
queries done by a few users. Of course, MySQL does not have
most of the features mentioned in the Features section above.
We are built for reliability and features, and we continue to
improve performance in every release.
Reliability Reliability
We realize that a DBMS must be reliable, or it is worthless. We We realize that a DBMS must be reliable, or it is worthless. We
@@ -297,12 +269,23 @@
We are free for all use, both commercial and non-commercial. We are free for all use, both commercial and non-commercial.
You can add our code to your product with no limitations, You can add our code to your product with no limitations,
except those outlined in our BSD-style license stated above. except those outlined in our BSD-style license stated above.
1.13) Who controls PostgreSQL?
If you are looking for a PostgreSQL gatekeeper, central committee, or
controlling company, give up --- there isn't one. We do have a core
committee and CVS committers, but these groups are more for
administrative purposes than control. The project is directed by the
community of developers and users, which anyone can join. All you need
to do is subscribe to the mailing lists and participate in the
discussions. (See the Developer's FAQ for information on how to get
involved in PostgreSQL development.)
_________________________________________________________________ _________________________________________________________________
User Client Questions User Client Questions
2.1) What interfaces are available for PostgreSQL? 2.1) What interfaces are available for PostgreSQL?
The PostgreSQL install includes only the C and embedded C interfaces. The PostgreSQL install includes only the C and embedded C interfaces.
All other interfaces are independent projects that are downloaded All other interfaces are independent projects that are downloaded
separately; being separate allows them to have their own release separately; being separate allows them to have their own release
@@ -313,8 +296,8 @@
others are available at http://gborg.postgresql.org in the others are available at http://gborg.postgresql.org in the
Drivers/Interfaces section and via Internet search. Drivers/Interfaces section and via Internet search.
2.2) What tools are available for using PostgreSQL with Web pages? 2.2) What tools are available for using PostgreSQL with Web pages?
A nice introduction to Database-backed Web pages can be seen at: A nice introduction to Database-backed Web pages can be seen at:
http://www.webreview.com http://www.webreview.com
@@ -324,36 +307,28 @@
For complex cases, many use the Perl and DBD::Pg with CGI.pm or For complex cases, many use the Perl and DBD::Pg with CGI.pm or
mod_perl. mod_perl.
2.3) Does PostgreSQL have a graphical user interface? 2.3) Does PostgreSQL have a graphical user interface?
Yes, there are several graphical interfaces to PostgreSQL available. Yes, see http://techdocs.postgresql.org/guides/GUITools for a detailed
These include pgAdmin III (http://www.pgadmin.org, PgAccess
http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhdb/
), TORA ( http://www.globecom.net/tora/, partly commercial), and
Rekall ( http://www.rekallrevealed.org/). There is also PhpPgAdmin (
http://phppgadmin.sourceforge.net/ ), a web-based interface to
PostgreSQL.
See http://techdocs.postgresql.org/guides/GUITools for a more detailed
list. list.
_________________________________________________________________ _________________________________________________________________
Administrative Questions Administrative Questions
3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql? 3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
Specify the --prefix option when running configure. Specify the --prefix option when running configure.
3.2) How do I control connections from other hosts? 3.2) How do I control connections from other hosts?
By default, PostgreSQL only allows connections from the local machine By default, PostgreSQL only allows connections from the local machine
using Unix domain sockets or TCP/IP connections. Other machines will using Unix domain sockets or TCP/IP connections. Other machines will
not be able to connect unless you modify listen_addresses in the not be able to connect unless you modify listen_addresses in the
postgresql.conf file, enable host-based authentication by modifying postgresql.conf file, enable host-based authentication by modifying
the $PGDATA/pg_hba.conf file, and restart the server. the $PGDATA/pg_hba.conf file, and restart the server.
3.3) How do I tune the database engine for better performance? 3.3) How do I tune the database engine for better performance?
There are three major areas for potential performance improvement: There are three major areas for potential performance improvement:
Query Changes Query Changes
@@ -384,69 +359,32 @@
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
x.html and http://www.powerpostgresql.com/PerfList/. x.html and http://www.powerpostgresql.com/PerfList/.
3.4) What debugging features are available? 3.4) What debugging features are available?
There are many log_* server configuration variables that enable There are many log_* server configuration variables that enable
printing of query and process statistics which can be very useful for printing of query and process statistics which can be very useful for
debugging and performance measurements. debugging and performance measurements.
The following detailed debug instructions are to be used to provide 3.5) Why do I get "Sorry, too many clients" when trying to connect?
more detailed information for server developers debugging a problem.
It is also possible to debug the server if it isn't operating
properly. First, by running configure with the --enable-cassert
option, many assert()s monitor the progress of the backend and halt
the program when something unexpected occurs.
The postmaster has a -d option that allows even more detailed
information to be reported. The -d option takes a number that
specifies the debug level. Be warned that high debug level values
generate large log files.
If postmaster is not running, you can actually run the postgres
backend from the command line, and type your SQL statement directly.
This is recommended only for debugging purposes. Note that a newline
terminates the query, not a semicolon. If you have compiled with
debugging symbols, you can use a debugger to see what is happening.
Because the backend was not started from postmaster, it is not running
in an identical environment and locking/backend interaction problems
may not be duplicated.
If postmaster is running, start psql in one window, then find the PID
of the postgres process used by psql using SELECT pg_backend_pid().
Use a debugger to attach to the postgres PID. You can set breakpoints
in the debugger and issue queries from psql. If you are debugging
postgres startup, you can set PGOPTIONS="-W n", then start psql. This
will cause startup to delay for n seconds so you can attach to the
process with the debugger, set any breakpoints, and continue through
the startup sequence.
You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put
in the client's current directory. Linux requires a compile with
-DLINUX_PROFILE for proper profiling.
3.5) Why do I get "Sorry, too many clients" when trying to connect?
You have reached the default limit is 100 database sessions. You need You have reached the default limit is 100 database sessions. You need
to increase the postmaster's limit on how many concurrent backend to increase the postmaster's limit on how many concurrent backend
processes it can start by changing the max_connections value in processes it can start by changing the max_connections value in
postgresql.conf and restarting the postmaster. postgresql.conf and restarting the postmaster.
3.6) Why do I need to do a dump and restore to upgrade between major 3.6) Why do I need to do a dump and restore to upgrade between major
PostgreSQL releases? PostgreSQL releases?
The PostgreSQL team makes only small changes between minor releases, The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4 to 7.4.1 does not require a dump and restore. so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the However, major releases (e.g. from 7.3 to 7.4) often change the
internal format of system tables and data files. These changes are internal format of system tables and data files. These changes are
often complex, so we don't maintain backward compatibility for data often complex, so we don't maintain backward compatibility for data
files. A dump outputs data in a generic format that can then be loaded files. A dump outputs data in a generic format that can then be loaded
in using the new internal format. in using the new internal format.
3.7) What computer hardware should I use? 3.7) What computer hardware should I use?
Because PC hardware is mostly compatible, people tend to believe that Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
quality motherboards are more reliable and have better performance quality motherboards are more reliable and have better performance
@@ -458,8 +396,8 @@
Operational Questions Operational Questions
4.1) How do I SELECT only the first few rows of a query? A random row? 4.1) How do I SELECT only the first few rows of a query? A random row?
To retrieve only a few rows, if you know at the number of rows needed To retrieve only a few rows, if you know at the number of rows needed
at the time of the SELECT use LIMIT . If an index matches the ORDER BY at the time of the SELECT use LIMIT . If an index matches the ORDER BY
it is possible the entire query does not have to be executed. If you it is possible the entire query does not have to be executed. If you
@@ -471,9 +409,9 @@
ORDER BY random() ORDER BY random()
LIMIT 1; LIMIT 1;
4.2) How do I find out what tables, indexes, databases, and users are 4.2) How do I find out what tables, indexes, databases, and users are
defined? How do I see the queries used by psql to display them? defined? How do I see the queries used by psql to display them?
Use the \dt command to see tables in psql. For a complete list of Use the \dt command to see tables in psql. For a complete list of
commands inside psql you can use \?. Alternatively you can read the commands inside psql you can use \?. Alternatively you can read the
source code for psql in file pgsql/src/bin/psql/describe.c, it source code for psql in file pgsql/src/bin/psql/describe.c, it
@@ -492,8 +430,8 @@
many of the SELECTs needed to get information from the database system many of the SELECTs needed to get information from the database system
tables. tables.
4.3) How do you change a column's data type? 4.3) How do you change a column's data type?
Changing the data type of a column can be done easily in 8.0 and later Changing the data type of a column can be done easily in 8.0 and later
with ALTER TABLE ALTER COLUMN TYPE. with ALTER TABLE ALTER COLUMN TYPE.
@@ -507,8 +445,8 @@
You might then want to do VACUUM FULL tab to reclaim the disk space You might then want to do VACUUM FULL tab to reclaim the disk space
used by the expired rows. used by the expired rows.
4.4) What is the maximum size for a row, a table, and a database? 4.4) What is the maximum size for a row, a table, and a database?
These are the limits: These are the limits:
Maximum size for a database? unlimited (32 TB databases exist) Maximum size for a database? unlimited (32 TB databases exist)
@@ -531,9 +469,9 @@
The maximum table size and maximum number of columns can be quadrupled The maximum table size and maximum number of columns can be quadrupled
by increasing the default block size to 32k. by increasing the default block size to 32k.
4.5) How much database disk space is required to store data from a typical 4.5) How much database disk space is required to store data from a typical
text file? text file?
A PostgreSQL database may require up to five times the disk space to A PostgreSQL database may require up to five times the disk space to
store data from a text file. store data from a text file.
@@ -565,13 +503,13 @@
NULLs are stored as bitmaps, so they use very little space. NULLs are stored as bitmaps, so they use very little space.
4.6) Why are my queries slow? Why don't they use my indexes? 4.6) Why are my queries slow? Why don't they use my indexes?
Indexes are not automatically used by every query. Indexes are only Indexes are not used by every query. Indexes are used only if the
used if the table is larger than a minimum size, and the query selects table is larger than a minimum size, and the query selects only a
only a small percentage of the rows in the table. This is because the small percentage of the rows in the table. This is because the random
random disk access caused by an index scan can be slower than a disk access caused by an index scan can be slower than a straight read
straight read through the table, or sequential scan. through the table, or sequential scan.
To determine if an index should be used, PostgreSQL must have To determine if an index should be used, PostgreSQL must have
statistics about the table. These statistics are collected using statistics about the table. These statistics are collected using
@@ -594,8 +532,8 @@
LIMIT 1; LIMIT 1;
If you believe the optimizer is incorrect in choosing a sequential If you believe the optimizer is incorrect in choosing a sequential
scan, use SET enable_seqscan TO 'off' and run tests to see if an index scan, use SET enable_seqscan TO 'off' and run query again to see if an
scan is indeed faster. index scan is indeed faster.
When using wild-card operators such as LIKE or ~, indexes can only be When using wild-card operators such as LIKE or ~, indexes can only be
used in certain circumstances: used in certain circumstances:
@@ -617,14 +555,14 @@
types exactly match the index's column types. This was particularly types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes. true of int2, int8, and numeric column indexes.
4.7) How do I see how the query optimizer is evaluating my query? 4.7) How do I see how the query optimizer is evaluating my query?
See the EXPLAIN manual page. See the EXPLAIN manual page.
4.8) How do I perform regular expression searches and case-insensitive 4.8) How do I perform regular expression searches and case-insensitive
regular expression searches? How do I use an index for case-insensitive regular expression searches? How do I use an index for case-insensitive
searches? searches?
The ~ operator does regular expression matching, and ~* does The ~ operator does regular expression matching, and ~* does
case-insensitive regular expression matching. The case-insensitive case-insensitive regular expression matching. The case-insensitive
variant of LIKE is called ILIKE. variant of LIKE is called ILIKE.
@@ -638,24 +576,24 @@
expresssion index, it will be used: expresssion index, it will be used:
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
4.9) In a query, how do I detect if a field is NULL? How can I sort on 4.9) In a query, how do I detect if a field is NULL? How can I sort on
whether a field is NULL or not? whether a field is NULL or not?
You test the column with IS NULL and IS NOT NULL. You test the column with IS NULL and IS NOT NULL, like this:
SELECT * SELECT *
FROM tab FROM tab
WHERE col IS NULL; WHERE col IS NULL;
To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers
in your WHERE clause. Things that are true will sort higher than in your ORDER BY clause. Things that are true will sort higher than
things that are false, so the following will put NULL entries at the things that are false, so the following will put NULL entries at the
top of the resulting list: top of the resulting list:
SELECT * SELECT *
FROM tab FROM tab
ORDER BY (col IS NOT NULL) ORDER BY (col IS NOT NULL)
4.10) What is the difference between the various character types? 4.10) What is the difference between the various character types?
Type Internal Name Notes Type Internal Name Notes
VARCHAR(n) varchar size specifies maximum length, no padding VARCHAR(n) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length CHAR(n) bpchar blank padded to the specified fixed length
@@ -668,9 +606,9 @@
The first four types above are "varlena" types (i.e., the first four The first four types above are "varlena" types (i.e., the first four
bytes on disk are the length, followed by the data). Thus the actual bytes on disk are the length, followed by the data). Thus the actual
space used is slightly greater than the declared size. However, these space used is slightly greater than the declared size. However, long
data types are also subject to compression or being stored out-of-line values are also subject to compression, so the space on disk might
by TOAST, so the space on disk might also be less than expected. also be less than expected.
VARCHAR(n) is best when storing variable-length strings and it limits VARCHAR(n) is best when storing variable-length strings and it limits
how long a string can be. TEXT is for strings of unlimited length, how long a string can be. TEXT is for strings of unlimited length,
with a maximum of one gigabyte. with a maximum of one gigabyte.
@@ -681,8 +619,8 @@
particularly values that include NULL bytes. All the types described particularly values that include NULL bytes. All the types described
here have similar performance characteristics. here have similar performance characteristics.
4.11.1) How do I create a serial/auto-incrementing field? 4.11.1) How do I create a serial/auto-incrementing field?
PostgreSQL supports a SERIAL data type. It auto-creates a sequence. PostgreSQL supports a SERIAL data type. It auto-creates a sequence.
For example, this: For example, this:
CREATE TABLE person ( CREATE TABLE person (
@@ -700,8 +638,8 @@
See the create_sequence manual page for more information about See the create_sequence manual page for more information about
sequences. sequences.
4.11.2) How do I get the value of a SERIAL insert? 4.11.2) How do I get the value of a SERIAL insert?
One approach is to retrieve the next SERIAL value from the sequence One approach is to retrieve the next SERIAL value from the sequence
object with the nextval() function before inserting and then insert it object with the nextval() function before inserting and then insert it
explicitly. Using the example table in 4.11.1, an example in a explicitly. Using the example table in 4.11.1, an example in a
@@ -712,28 +650,28 @@
You would then also have the new value stored in new_id for use in You would then also have the new value stored in new_id for use in
other queries (e.g., as a foreign key to the person table). Note that other queries (e.g., as a foreign key to the person table). Note that
the name of the automatically created SEQUENCE object will be named the name of the automatically created SEQUENCE object will be named
<table>_<serialcolumn>_seq, where table and serialcolumn are the names <table>_< serialcolumn>_seq, where table and serialcolumn are the
of your table and your SERIAL column, respectively. names of your table and your SERIAL column, respectively.
Alternatively, you could retrieve the assigned SERIAL value with the Alternatively, you could retrieve the assigned SERIAL value with the
currval() function after it was inserted by default, e.g., currval() function after it was inserted by default, e.g.,
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
4.11.3) Doesn't currval() lead to a race condition with other users? 4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your session, not No. currval() returns the current value assigned by your session, not
by all sessions. by all sessions.
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are 4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are
there gaps in the numbering of my sequence/SERIAL column? there gaps in the numbering of my sequence/SERIAL column?
To improve concurrency, sequence values are given out to running To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted transactions. completes. This causes gaps in numbering from aborted transactions.
4.12) What is an OID? What is a TID? 4.12) What is an OID? What is a CTID?
Every row that is created in PostgreSQL gets a unique OID unless Every row that is created in PostgreSQL gets a unique OID unless
created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
integers that are unique across the entire installation. However, they integers that are unique across the entire installation. However, they
@@ -745,12 +683,12 @@
single table. and are therefore less likely to overflow. SERIAL8 is single table. and are therefore less likely to overflow. SERIAL8 is
available for storing eight-byte sequence values. available for storing eight-byte sequence values.
TIDs are used to identify specific physical rows with block and offset CTIDs are used to identify specific physical rows with block and
values. TIDs change after rows are modified or reloaded. They are used offset values. CTIDs change after rows are modified or reloaded. They
by index entries to point to physical rows. are used by index entries to point to physical rows.
4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your You probably have run out of virtual memory on your system, or your
kernel has a low limit for certain resources. Try this before starting kernel has a low limit for certain resources. Try this before starting
postmaster: postmaster:
@@ -764,32 +702,17 @@
problem with the SQL client because the backend is returning too much problem with the SQL client because the backend is returning too much
data, try it before starting the client. data, try it before starting the client.
4.14) How do I tell what PostgreSQL version I am running? 4.14) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version(); From psql, type SELECT version();
4.15) Why does my large-object operations get "invalid large obj 4.15) How do I create a column that will default to the current time?
descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close.
Currently PostgreSQL enforces the rule by closing large object handles
at transaction commit. So the first attempt to do anything with the
handle will draw invalid large obj descriptor. So code that used to
work (at least most of the time) will now generate that error message
if you fail to use a transaction.
If you are using a client interface like ODBC you may need to set
auto-commit off.
4.16) How do I create a column that will default to the current time?
Use CURRENT_TIMESTAMP: Use CURRENT_TIMESTAMP:
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
4.17) How do I perform an outer join? 4.16) How do I perform an outer join?
PostgreSQL supports outer joins using the SQL standard syntax. Here PostgreSQL supports outer joins using the SQL standard syntax. Here
are two examples: are two examples:
SELECT * SELECT *
@@ -806,8 +729,8 @@
is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called
INNER joins. INNER joins.
4.18) How do I perform queries using multiple databases? 4.17) How do I perform queries using multiple databases?
There is no way to query a database other than the current one. There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is Because PostgreSQL loads database-specific system catalogs, it is
uncertain how a cross-database query should even behave. uncertain how a cross-database query should even behave.
@@ -816,15 +739,15 @@
course, a client can also make simultaneous connections to different course, a client can also make simultaneous connections to different
databases and merge the results on the client side. databases and merge the results on the client side.
4.19) How do I return multiple rows or columns from a function? 4.18) How do I return multiple rows or columns from a function?
It is easy using set-returning functions, It is easy using set-returning functions,
http://techdocs.postgresql.org/guides/SetReturningFunctions http://techdocs.postgresql.org/guides/SetReturningFunctions
. .
4.20) Why do I get "relation with OID ##### does not exist" errors when 4.19) Why do I get "relation with OID ##### does not exist" errors when
accessing temporary tables in PL/PgSQL functions? accessing temporary tables in PL/PgSQL functions?
PL/PgSQL caches function scripts, and an unfortunate side effect is PL/PgSQL caches function scripts, and an unfortunate side effect is
that if a PL/PgSQL function accesses a temporary table, and that table that if a PL/PgSQL function accesses a temporary table, and that table
is later dropped and recreated, and the function called again, the is later dropped and recreated, and the function called again, the
@@ -833,46 +756,21 @@
table access in PL/PgSQL. This will cause the query to be reparsed table access in PL/PgSQL. This will cause the query to be reparsed
every time. every time.
4.21) What encryption options are available? 4.20) What replication solutions are available?
* contrib/pgcrypto contains many encryption functions for use in SQL Though "replication" is a single term, there are several technologies
queries. for doing replication, with advantages and disadvantages for each.
* To encrypt transmission from the client to the server, the server
must have the ssl option set to true in postgresql.conf, and an
applicable host or hostssl record must exist in pg_hba.conf, and
the client sslmode must not be disable. (Note that it is also
possible to use a third-party encrypted transport, such as stunnel
or ssh, rather than PostgreSQL's native SSL connections.)
* Database user passwords are automatically encrypted when stored in
the system tables.
* The server can run using an encrypted file system.
_________________________________________________________________
Extending PostgreSQL Master/slave replication allows a single master to receive read/write
queries, while slaves can only accept read/SELECT queries. The most
5.1) I wrote a user-defined function. When I run it in psql, why does it popular freely available master-slave PostgreSQL replication solution
dump core? is Slony-I.
The problem could be a number of things. Try testing your user-defined
function in a stand-alone test program first.
5.2) How can I contribute some nifty new types and functions to PostgreSQL? Multi-master replication allows read/write queries to be sent to
multiple replicated computers. This capability also has a severe
Send your extensions to the pgsql-hackers mailing list, and they will impact on performance due to the need to synchronize changes between
eventually end up in the contrib/ subdirectory. servers. Pgcluster is the most popular such solution freely available
for PostgreSQL.
5.3) How do I write a C function to return a tuple? There are also commercial and hardware-based replication solutions
available supporting a variety of replication models.
In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the
Programmer's Guide for more information. An example of a
table-returning function defined in C can be found in
contrib/tablefunc.
5.4) I have changed a source file. Why does the recompile not see the
change?
The Makefiles do not have the proper dependencies for include files.
You have to do a make clean and then another make. If you are using
GCC you can use the --enable-depend option of configure to have the
compiler compute the dependencies automatically.

48
doc/FAQ_DEV
View File

@@ -1,7 +1,7 @@
Developer's Frequently Asked Questions (FAQ) for PostgreSQL Developer's Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sun Mar 13 22:07:18 EST 2005 Last updated: Fri May 6 13:47:54 EDT 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -23,7 +23,7 @@ General Questions
1.10) What books are good for developers? 1.10) What books are good for developers?
1.11) What is configure all about? 1.11) What is configure all about?
1.12) How do I add a new port? 1.12) How do I add a new port?
1.13) Why don't you use threads/raw devices/async-I/O, <insert your 1.13) Why don't you use threads, raw devices, async-I/O, <insert your
favorite wizz-bang feature here>? favorite wizz-bang feature here>?
1.14) How are RPM's packaged? 1.14) How are RPM's packaged?
1.15) How are CVS branches handled? 1.15) How are CVS branches handled?
@@ -42,6 +42,7 @@ Technical Questions
2.5) Why do we use palloc() and pfree() to allocate memory? 2.5) Why do we use palloc() and pfree() to allocate memory?
2.6) What is ereport()? 2.6) What is ereport()?
2.7) What is CommandCounterIncrement()? 2.7) What is CommandCounterIncrement()?
2.8) What debugging features are available?
_________________________________________________________________ _________________________________________________________________
General Questions General Questions
@@ -172,6 +173,11 @@ General Questions
There have been a number of discussions about other testing frameworks There have been a number of discussions about other testing frameworks
and some developers are exploring these ideas. and some developers are exploring these ideas.
Keep in mind the Makefiles do not have the proper dependencies for
include files. You have to do a make clean and then another make. If
you are using GCC you can use the --enable-depend option of configure
to have the compiler compute the dependencies automatically.
1.9) What tools are available for developers? 1.9) What tools are available for developers?
First, all the files in the src/tools directory are designed for First, all the files in the src/tools directory are designed for
@@ -343,8 +349,8 @@ General Questions
src/makefiles directory for port-specific Makefile handling. There is src/makefiles directory for port-specific Makefile handling. There is
a backend/port directory if you need special files for your OS. a backend/port directory if you need special files for your OS.
1.13) Why don't you use threads/raw devices/async-I/O, <insert your favorite 1.13) Why don't you use threads, raw devices, async-I/O, <insert your
wizz-bang feature here>? favorite wizz-bang feature here>?
There is always a temptation to use the newest operating system There is always a temptation to use the newest operating system
features as soon as they arrive. We resist that temptation. features as soon as they arrive. We resist that temptation.
@@ -762,3 +768,37 @@ typedef struct nameData
to be broken into pieces so each piece can see rows modified by to be broken into pieces so each piece can see rows modified by
previous pieces. CommandCounterIncrement() increments the Command previous pieces. CommandCounterIncrement() increments the Command
Counter, creating a new part of the transaction. Counter, creating a new part of the transaction.
2.8) What debugging features are available?
First, try running configure with the --enable-cassert option, many
assert()s monitor the progress of the backend and halt the program
when something unexpected occurs.
The postmaster has a -d option that allows even more detailed
information to be reported. The -d option takes a number that
specifies the debug level. Be warned that high debug level values
generate large log files.
If the postmaster is not running, you can actually run the postgres
backend from the command line, and type your SQL statement directly.
This is recommended only for debugging purposes. If you have compiled
with debugging symbols, you can use a debugger to see what is
happening. Because the backend was not started from postmaster, it is
not running in an identical environment and locking/backend
interaction problems may not be duplicated.
If the postmaster is running, start psql in one window, then find the
PID of the postgres process used by psql using SELECT
pg_backend_pid(). Use a debugger to attach to the postgres PID. You
can set breakpoints in the debugger and issue queries from psql. If
you are debugging postgres startup, you can set PGOPTIONS="-W n", then
start psql. This will cause startup to delay for n seconds so you can
attach to the process with the debugger, set any breakpoints, and
continue through the startup sequence.
You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put
in the client's current directory. Linux requires a compile with
-DLINUX_PROFILE for proper profiling.

468
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View File

@@ -10,7 +10,7 @@
alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Mar 15 17:38:18 EST 2005</P> <P>Last updated: Mon May 9 13:15:04 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>) "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
@@ -40,6 +40,7 @@
<A href="#1.11">1.11</A>) How do I join the development team?<BR> <A href="#1.11">1.11</A>) How do I join the development team?<BR>
<A href="#1.12">1.12</A>) How does PostgreSQL compare to other <A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR> <SMALL>DBMS</SMALL>s?<BR>
<A href="#1.13">1.13</A>) Who controls PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
@@ -99,41 +100,29 @@
reused on transaction abort? Why are there gaps in the numbering of reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?<BR> my sequence/SERIAL column?<BR>
<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a <A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR> <SMALL>CTID</SMALL>?<BR>
<A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory <A href="#4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR> exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I <A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR> am running?<BR>
<A href="#4.15">4.15</A>) Why does my large-object operations get <A href="#4.15">4.15</A>) How do I create a column that will
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.16">4.16</A>) How do I create a column that will
default to the current time?<BR> default to the current time?<BR>
<A href="#4.17">4.17</A>) How do I perform an outer join?<BR> <A href="#4.16">4.16</A>) How do I perform an outer join?<BR>
<A href="#4.18">4.18</A>) How do I perform queries using multiple <A href="#4.17">4.17</A>) How do I perform queries using multiple
databases?<BR> databases?<BR>
<A href="#4.19">4.19</A>) How do I return multiple rows or columns <A href="#4.18">4.18</A>) How do I return multiple rows or columns
from a function?<BR> from a function?<BR>
<A href="#4.20">4.20</A>) Why do I get "relation with OID ##### <A href="#4.19">4.19</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL does not exist" errors when accessing temporary tables in PL/PgSQL
functions?<BR> functions?<BR>
<A href="#4.21">4.21</A>) What encryption options are available?<BR> <A href="#4.20">4.20</A>) What replication solutions are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
it in <I>psql</I>, why does it dump core?<BR>
<A href="#5.2">5.2</A>) How can I contribute some nifty new types
and functions to PostgreSQL?<BR>
<A href="#5.3">5.3</A>) How do I write a C function to return a
tuple?<BR>
<A href="#5.4">5.4</A>) I have changed a source file. Why does the
recompile not see the change?<BR>
<HR> <HR>
<H2 align="center">General Questions</H2> <H2 align="center">General Questions</H2>
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4> <H3><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H3>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P> <I>Postgres</I>.</P>
@@ -152,8 +141,8 @@
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A> http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P> </P>
<H4><A name="1.2">1.2</A>) What is the copyright of <H3><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4> PostgreSQL?</H3>
<P>PostgreSQL is distributed under the classic BSD license. It has <P>PostgreSQL is distributed under the classic BSD license. It has
no restrictions on how the source code can be used. We like it and no restrictions on how the source code can be used. We like it and
@@ -185,17 +174,16 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P> SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4> <H3><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H3>
<P>In general, any modern Unix-compatible platform should be able to <P>In general, any modern Unix-compatible platform should be able to
run PostgreSQL. The platforms that had received explicit testing at run PostgreSQL. The platforms that had received explicit testing at
the time of release are listed in the installation the time of release are listed in the installation
instructions.</P> instructions.</P>
<P>Starting with version 8.0, PostgreSQL now runs natively on <P>PostgreSQL also runs natively on Microsoft Windows NT-based operating
Microsoft Windows NT-based operating systems like Win2000, WinXP, systems like Win2000, WinXP, and Win2003. A prepackaged installer is
and Win2003. A prepackaged installer is available at <a href= available at <a href= "http://pgfoundry.org/projects/pginstaller">
"http://pgfoundry.org/projects/pginstaller">
http://pgfoundry.org/projects/pginstaller</a>. MSDOS-based versions http://pgfoundry.org/projects/pginstaller</a>. MSDOS-based versions
of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.</P> of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.</P>
@@ -205,13 +193,14 @@
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F"> "http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p> http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p>
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4> <H3><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H3>
<P>The primary anonymous ftp site for PostgreSQL is <A href= <P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A>. http://www.postgresql.org/ftp/</a>, and via ftp, use
For mirror sites, see our main web site.</P> <A href="ftp://ftp.PostgreSQL.org/pub/">
ftp://ftp.PostgreSQL.org/pub/</A>.</P>
<H4><A name="1.5">1.5</A>) Where can I get support?</H4> <H3><A name="1.5">1.5</A>) Where can I get support?</H3>
<P>The PostgreSQL community provides assistance to many of its users <P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is via email. The main web site to subscribe to the email lists is
@@ -222,7 +211,7 @@
<P>The major IRC channel is <I>#postgresql</I> on Freenode <P>The major IRC channel is <I>#postgresql</I> on Freenode
(<I>irc.freenode.net</I>). To connect you can use the Unix (<I>irc.freenode.net</I>). To connect you can use the Unix
program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE> program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
or use any of the other popular IRC clients. A Spanish one also exists or use any other IRC clients. A Spanish one also exists
on the same network, (<I>#postgresql-es</I>), and a French one, on the same network, (<I>#postgresql-es</I>), and a French one,
(<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.</P> (<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.</P>
@@ -230,7 +219,7 @@
"http://techdocs.postgresql.org/companies.php"> "http://techdocs.postgresql.org/companies.php">
http://techdocs.postgresql.org/companies.php</A>.</P> http://techdocs.postgresql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4> <H3><A name="1.6">1.6</A>) How do I submit a bug report?</H3>
<P>Visit the PostgreSQL bug form at <A href= <P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug"> "http://www.postgresql.org/support/submitbug">
@@ -240,13 +229,14 @@
"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to "ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A> to
see if there is a more recent PostgreSQL version.</P> see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4> <H3><A name="1.7">1.7</A>) What is the latest release?</H3>
<P>The latest release of PostgreSQL is version 8.0.1.</P> <P>The latest release of PostgreSQL is version 8.0.2.</P>
<P>We plan to have major releases every ten to twelve months.</P> <P>We plan to have a major release every year, with minor releases
every few months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4> <H3><A name="1.8">1.8</A>) What documentation is available?</H3>
<P>PostgreSQL includes extensive documentation, including a large <P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I> manual, manual pages, and some test examples. See the <I>/doc</I>
@@ -258,8 +248,9 @@
"http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A> "http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
and <A href= and <A href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>. "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
There is a list of PostgreSQL books available for purchase at <A There are a number of PostgreSQL books available for purchase. One
href= of the most popular ones is by Korry Douglas. A list of book reviews
can be found at <A href=
"http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>. "http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
There is also a collection of PostgreSQL technical articles at <A There is also a collection of PostgreSQL technical articles at <A
href= href=
@@ -271,22 +262,25 @@
<P>Our web site contains even more documentation.</P> <P>Our web site contains even more documentation.</P>
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or <H3><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4> missing features?</H3>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92. <P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A> See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list for known bugs, missing features, and future plans.</P> list for known bugs, missing features, and future plans.</P>
<H4><A name="1.10">1.10</A>) How can I learn <H3><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4> <SMALL>SQL</SMALL>?</H3>
<P>The PostgreSQL book at <A href= <P>First, consider the PostgreSQL-specific books mentioned above.
"http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A> Another one is "Teach Yourself SQL in 21 Days, Second Edition"
teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A at <A href=
href= "http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A>.
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A> Many of our users like <I>The Practical SQL Handbook</I>,
There is a nice tutorial at <A href= Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<P>There is also a nice tutorial at <A href=
"http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A> "http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
at <A href= at <A href=
"http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM"> "http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
@@ -294,31 +288,14 @@
and at <A href= and at <A href=
"http://sqlcourse.com/">http://sqlcourse.com.</A></P> "http://sqlcourse.com/">http://sqlcourse.com.</A></P>
<P>Another one is "Teach Yourself SQL in 21 Days, Second Edition" <H3><A name="1.11">1.11</A>) How do I join the development
at <A href= team?</H3>
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
<P>Many of our users like <I>The Practical SQL Handbook</I>, <P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The Developer's FAQ</A>.
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.11">1.11</A>) How do I join the development <H3><A name="1.12">1.12</A>) How does PostgreSQL compare to other
team?</H4> <SMALL>DBMS</SMALL>s?</H3>
<P>First, download the latest source and read the PostgreSQL
Developers FAQ and documentation on our web site, or in the
distribution. Second, subscribe to the <I>pgsql-hackers</I> and
<I>pgsql-patches</I> mailing lists. Third, submit high quality
patches to pgsql-patches.</P>
<P>There are about a dozen people who have commit privileges to the
PostgreSQL <SMALL>CVS</SMALL> archive. They each have submitted so
many high-quality patches that it was impossible for the existing
committers to keep up, and we had confidence that patches they
committed were of high quality.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
<P>There are several ways of measuring software: features, <P>There are several ways of measuring software: features,
performance, reliability, support, and price.</P> performance, reliability, support, and price.</P>
@@ -339,13 +316,7 @@
<DD>PostgreSQL's performance is comparable to other commercial and <DD>PostgreSQL's performance is comparable to other commercial and
open source databases. It is faster for some things, slower for open source databases. It is faster for some things, slower for
others. In comparison to MySQL or leaner database systems, we are others. Our performance is usually +/-10% compared to other databases.
faster for multiple users, complex queries, and a read/write query
load. MySQL is faster for simple SELECT queries done by a few users.
Of course, MySQL does not have most of the features mentioned in the
<I>Features</I> section above. We are built for reliability and
features, and we continue to improve performance in every
release. <BR>
<BR> <BR>
</DD> </DD>
@@ -383,13 +354,24 @@
</DD> </DD>
</DL> </DL>
<H3><A name="1.13">1.13</A>) Who controls PostgreSQL?<BR>
<P>If you are looking for a PostgreSQL gatekeeper, central committee,
or controlling company, give up --- there isn't one. We do have a
core committee and CVS committers, but these groups are more for
administrative purposes than control. The project is directed by
the community of developers and users, which anyone can join. All
you need to do is subscribe to the mailing lists and participate in the
discussions. (See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A> for information on how to get involved in PostgreSQL
development.)</P>
<HR> <HR>
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) What interfaces are available for <H3><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H4> PostgreSQL?</H3>
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded <P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects <SMALL>C</SMALL> interfaces. All other interfaces are independent projects
@@ -403,8 +385,8 @@
in the <I>Drivers/Interfaces</I> section and via Internet search. in the <I>Drivers/Interfaces</I> section and via Internet search.
</P> </P>
<H4><A name="2.2">2.2</A>) What tools are available for using <H3><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4> PostgreSQL with Web pages?</H3>
<P>A nice introduction to Database-backed Web pages can be seen at: <P>A nice introduction to Database-backed Web pages can be seen at:
<A href="http://www.webreview.com">http://www.webreview.com</A></P> <A href="http://www.webreview.com">http://www.webreview.com</A></P>
@@ -416,25 +398,11 @@
<P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or <P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
mod_perl.</P> mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user <H3><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4> interface?</H3>
<P>Yes, there are several graphical interfaces to PostgreSQL <P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools">
available. These include pgAdmin III (<a http://techdocs.postgresql.org/guides/GUITools</a> for a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, PgAccess
<a href="http://www.pgaccess.org"> http://www.pgaccess.org</a>),
RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
</a>), TORA (<a href="http://www.globecom.net/tora/">
http://www.globecom.net/tora/</a>, partly commercial), and Rekall (<a
href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin (<a
href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
PostgreSQL.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
detailed list.</P> detailed list.</P>
<HR> <HR>
@@ -442,14 +410,14 @@
<H2 align="center">Administrative Questions</H2> <H2 align="center">Administrative Questions</H2>
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere <H3><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H4> other than <I>/usr/local/pgsql</I>?</H3>
<P>Specify the <I>--prefix</I> option when running <P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P> <I>configure</I>.</P>
<H4><A name="3.2">3.2</A>) How do I control connections from other <H3><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H4> hosts?</H3>
<P>By default, PostgreSQL only allows connections from the local <P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets or TCP/IP connections. Other machine using Unix domain sockets or TCP/IP connections. Other
@@ -458,8 +426,8 @@
host-based authentication by modifying the host-based authentication by modifying the
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P> <I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
<H4><A name="3.3">3.3</A>) How do I tune the database engine for <H3><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4> better performance?</H3>
<P>There are three major areas for potential performance <P>There are three major areas for potential performance
improvement:</P> improvement:</P>
@@ -515,57 +483,15 @@
</DD> </DD>
</DL> </DL>
<H4><A name="3.4">3.4</A>) What debugging features are <H3><A name="3.4">3.4</A>) What debugging features are
available?</H4> available?</H3>
<P>There are many <CODE>log_*</CODE> server configuration variables <P>There are many <CODE>log_*</CODE> server configuration variables
that enable printing of query and process statistics which can be that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P> very useful for debugging and performance measurements.</P>
<P><B>The following detailed debug instructions are to be used to <H3><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
provide more detailed information for server developers debugging a clients"</I> when trying to connect?</H3>
problem.</B></P>
<P>It is also possible to debug the server if it isn't operating
properly. First, by running <I>configure</I> with the --enable-cassert
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
<P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
detailed information to be reported. The <I>-d</I> option takes a
number that specifies the debug level. Be warned that high debug
level values generate large log files.</P>
<P>If <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
<SMALL>SQL</SMALL> statement directly. This is recommended
<B>only</B> for debugging purposes. Note that a newline terminates
the query, not a semicolon. If you have compiled with debugging
symbols, you can use a debugger to see what is happening. Because
the backend was not started from <I>postmaster</I>, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.</P>
<P>If <I>postmaster</I> is running, start <I>psql</I> in one
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
You can set breakpoints in the debugger and issue queries from
<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
to delay for <I>n</I> seconds so you can attach to the process with
the debugger, set any breakpoints, and continue through the startup
sequence.</P>
<P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<P>You have reached the default limit is 100 database sessions. You <P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many need to increase the <I>postmaster</I>'s limit on how many
@@ -573,18 +499,18 @@
<I>max_connections</I> value in <I>postgresql.conf</I> and <I>max_connections</I> value in <I>postgresql.conf</I> and
restarting the <I>postmaster</I>.</P> restarting the <I>postmaster</I>.</P>
<H4><A name="3.6">3.6</A>) Why do I need to do a dump and restore <H3><A name="3.6">3.6</A>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H4> to upgrade between major PostgreSQL releases?</H3>
<P>The PostgreSQL team makes only small changes between minor releases, <P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4 to 7.4.1 does not require a dump and restore. so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the internal However, major releases (e.g. from 7.3 to 7.4) often change the internal
format of system tables and data files. These changes are often complex, format of system tables and data files. These changes are often complex,
so we don't maintain backward compatibility for data files. A dump outputs so we don't maintain backward compatibility for data files. A dump outputs
data in a generic format that can then be loaded in using the new internal data in a generic format that can then be loaded in using the new internal
format.</P> format.</P>
<H4><A name="3.7">3.7</A>) What computer hardware should I use?</H4> <H3><A name="3.7">3.7</A>) What computer hardware should I use?</H3>
<P>Because PC hardware is mostly compatible, people tend to believe that <P>Because PC hardware is mostly compatible, people tend to believe that
all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and all PC hardware is of equal quality. It is not. ECC RAM, SCSI, and
@@ -598,8 +524,8 @@
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the <H3><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4> first few rows of a query? A random row?</H3>
<P>To retrieve only a few rows, if you know at the number of rows <P>To retrieve only a few rows, if you know at the number of rows
needed at the time of the <SMALL>SELECT</SMALL> use needed at the time of the <SMALL>SELECT</SMALL> use
@@ -617,9 +543,9 @@
LIMIT 1; LIMIT 1;
</PRE> </PRE>
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes, <H3><A name="4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?</H4> by <I>psql</I> to display them?</H3>
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of <P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
commands inside psql you can use \?. Alternatively you can read the source commands inside psql you can use \?. Alternatively you can read the source
@@ -640,7 +566,7 @@
illustrates many of the <SMALL>SELECT</SMALL>s needed to get illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P> information from the database system tables.</P>
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4> <H3><A name="4.3">4.3</A>) How do you change a column's data type?</H3>
<P>Changing the data type of a column can be done easily in 8.0 <P>Changing the data type of a column can be done easily in 8.0
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P> and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.</P>
@@ -656,12 +582,12 @@
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the <P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P> disk space used by the expired rows.</P>
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a <H3><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4> table, and a database?</H3>
<P>These are the limits:</P> <P>These are the limits:</P>
<BLOCKQUOTE> <BLOCKQUOTE>
<TABLE > <TABLE>
<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases <TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
exist)</TD></TR> exist)</TD></TR>
<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR> <TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
@@ -687,8 +613,8 @@ table?</TD><TD>unlimited</TD></TR>
<P>The maximum table size and maximum number of columns can be <P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P> quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.5">4.5</A>) How much database disk space is required <H3><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4> to store data from a typical text file?</H3>
<P>A PostgreSQL database may require up to five times the disk <P>A PostgreSQL database may require up to five times the disk
space to store data from a text file.</P> space to store data from a text file.</P>
@@ -724,14 +650,14 @@ table?</TD><TD>unlimited</TD></TR>
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they <P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P> use very little space.</P>
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they <H3><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H4> use my indexes?</H3>
<P>Indexes are not automatically used by every query. Indexes are only <P>Indexes are not used by every query. Indexes are used only if the
used if the table is larger than a minimum size, and the query table is larger than a minimum size, and the query selects only a
selects only a small percentage of the rows in the table. This is small percentage of the rows in the table. This is because the random
because the random disk access caused by an index scan can be disk access caused by an index scan can be slower than a straight read
slower than a straight read through the table, or sequential scan. </P> through the table, or sequential scan. </P>
<P>To determine if an index should be used, PostgreSQL must have <P>To determine if an index should be used, PostgreSQL must have
statistics about the table. These statistics are collected using statistics about the table. These statistics are collected using
@@ -759,7 +685,7 @@ table?</TD><TD>unlimited</TD></TR>
<P>If you believe the optimizer is incorrect in choosing a <P>If you believe the optimizer is incorrect in choosing a
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
run tests to see if an index scan is indeed faster.</P> run query again to see if an index scan is indeed faster.</P>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or <P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used in certain circumstances:</P> <I>~</I>, indexes can only be used in certain circumstances:</P>
@@ -788,14 +714,14 @@ table?</TD><TD>unlimited</TD></TR>
types exactly match the index's column types. This was particularly types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.</P> true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is <H3><A name="4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?</H4> evaluating my query?</H3>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P> <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.8">4.8</A>) How do I perform regular expression <H3><A name="4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4> use an index for case-insensitive searches?</H3>
<P>The <I>~</I> operator does regular expression matching, and <P>The <I>~</I> operator does regular expression matching, and
<I>~*</I> does case-insensitive regular expression matching. The <I>~*</I> does case-insensitive regular expression matching. The
@@ -809,19 +735,18 @@ table?</TD><TD>unlimited</TD></TR>
FROM tab FROM tab
WHERE lower(col) = 'abc'; WHERE lower(col) = 'abc';
</PRE> </PRE>
This will not use an standard index. However, if you create a This will not use an standard index. However, if you create a
expresssion index, it will be used: expresssion index, it will be used:
<PRE> <PRE>
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
</PRE> </PRE>
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field <H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL> is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H4> NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS <P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P> NOT NULL</SMALL>, like this:</P>
<PRE> <PRE>
SELECT * SELECT *
@@ -830,7 +755,7 @@ table?</TD><TD>unlimited</TD></TR>
</PRE> </PRE>
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL> <P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>WHERE</SMALL> clause. and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
Things that are <I>true</I> will sort higher than things that are <I>false</I>, Things that are <I>true</I> will sort higher than things that are <I>false</I>,
so the following will put NULL entries at the top of the resulting list:</P> so the following will put NULL entries at the top of the resulting list:</P>
@@ -840,10 +765,10 @@ table?</TD><TD>unlimited</TD></TR>
ORDER BY (col IS NOT NULL) ORDER BY (col IS NOT NULL)
</PRE> </PRE>
<H4><A name="4.10">4.10</A>) What is the difference between the <H3><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4> various character types?</H3>
<BLOCKQUOTE> <BLOCKQUOTE>
<TABLE > <TABLE>
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR> <TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum <TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
length, no padding</TD></TR> length, no padding</TD></TR>
@@ -863,9 +788,8 @@ length</TD></TR>
<P>The first four types above are "varlena" types (i.e., the first <P>The first four types above are "varlena" types (i.e., the first
four bytes on disk are the length, followed by the data). Thus the four bytes on disk are the length, followed by the data). Thus the
actual space used is slightly greater than the declared size. actual space used is slightly greater than the declared size.
However, these data types are also subject to compression or being However, long values are also subject to compression, so the space
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk on disk might also be less than expected.</P>
might also be less than expected.</P>
<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length <SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
strings and it limits how long a string can be. <SMALL>TEXT</SMALL> strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
@@ -877,12 +801,11 @@ length</TD></TR>
particularly values that include <SMALL>NULL</SMALL> bytes. All the particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P> types described here have similar performance characteristics.</P>
<H4><A name="4.11.1">4.11.1</A>) How do I create a <H3><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4> serial/auto-incrementing field?</H3>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
auto-creates a sequence. For example, auto-creates a sequence. For example, this:</P>
this:</P>
<PRE> <PRE>
CREATE TABLE person ( CREATE TABLE person (
id SERIAL, id SERIAL,
@@ -891,19 +814,20 @@ length</TD></TR>
</PRE> </PRE>
is automatically translated into this: is automatically translated into this:
<PRE> <PRE>
CREATE SEQUENCE person_id_seq; CREATE SEQUENCE person_id_seq;
CREATE TABLE person ( CREATE TABLE person (
id INT4 NOT NULL DEFAULT nextval('person_id_seq'), id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
name TEXT name TEXT
); );
</PRE> </PRE>
See the <I>create_sequence</I> manual page for more information See the <I>create_sequence</I> manual page for more information
about sequences. about sequences.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a <H3><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4> <SMALL>SERIAL</SMALL> insert?</H3>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value <P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function from the sequence object with the <I>nextval()</I> function
@@ -915,39 +839,39 @@ length</TD></TR>
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')"); execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
</PRE> </PRE>
You would then also have the new value stored in You would then also have the new value stored in <CODE>new_id</CODE>
<CODE>new_id</CODE> for use in other queries (e.g., as a foreign for use in other queries (e.g., as a foreign key to the <CODE>person
key to the <CODE>person</CODE> table). Note that the name of the </CODE> table). Note that the name of the automatically created
automatically created <SMALL>SEQUENCE</SMALL> object will be named <SMALL>SEQUENCE</SMALL> object will be named &lt;<I>table</I>&gt;_&lt;<I>
&lt;<I>table</I>&gt;_&lt;<I>serialcolumn</I>&gt;_<I>seq</I>, where serialcolumn</I>&gt;_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
<I>table</I> and <I>serialcolumn</I> are the names of your table are the names of your table and your <SMALL>SERIAL</SMALL> column,
and your <SMALL>SERIAL</SMALL> column, respectively. respectively.
<P>Alternatively, you could retrieve the assigned <P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
<SMALL>SERIAL</SMALL> value with the <I>currval()</I> function value with the <I>currval()</I> function <I>after</I> it was inserted by
<I>after</I> it was inserted by default, e.g.,</P> default, e.g.,</P>
<PRE> <PRE>
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
</PRE> </PRE>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I> <H3><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4> lead to a race condition with other users?</H3>
<P>No. <I>currval()</I> returns the current value assigned by your <P>No. <I>currval()</I> returns the current value assigned by your
session, not by all sessions.</P> session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers <H3><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of reused on transaction abort? Why are there gaps in the numbering of
my sequence/SERIAL column?</H4> my sequence/SERIAL column?</H3>
<P>To improve concurrency, sequence values are given out to running <P>To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction transactions as needed and are not locked until the transaction
completes. This causes gaps in numbering from aborted completes. This causes gaps in numbering from aborted
transactions.</P> transactions.</P>
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is <H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4> a <SMALL>CTID</SMALL>?</H3>
<P>Every row that is created in PostgreSQL gets a unique <P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>. <SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
@@ -964,13 +888,13 @@ length</TD></TR>
<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence <SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
values.</P> values.</P>
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows <P>C<SMALL>TID</SMALL>s are used to identify specific physical rows
with block and offset values. T<SMALL>ID</SMALL>s change after rows with block and offset values. C<SMALL>TID</SMALL>s change after rows
are modified or reloaded. They are used by index entries to point are modified or reloaded. They are used by index entries to point
to physical rows.</P> to physical rows.</P>
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR: <H3><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4> Memory exhausted in AllocSetAlloc()"</I>?</H3>
<P>You probably have run out of virtual memory on your system, <P>You probably have run out of virtual memory on your system,
or your kernel has a low limit for certain resources. Try this or your kernel has a low limit for certain resources. Try this
@@ -988,36 +912,20 @@ length</TD></TR>
backend is returning too much data, try it before starting the backend is returning too much data, try it before starting the
client. client.
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version <H3><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4> I am running?</H3>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P> <P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.15">4.15</A>) Why does my large-object operations <H3><A name="4.15">4.15</A>) How do I create a column that will
get <I>"invalid large obj descriptor"</I>?</H4> default to the current time?</H3>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
around any use of a large object handle, that is, surrounding
<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
<P>Currently PostgreSQL enforces the rule by closing large object
handles at transaction commit. So the first attempt to do anything
with the handle will draw <I>invalid large obj descriptor</I>. So
code that used to work (at least most of the time) will now
generate that error message if you fail to use a transaction.</P>
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P> <P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE> <PRE>
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE> </PRE>
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4> <H3><A name="4.16">4.16</A>) How do I perform an outer join?</H3>
<P>PostgreSQL supports outer joins using the SQL standard syntax. <P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P> Here are two examples:</P>
@@ -1040,8 +948,8 @@ length</TD></TR>
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins <SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P> are called <SMALL>INNER</SMALL> joins.</P>
<H4><A name="4.18">4.18</A>) How do I perform queries using <H3><A name="4.17">4.17</A>) How do I perform queries using
multiple databases?</H4> multiple databases?</H3>
<P>There is no way to query a database other than the current one. <P>There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is Because PostgreSQL loads database-specific system catalogs, it is
@@ -1052,16 +960,16 @@ length</TD></TR>
connections to different databases and merge the results on the connections to different databases and merge the results on the
client side.</P> client side.</P>
<H4><A name="4.19">4.19</A>) How do I return multiple rows or <H3><A name="4.18">4.18</A>) How do I return multiple rows or
columns from a function?</H4> columns from a function?</H3>
<P>It is easy using set-returning functions, <P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions"> <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>. http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
<H4><A name="4.20">4.20</A>) Why do I get "relation with OID ##### <H3><A name="4.19">4.19</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL does not exist" errors when accessing temporary tables in PL/PgSQL
functions?</H4> functions?</H3>
<P>PL/PgSQL caches function scripts, and an unfortunate side effect <P>PL/PgSQL caches function scripts, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that is that if a PL/PgSQL function accesses a temporary table, and that
@@ -1071,56 +979,26 @@ length</TD></TR>
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This <SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P> will cause the query to be reparsed every time.</P>
<H4><A name="4.21">4.21</A>) What encryption options are available? <H3><A name="4.20">4.20</A>) What replication solutions are available?
</H4> </H3>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
use in <SMALL>SQL</SMALL> queries.</LI>
<LI>To encrypt transmission from the client to the server, the server
must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
</I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
<I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
<I>disable.</I> (Note that it is also possible to use a third-party
encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
native SSL connections.)</LI>
<LI>Database user passwords are automatically encrypted when stored in
the system tables.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
<HR> <P>Though "replication" is a single term, there are several technologies
for doing replication, with advantages and disadvantages for each.</P>
<H2 align="center">Extending PostgreSQL</H2> <P>Master/slave replication allows a single master to receive read/write
queries, while slaves can only accept read/<SMALL>SELECT</SMALL>
queries. The most popular freely available master-slave PostgreSQL
replication solution is <A
href="http://gborg.postgresql.org/project/slony1/projdisplay.php">
Slony-I</A>.</P>
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I <P>Multi-master replication allows read/write queries to be sent to
run it in <I>psql</I>, why does it dump core?</H4> multiple replicated computers. This capability also has a severe impact
on performance due to the need to synchronize changes between servers.
<A href="http://pgfoundry.org/projects/pgcluster/">Pgcluster</a> is the
most popular such solution freely available for PostgreSQL.</P>
<P>The problem could be a number of things. Try testing your <P>There are also commercial and hardware-based replication solutions
user-defined function in a stand-alone test program first.</P> available supporting a variety of replication models.</P>
<H4><A name="5.2">5.2</A>) How can I contribute some nifty new
types and functions to PostgreSQL?</H4>
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
and they will eventually end up in the <I>contrib/</I>
subdirectory.</P>
<H4><A name="5.3">5.3</A>) How do I write a C function to return a
tuple?</H4>
<P>In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the
Programmer's Guide for more information. An example of a
table-returning function defined in C can be found in
<I>contrib/tablefunc</I>.</P>
<H4><A name="5.4">5.4</A>) I have changed a source file. Why does
the recompile not see the change?</H4>
<P>The <I>Makefiles</I> do not have the proper dependencies for
include files. You have to do a <I>make clean</I> and then another
<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
<I>--enable-depend</I> option of <I>configure</I> to have the
compiler compute the dependencies automatically.</P>
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@@ -13,7 +13,7 @@
<H1>Developer's Frequently Asked Questions (FAQ) for <H1>Developer's Frequently Asked Questions (FAQ) for
PostgreSQL</H1> PostgreSQL</H1>
<P>Last updated: Sun Mar 13 22:07:18 EST 2005</P> <P>Last updated: Fri May 6 13:47:54 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -44,8 +44,8 @@
<A href="#1.10">1.10</A>) What books are good for developers?<BR> <A href="#1.10">1.10</A>) What books are good for developers?<BR>
<A href="#1.11">1.11</A>) What is configure all about?<BR> <A href="#1.11">1.11</A>) What is configure all about?<BR>
<A href="#1.12">1.12</A>) How do I add a new port?<BR> <A href="#1.12">1.12</A>) How do I add a new port?<BR>
<A href="#1.13">1.13</A>) Why don't you use threads/raw <A href="#1.13">1.13</A>) Why don't you use threads, raw
devices/async-I/O, &lt;insert your favorite wizz-bang feature devices, async-I/O, &lt;insert your favorite wizz-bang feature
here&gt;?<BR> here&gt;?<BR>
<A href="#1.14">1.14</A>) How are RPM's packaged?<BR> <A href="#1.14">1.14</A>) How are RPM's packaged?<BR>
<A href="#1.15">1.15</A>) How are CVS branches handled?<BR> <A href="#1.15">1.15</A>) How are CVS branches handled?<BR>
@@ -71,6 +71,8 @@
<I>pfree</I>() to allocate memory?<BR> <I>pfree</I>() to allocate memory?<BR>
<A href="#2.6">2.6</A>) What is ereport()?<BR> <A href="#2.6">2.6</A>) What is ereport()?<BR>
<A href="#2.7">2.7</A>) What is CommandCounterIncrement()?<BR> <A href="#2.7">2.7</A>) What is CommandCounterIncrement()?<BR>
<A href="#2.8">2.8</A>) What debugging features are available?<BR>
<BR> <BR>
<HR> <HR>
@@ -230,6 +232,12 @@
<P>There have been a number of discussions about other testing <P>There have been a number of discussions about other testing
frameworks and some developers are exploring these ideas.</P> frameworks and some developers are exploring these ideas.</P>
<P>Keep in mind the <I>Makefiles</I> do not have the proper
dependencies for include files. You have to do a <I>make clean</I>
and then another <I>make</I>. If you are using <SMALL>GCC</SMALL>
you can use the <I>--enable-depend</I> option of <I>configure</I>
to have the compiler compute the dependencies automatically.</P>
<H3><A name="1.9">1.9</A>) What tools are available for <H3><A name="1.9">1.9</A>) What tools are available for
developers?</H3> developers?</H3>
@@ -422,8 +430,8 @@
handling. There is a <I>backend/port</I> directory if you need handling. There is a <I>backend/port</I> directory if you need
special files for your OS.</P> special files for your OS.</P>
<H3><A name="1.13">1.13</A>) Why don't you use threads/raw <H3><A name="1.13">1.13</A>) Why don't you use threads, raw
devices/async-I/O, &lt;insert your favorite wizz-bang feature devices, async-I/O, &lt;insert your favorite wizz-bang feature
here&gt;?</H3> here&gt;?</H3>
<P>There is always a temptation to use the newest operating system <P>There is always a temptation to use the newest operating system
@@ -941,6 +949,45 @@
modified by previous pieces. <I>CommandCounterIncrement()</I> modified by previous pieces. <I>CommandCounterIncrement()</I>
increments the Command Counter, creating a new part of the increments the Command Counter, creating a new part of the
transaction.</P> transaction.</P>
<H3><A name="2.8">2.8</A>) What debugging features are
available?</H3>
<P>First, try running <I>configure</I> with the --enable-cassert
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
<P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
detailed information to be reported. The <I>-d</I> option takes a
number that specifies the debug level. Be warned that high debug
level values generate large log files.</P>
<P>If the <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
<SMALL>SQL</SMALL> statement directly. This is recommended
<B>only</B> for debugging purposes. If you have compiled with debugging
symbols, you can use a debugger to see what is happening. Because
the backend was not started from <I>postmaster</I>, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.</P>
<P>If the <I>postmaster</I> is running, start <I>psql</I> in one
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
You can set breakpoints in the debugger and issue queries from
<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
to delay for <I>n</I> seconds so you can attach to the process with
the debugger, set any breakpoints, and continue through the startup
sequence.</P>
<P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
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