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@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Jan 29 23:44:48 EST 2005 Last updated: Mon Jan 31 21:40:28 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -15,18 +15,17 @@
General Questions General Questions
1.1) What is PostgreSQL? How is it pronounced? 1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright on PostgreSQL? 1.2) What is the copyright of PostgreSQL?
1.3) What platforms does PostgreSQL support? 1.3) What platforms does PostgreSQL support?
1.4) Where can I get PostgreSQL? 1.4) Where can I get PostgreSQL?
1.5) Where can I get support? 1.5) Where can I get support?
1.6) What is the latest release? 1.6) How do I submit a bug report?
1.7) What documentation is available? 1.7) What is the latest release?
1.8) How do I find out about known bugs or missing features? 1.8) What documentation is available?
1.9) How can I learn SQL? 1.9) How do I find out about known bugs or missing features?
1.10) How do I join the development team? 1.10) How can I learn SQL?
1.11) How do I submit a bug report? 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) How can I financially assist PostgreSQL?
User Client Questions User Client Questions
@@ -52,7 +51,7 @@
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?
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?
4.3) How do you remove a column from a table, or change its data type? 4.3) How do you change a column's data type?
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?
4.5) How much database disk space is required to store data from a 4.5) How much database disk space is required to store data from a
typical text file? typical text file?
@@ -69,19 +68,18 @@
4.11.3) Why aren't my sequence numbers reused on transaction abort? 4.11.3) 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 TID?
4.13) What is the meaning of some of the terms used in PostgreSQL? 4.13) Why do I get the error "ERROR: Memory exhausted in
4.14) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"? AllocSetAlloc()"?
4.15) How do I tell what PostgreSQL version I am running? 4.14) How do I tell what PostgreSQL version I am running?
4.16) Why does my large-object operations get "invalid large obj 4.15) Why does my large-object operations get "invalid large obj
descriptor"? descriptor"?
4.17) How do I create a column that will default to the current time? 4.16) How do I create a column that will default to the current time?
4.18) How do I perform an outer join? 4.17) How do I perform an outer join?
4.19) How do I perform queries using multiple databases? 4.18) How do I perform queries using multiple databases?
4.20) How do I return multiple rows or columns from a function? 4.19) How do I return multiple rows or columns from a function?
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions? functions?
4.22) What encryption options are available? 4.21) What encryption options are available?
Extending PostgreSQL Extending PostgreSQL
@@ -98,38 +96,26 @@
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. PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.
PostgreSQL is an enhancement of the POSTGRES database management PostgreSQL is an object-relational database system that has the
system (and is still sometimes reffered to as simply "Postgres"), a features of traditional commercial database systems with enhancements
next-generation DBMS research prototype. While PostgreSQL retains the to be found in next-generation DBMS systems. PostgreSQL is free and
powerful data model and rich data types of POSTGRES, it replaces the the complete source code is available.
PostQuel query language with an extended subset of SQL. PostgreSQL is
free and the complete source is available.
PostgreSQL development is performed by a team of developers who all PostgreSQL development is performed by a team of mostly volunteer
subscribe to the PostgreSQL development mailing list. The current developers spread throughout the world and communicating via the
coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See section Internet. It is a community project and is not controlled by any
1.6 on how to join). This team is now responsible for all development
of PostgreSQL. It is a community project and is not controlled by any
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
The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many 1.2) What is the copyright of PostgreSQL?
others have contributed to the porting, testing, debugging, and
enhancement of the code. The original Postgres code, from which
PostgreSQL is derived, was the effort of many graduate students,
undergraduate students, and staff programmers working under the
direction of Professor Michael Stonebraker at the University of
California, Berkeley.
The original name of the software at Berkeley was Postgres. When SQL
functionality was added in 1995, its name was changed to Postgres95.
The name was changed at the end of 1996 to PostgreSQL.
1.2) What is the copyright on PostgreSQL?
PostgreSQL is subject to the following COPYRIGHT: 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
no intention of changing it.
This is the BSD license we use:
PostgreSQL Data Base Management System PostgreSQL Data Base Management System
@@ -155,10 +141,6 @@
CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT,
UPDATES, ENHANCEMENTS, OR MODIFICATIONS. UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
The above is the BSD license, the classic open-source license. It has
no restrictions on how the source code may be used. We like it and
have no intention of changing it.
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
@@ -183,60 +165,40 @@
1.5) Where can I get support? 1.5) Where can I get support?
The main mailing list is: pgsql-general@PostgreSQL.org. It is The PostgreSQL community provides assistance to many of its users via
available for discussion of matters pertaining to PostgreSQL. To email. The main web site to subscribe to the email lists is
subscribe, send mail with the following lines in the body (not the http://www.postgresql.org/community/lists/. The general or bugs lists
subject line): are a good place to start.
subscribe
end
to pgsql-general-request@PostgreSQL.org.
There is also a digest list available. To subscribe to this list, send
email to: pgsql-general-digest-request@PostgreSQL.org with a body of:
subscribe
end
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
The bugs mailing list is available. To subscribe to this list, send
email to pgsql-bugs-request@PostgreSQL.org with a body of:
subscribe
end
There is also a developers discussion mailing list available. To
subscribe to this list, send email to
pgsql-hackers-request@PostgreSQL.org with a body of:
subscribe
end
Additional mailing lists and information about PostgreSQL can be found
via the PostgreSQL WWW home page at:
http://www.PostgreSQL.org
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 command 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 of the other popular IRC clients. A
Spanish one also exists on the same network, (#postgresql-es), and a Spanish one also exists on the same network, (#postgresql-es), and a
French one, (#postgresqlfr). There is also a PostgreSQL channel on French one, (#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.postg resql.org/companies.php.
1.6) What is the latest release? 1.6) How do I submit a bug report?
Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version.
1.7) What is the latest release?
The latest release of PostgreSQL is version 8.0.0. The latest release of PostgreSQL is version 8.0.0.
We plan to have major releases every six to eight months. We plan to have major releases every ten to twelve months.
1.7) What documentation is available? 1.8) What documentation is available?
Several manuals, manual pages, and some small test examples are PostgreSQL includes extensive documentation, including a large manual,
included in the distribution. See the /doc directory. You can also manual pages, and some test examples. See the /doc directory. You can
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/awbook.html and http://www.PostgreSQL.org/docs/awbook.html and
@@ -252,12 +214,12 @@
Our web site contains even more documentation. Our web site contains even more documentation.
1.8) 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.
1.9) How can I learn SQL? 1.10) How can I learn SQL?
The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html
teaches SQL. There is another PostgreSQL book at teaches SQL. There is another PostgreSQL book at
@@ -273,10 +235,10 @@
et al., Addison-Wesley. Others like The Complete Reference SQL, Groff et al., Addison-Wesley. Others like The Complete Reference SQL, Groff
et al., McGraw-Hill. et al., McGraw-Hill.
1.10) How do I join the development team? 1.11) How do I join the development team?
First, download the latest source and read the PostgreSQL Developers First, download the latest source and read the PostgreSQL Developers
documentation on our web site, or in the distribution. Second, FAQ and documentation on our web site, or in the distribution. Second,
subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third,
submit high quality patches to pgsql-patches. submit high quality patches to pgsql-patches.
@@ -286,14 +248,6 @@
and we had confidence that patches they committed were of high and we had confidence that patches they committed were of high
quality. quality.
1.11) How do I submit a bug report?
Visit the PostgreSQL bug form at
http://www.postgresql.org/support/submitbug.
Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if
there is a more recent PostgreSQL version or patches.
1.12) How does PostgreSQL compare to other DBMSs? 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,
@@ -340,29 +294,6 @@
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) How can I financially assist PostgreSQL?
PostgreSQL has had a first-class infrastructure since we started in
1996. This is all thanks to Marc Fournier, who has created and managed
this infrastructure over the years.
Quality infrastructure is very important to an open-source project. It
prevents disruptions that can greatly delay forward movement of the
project.
Of course, this infrastructure is not cheap. There are a variety of
monthly and one-time expenses that are required to keep it going. If
you or your company has money it can donate to help fund this effort,
please go to http://store.pgsql.com/shopping/ and make a donation.
Although the web page mentions PostgreSQL, Inc, the "contributions"
item is solely to support the PostgreSQL project and does not fund any
specific company. If you prefer, you can also send a check to the
contact address.
Also, if you have a success story about PostgreSQL, please email it to
our advocacy list at pgsql-advocacy@postgresql.org.
_________________________________________________________________ _________________________________________________________________
User Client Questions User Client Questions
@@ -384,16 +315,16 @@
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
For Web integration, PHP is an excellent interface. It is at For Web integration, PHP (http://www.php.net) is an excellent
http://www.php.net. interface.
For complex cases, many use the Perl interface and CGI.pm or mod_perl. For complex cases, many use the Perl and CGI.pm or 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, there are several graphical interfaces to PostgreSQL available.
These include PgAccess http://www.pgaccess.org), pgAdmin III These include pgAdmin III (http://www.pgadmin.org, PgAccess
(http://www.pgadmin.org, RHDB Admin (http://sources.redhat.com/rhdb/ http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhd b/
), TORA (http://www.globecom.net/tora/, partly commercial), and Rekall ), TORA (http://www.globecom.net/tora/, partly commercial), and Rekall
( http://www.rekallrevealed.org/). There is also PhpPgAdmin ( ( http://www.rekallrevealed.org/). There is also PhpPgAdmin (
http://phppgadmin.sourceforge.net/ ), a web-based interface to http://phppgadmin.sourceforge.net/ ), a web-based interface to
@@ -414,61 +345,58 @@
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 and enable host-based authentication by modifying the postgresql.conf file, enable host-based authentication by modifying
file $PGDATA/pg_hba.conf accordingly. 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?
Certainly, indexes can speed up queries. The EXPLAIN ANALYZE command There are three major areas for potential performance improvement:
allows you to see how PostgreSQL is interpreting your query, and which
indexes are being used.
If you are doing many INSERTs, consider doing them in a large batch
using the COPY command. This is much faster than individual INSERTS.
Second, statements not in a BEGIN WORK/COMMIT transaction block are
considered to be in their own transaction. Consider performing several
statements in a single transaction block. This reduces the transaction
overhead. Also, consider dropping and recreating indexes when making
large data changes.
There are several tuning options in the Administration Guide/Server
Run-time Environment/Run-time Configuration. You can disable fsync()
by using fsync option. This will prevent fsync()s from flushing to
disk after every transaction.
You can use the shared_buffers option to increase the number of shared
memory buffers used by the backend processes. If you make this
parameter too high, the postmaster may not start because you have
exceeded your kernel's limit on shared memory space. Each buffer is 8K
and the default is 1000 buffers.
You can also use the sort_mem (from PostgreSQL 8.0: work_mem) options
to increase the maximum amount of memory used by the backend processes
for each temporary sort. The default is 1024 (i.e. 1MB).
You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details.
Query Changes
This involves modifying queries to obtain better performance:
+ Creation of indexes, including expression and partial indexes
+ Use of COPY instead of multiple INSERTs
+ Grouping of multiple statements into a single transaction to
reduce commit overhead
+ Use of CLUSTER when retrieving many rows from an index
+ Use of LIMIT for returning a subset of a query's output
+ Use of Prepared queries
+ Use of ANALYZE to maintain accurate optimizer statistics
+ Regular use of VACUUM or pg_autovacuum
+ Dropping of indexes during large data changes
Server Configuration
A number of postgresql.conf settings affect performance. For
more details, see Administration Guide/Server Run-time
Environment/Run-time Configuration for a full listing, and for
commentary see
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_co
nf_e.html and
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
Hardware Selection
The effect of hardware on performance is detailed in
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
x.html.
3.4) What debugging features are available? 3.4) What debugging features are available?
PostgreSQL has several features that report status information that There are many log_* server configuration variables that enable
can be valuable for debugging purposes. printing of query and process statistics which can be very useful for
debugging and performance measurements.
First, by running configure with the --enable-cassert option, many The following detailed debug instructions are to be used to provide
assert()s monitor the progress of the backend and halt the program more detailed information for server developers debugging a problem.
when something unexpected occurs.
Both postmaster and postgres have several debug options available. It is also possible to debug the server if it isn't operating
First, whenever you start postmaster, make sure you send the standard properly. First, by running configure with the --enable-cassert
output and error to a log file, like: option, many assert()s monitor the progress of the backend and halt
cd /usr/local/pgsql the program when something unexpected occurs.
./bin/postmaster >server.log 2>&1 &
The postmaster has a -d option that allows even more detailed
This will put a server.log file in the top-level PostgreSQL directory. information to be reported. The -d option takes a number that
This file contains useful information about problems or errors specifies the debug level. Be warned that high debug level values
encountered by the server. 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. generate large log files.
If postmaster is not running, you can actually run the postgres If postmaster is not running, you can actually run the postgres
@@ -489,10 +417,6 @@
process with the debugger, set any breakpoints, and continue through process with the debugger, set any breakpoints, and continue through
the startup sequence. the startup sequence.
There are several log_* server configuration variables that enable
printing of process statistics which can be very useful for debugging
and performance measurements.
You can also compile with profiling to see what functions are taking You can also compile with profiling to see what functions are taking
execution time. The backend profile files will be deposited in the execution time. The backend profile files will be deposited in the
pgsql/data/base/dbname directory. The client profile file will be put pgsql/data/base/dbname directory. The client profile file will be put
@@ -501,23 +425,10 @@
3.5) Why do I get "Sorry, too many clients" when trying to connect? 3.5) Why do I get "Sorry, too many clients" when trying to connect?
You need to increase postmaster's limit on how many concurrent backend You have reached the default limit is 100 database sessions. You need
processes it can start. to increase the postmaster's limit on how many concurrent backend
processes it can start by changing the max_connections value in
The default limit is 32 processes. You can increase it by restarting postgresql.conf and restarting the postmaster.
postmaster with a suitable -N value or modifying postgresql.conf.
Note that if you make -N larger than 32, you must also increase -B
beyond its default of 64; -B must be at least twice -N, and probably
should be more than that for best performance. For large numbers of
backend processes, you are also likely to find that you need to
increase various Unix kernel configuration parameters. Things to check
include the maximum size of shared memory blocks, SHMMAX; the maximum
number of semaphores, SEMMNS and SEMMNI; the maximum number of
processes, NPROC; the maximum number of processes per user, MAXUPRC;
and the maximum number of open files, NFILE and NINODE. The reason
that PostgreSQL has a limit on the number of allowed backend processes
is so your system won't run out of resources.
3.6) What is in the pgsql_tmp directory? 3.6) What is in the pgsql_tmp directory?
@@ -534,17 +445,13 @@
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 7.2 to 7.2.1 does not require a dump and restore. so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) 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 compatability 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.
In releases where the on-disk format does not change, the pg_upgrade
script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release.
3.8) What computer hardware should I use? 3.8) 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
@@ -560,13 +467,10 @@
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?
See the FETCH manual page, or use SELECT ... LIMIT.... 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
The entire query may have to be evaluated, even if you only want the it is possible the entire query does not have to be executed. If you
first few rows. Consider using a query that has an ORDER BY. If there don't know the number of rows at SELECT time, use a cursor and FETCH.
is an index that matches the ORDER BY, PostgreSQL may be able to
evaluate only the first few records requested, or the entire query may
have to be evaluated until the desired rows have been generated.
To SELECT a random row, use: To SELECT a random row, use:
SELECT col SELECT col
@@ -587,25 +491,16 @@
query to get information about the database. query to get information about the database.
There are also system tables beginning with pg_ that describe these There are also system tables beginning with pg_ that describe these
too. Use psql -l will list all databases. too.
Use psql -l will list all databases.
Also try the file pgsql/src/tutorial/syscat.source. It illustrates Also try the file pgsql/src/tutorial/syscat.source. It illustrates
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 remove a column from a table, or change its data type? 4.3) How do you change a column's data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can do this:
BEGIN;
LOCK TABLE old_table;
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;
COMMIT;
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.
@@ -622,14 +517,16 @@
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 table? 32 TB Maximum size for a database? unlimited (32 TB databases exist)
Maximum size for a row? 1.6TB Maximum size for a table? 32 TB
Maximum size for a field? 1 GB Maximum size for a row? 1.6TB
Maximum number of rows in a table? unlimited Maximum size for a field? 1 GB
Maximum number of columns in a table? 250-1600 depending on column types Maximum number of rows in a table? unlimited
Maximum number of indexes on a table? unlimited Maximum number of columns in a table? 250-1600 depending on column
types
Maximum number of indexes on a table? unlimited
Of course, these are not actually unlimited, but limited to available Of course, these are not actually unlimited, but limited to available
disk space and memory/swap space. Performance may suffer when these disk space and memory/swap space. Performance may suffer when these
values get unusually large. values get unusually large.
@@ -716,15 +613,15 @@
* The search string can not start with a character class, e.g. * The search string can not start with a character class, e.g.
[a-e]. [a-e].
* Case-insensitive searches such as ILIKE and ~* do not utilize * Case-insensitive searches such as ILIKE and ~* do not utilize
indexes. Instead, use functional indexes, which are described in indexes. Instead, use expression indexes, which are described in
section 4.10. section 4.8.
* The default C locale must be used during initdb because it is not * The default C locale must be used during initdb because it is not
possible to know the next-greater character in a non-C locale. You possible to know the next-greatest character in a non-C locale.
can create a special text_pattern_ops index for such cases that You can create a special text_pattern_ops index for such cases
work only for LIKE indexing. that work only for LIKE indexing.
In pre-8.0 releases, indexes often can not be used unless the data In pre-8.0 releases, indexes often can not be used unless the data
types exactly match the index's column types. This is 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?
@@ -745,7 +642,7 @@
WHERE lower(col) = 'abc'; WHERE lower(col) = 'abc';
This will not use an standard index. However, if you create a This will not use an standard index. However, if you create a
functional 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? 4.9) In a query, how do I detect if a field is NULL?
@@ -754,14 +651,13 @@
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 TEXT text no specific upper limit on length
TEXT text no specific upper limit on length BYTEA bytea variable-length byte array (null-byte safe)
BYTEA bytea variable-length byte array (null-byte safe) "char" char one character
"char" char one character
You will see the internal name when examining system catalogs and in You will see the internal name when examining system catalogs and in
some error messages. some error messages.
@@ -797,9 +693,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
); );
See the create_sequence manual page for more information about See the create_sequence manual page for more information about
sequences. You can also use each row's OID field as a unique value. sequences.
However, if you need to dump and reload the database, you need to use
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
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?
@@ -821,16 +715,10 @@ BYTEA bytea variable-length byte array (null-byte safe)
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')");
Finally, you could use the OID returned from the INSERT statement to
look up the default value, though this is probably the least portable
approach, and the oid value will wrap around when it reaches 4
billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute().
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 backend, not No. currval() returns the current value assigned by your session, not
by all users. 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?
@@ -856,25 +744,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
values. TIDs change after rows are modified or reloaded. They are used values. TIDs change after rows are modified or reloaded. They are used
by index entries to point to physical rows. by index entries to point to physical rows.
4.13) What is the meaning of some of the terms used in PostgreSQL? 4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
Some of the source code and older documentation use terms that have
more common usage. Here are some:
* table, relation, class
* row, record, tuple
* column, field, attribute
* retrieve, select
* replace, update
* append, insert
* OID, serial value
* portal, cursor
* range variable, table name, table alias
A list of general database terms can be found at:
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
4.14) 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
@@ -889,11 +759,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
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.
4.15) 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();
4.16) Why does my large-object operations get "invalid large obj 4.15) Why does my large-object operations get "invalid large obj
descriptor"? descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object You need to put BEGIN WORK and COMMIT around any use of a large object
@@ -908,12 +778,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
If you are using a client interface like ODBC you may need to set If you are using a client interface like ODBC you may need to set
auto-commit off. auto-commit off.
4.17) How do I create a column that will default to the current time? 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 );
4.18) How do I perform an outer join? 4.17) 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:
@@ -931,37 +801,25 @@ BYTEA bytea variable-length byte array (null-byte safe)
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.
In previous releases, outer joins can be simulated using UNION and NOT 4.18) How do I perform queries using multiple databases?
IN. For example, when joining tab1 and tab2, the following query does
an outer join of the two tables:
SELECT tab1.col1, tab2.col2
FROM tab1, tab2
WHERE tab1.col1 = tab2.col1
UNION ALL
SELECT tab1.col1, NULL
FROM tab1
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
4.19) 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.
contrib/dblink allows cross-database queries using function calls. Of contrib/dblink allows cross-database queries using function calls. Of
course, a client can 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.
4.20) How do I return multiple rows or columns from a function? 4.19) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a It is easy using set-returning functions,
function, http://techdocs.postgresql.org/guides/SetReturningFunctions. http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL 4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions? functions?
PL/PgSQL caches function contents, 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
function will fail because the cached function contents still point to function will fail because the cached function contents still point to
@@ -969,7 +827,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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.
4.22) What encryption options are available? 4.21) What encryption options are available?
* contrib/pgcrypto contains many encryption functions for use in SQL * contrib/pgcrypto contains many encryption functions for use in SQL
queries. queries.
@@ -980,8 +838,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
possible to use a third-party encrypted transport, such as stunnel possible to use a third-party encrypted transport, such as stunnel
or ssh, rather than PostgreSQL's native SSL connections.) or ssh, rather than PostgreSQL's native SSL connections.)
* Database user passwords are automatically encrypted when stored in * Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable the option the system tables.
PASSWORD_ENCRYPTION in postgresql.conf.
* The server can run using an encrypted file system. * The server can run using an encrypted file system.
_________________________________________________________________ _________________________________________________________________

609
doc/src/FAQ/FAQ.html
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: Sat Jan 29 23:44:48 EST 2005</P> <P>Last updated: Mon Jan 31 21:40:28 EST 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>)
@@ -27,21 +27,19 @@
<H2 align="center">General Questions</H2> <H2 align="center">General Questions</H2>
<A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR> <A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR> <A href="#1.2">1.2</A>) What is the copyright of PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR> <A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
<A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR> <A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.5">1.5</A>) Where can I get support?<BR> <A href="#1.5">1.5</A>) Where can I get support?<BR>
<A href="#1.6">1.6</A>) What is the latest release?<BR> <A href="#1.6">1.6</A>) How do I submit a bug report?<BR>
<A href="#1.7">1.7</A>) What documentation is available?<BR> <A href="#1.7">1.7</A>) What is the latest release?<BR>
<A href="#1.8">1.8</A>) How do I find out about known bugs or <A href="#1.8">1.8</A>) What documentation is available?<BR>
<A href="#1.9">1.9</A>) How do I find out about known bugs or
missing features?<BR> missing features?<BR>
<A href="#1.9">1.9</A>) How can I learn <SMALL>SQL</SMALL>?<BR> <A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.10">1.10</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="#1.11">1.11</A>) How do I submit a bug report?<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>) How can I financially assist
PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
@@ -76,8 +74,7 @@
<A href="#4.2">4.2</A>) How do I find out what tables, indexes, <A href="#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?<BR> by <I>psql</I> to display them?<BR>
<A href="#4.3">4.3</A>) How do you remove a column from a <A href="#4.3">4.3</A>) How do you change a column's data type?<BR>
table, or change its data type?<BR>
<A href="#4.4">4.4</A>) What is the maximum size for a row, a <A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR> table, and a database?<BR>
<A href="#4.5">4.5</A>) How much database disk space is required <A href="#4.5">4.5</A>) How much database disk space is required
@@ -104,24 +101,22 @@
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>TID</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the meaning of some of the terms <A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
used in PostgreSQL?<BR>
<A href="#4.14">4.14</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR> exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.15">4.15</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="#4.16">4.16</A>) Why does my large-object operations get <A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR> <I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.17">4.17</A>) How do I create a column that will <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="#4.18">4.18</A>) How do I perform an outer join?<BR> <A href="#4.17">4.17</A>) How do I perform an outer join?<BR>
<A href="#4.19">4.19</A>) How do I perform queries using multiple <A href="#4.18">4.18</A>) How do I perform queries using multiple
databases?<BR> databases?<BR>
<A href="#4.20">4.20</A>) How do I return multiple rows or columns <A href="#4.19">4.19</A>) How do I return multiple rows or columns
from a function?<BR> from a function?<BR>
<A href="#4.21">4.21</A>) Why can't I reliably create/drop <A href="#4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR> temporary tables in PL/PgSQL functions?<BR>
<A href="#4.22">4.22</A>) What encryption options are available?<BR> <A href="#4.21">4.21</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2> <H2 align="center">Extending PostgreSQL</H2>
@@ -140,45 +135,31 @@
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4> <H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P> <P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P>
<P>PostgreSQL is an enhancement of the POSTGRES database management <P>PostgreSQL is an object-relational database system that has the
system (and is still sometimes reffered to as simply "Postgres"), features of traditional commercial database systems with
a next-generation <SMALL>DBMS</SMALL> research prototype. enhancements to be found in next-generation <SMALL>DBMS</SMALL>
While PostgreSQL retains the powerful data model and rich data systems. PostgreSQL is free and the complete source code is
types of POSTGRES, it replaces the PostQuel query language with an available.</P>
extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
complete source is available.</P>
<P>PostgreSQL development is performed by a team of <P>PostgreSQL development is performed by a team of mostly volunteer
developers who all subscribe to the PostgreSQL development mailing developers spread throughout the world and communicating via the
list. The current coordinator is Marc G. Fournier (<A href= Internet. It is a community project and is not controlled by any
"mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See company. To get involved, see the developer's FAQ at <A href=
section <a href="#1.6">1.6</a> on how to join). This team is now
responsible for all development of PostgreSQL. It is a community
project and is not controlled by any company. To get involved, see
the developer's FAQ at <A href=
"http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html"> "http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html">
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A> http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P> </P>
<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. <H4><A name="1.2">1.2</A>) What is the copyright of
Many others have contributed to the porting, testing, debugging,
and enhancement of the code. The original Postgres code, from which
PostgreSQL is derived, was the effort of many graduate students,
undergraduate students, and staff programmers working under the
direction of Professor Michael Stonebraker at the University of
California, Berkeley.</P>
<P>The original name of the software at Berkeley was Postgres. When
<SMALL>SQL</SMALL> functionality was added in 1995, its name was
changed to Postgres95. The name was changed at the end of 1996 to
PostgreSQL.</P>
<H4><A name="1.2">1.2</A>) What is the copyright on
PostgreSQL?</H4> PostgreSQL?</H4>
<P>PostgreSQL is subject to the following COPYRIGHT:</P> <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
have no intention of changing it.</P>
<P>This is the BSD license we use:</P>
<P>PostgreSQL Data Base Management System</P> <P>PostgreSQL Data Base Management System</P>
@@ -204,10 +185,6 @@
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>
<P>The above is the BSD license, the classic open-source license.
It has no restrictions on how the source code may be used. We like
it and have no intention of changing it.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4> <H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<P>In general, any modern Unix-compatible platform should be able to <P>In general, any modern Unix-compatible platform should be able to
@@ -236,78 +213,46 @@
<H4><A name="1.5">1.5</A>) Where can I get support?</H4> <H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<P>The main mailing list is: <A href= <P>The PostgreSQL community provides assistance to many of its users
"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>. via email. The main web site to subscribe to the email lists is
It is available for discussion of matters pertaining to PostgreSQL. <a href="http://www.postgresql.org/community/lists/">
To subscribe, send mail with the following lines in the body (not http://www.postgresql.org/community/lists/</a>. The <I>general</I>
the subject line):</P> or <I>bugs</I> lists are a good place to start.
<PRE>
subscribe
end
</PRE>
<P>to <A href=
"mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
<P>There is also a digest list available. To subscribe to this
list, send email to: <A href=
"mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
<P>The bugs mailing list is available. To subscribe to this list,
send email to <A href=
"mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
There is also a developers discussion mailing list available. To
subscribe to this list, send email to <A href=
"mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
with a body of:
<PRE>
subscribe
end
</PRE>
<P>Additional mailing lists and information about PostgreSQL can be
found via the PostgreSQL WWW home page at:</P>
<BLOCKQUOTE>
<A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
</BLOCKQUOTE>
<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
command <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 of the other popular 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. (<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.
<P>A list of commercial support companies is available at <A href= <P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P> "http://techdocs.postgresql.org/companies.php">http://techdocs.postg
resql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) What is the latest release?</H4> <H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 8.0.0.</P> <P>The latest release of PostgreSQL is version 8.0.0.</P>
<P>We plan to have major releases every six to eight months.</P> <P>We plan to have major releases every ten to twelve months.</P>
<H4><A name="1.7">1.7</A>) What documentation is available?</H4> <H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<P>Several manuals, manual pages, and some small test examples are <P>PostgreSQL includes extensive documentation, including a large
included in the distribution. See the <I>/doc</I> directory. You manual, manual pages, and some test examples. See the <I>/doc</I>
can also browse the manuals online at <A href= directory. You can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P> "http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.
</P>
<P>There are two PostgreSQL books available online at <A href= <P>There are two PostgreSQL books available online at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A> "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
@@ -326,14 +271,14 @@
<P>Our web site contains even more documentation.</P> <P>Our web site contains even more documentation.</P>
<H4><A name="1.8">1.8</A>) How do I find out about known bugs or <H4><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4> missing features?</H4>
<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://developer.PostgreSQL.org/todo.php">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="1.9">1.9</A>) How can I learn <H4><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4> <SMALL>SQL</SMALL>?</H4>
<P>The PostgreSQL book at <A href= <P>The PostgreSQL book at <A href=
@@ -357,12 +302,12 @@
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P> Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.10">1.10</A>) How do I join the development <H4><A name="1.11">1.11</A>) How do I join the development
team?</H4> team?</H4>
<P>First, download the latest source and read the PostgreSQL <P>First, download the latest source and read the PostgreSQL
Developers documentation on our web site, or in the distribution. Developers FAQ and documentation on our web site, or in the
Second, subscribe to the <I>pgsql-hackers</I> and distribution. Second, subscribe to the <I>pgsql-hackers</I> and
<I>pgsql-patches</I> mailing lists. Third, submit high quality <I>pgsql-patches</I> mailing lists. Third, submit high quality
patches to pgsql-patches.</P> patches to pgsql-patches.</P>
@@ -372,16 +317,6 @@
committers to keep up, and we had confidence that patches they committers to keep up, and we had confidence that patches they
committed were of high quality.</P> committed were of high quality.</P>
<H4><A name="1.11">1.11</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version or patches.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other <H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4> <SMALL>DBMS</SMALL>s?</H4>
@@ -448,31 +383,6 @@
</DD> </DD>
</DL> </DL>
<H4><A name="1.13">1.13</A>) How can I financially assist
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
in 1996. This is all thanks to Marc Fournier, who has created
and managed this infrastructure over the years.</P>
<P>Quality infrastructure is very important to an open-source
project. It prevents disruptions that can greatly delay forward
movement of the project.</P>
<P>Of course, this infrastructure is not cheap. There are a variety
of monthly and one-time expenses that are required to keep it
going. If you or your company has money it can donate to help fund
this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
and make a donation.</P>
<P>Although the web page mentions PostgreSQL, Inc, the
"contributions" item is solely to support the PostgreSQL project
and does not fund any specific company. If you prefer, you can also
send a check to the contact address.</P>
<P>Also, if you have a success story about PostgreSQL, please email
it to our advocacy list at <a href="mailto:pgsql-advocacy@postgresql.org">
pgsql-advocacy@postgresql.org</a>.</P>
<HR> <HR>
@@ -499,27 +409,33 @@
<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>
<P>For Web integration, PHP is an excellent interface. It is at <A <P>For Web integration, PHP (<A
href="http://www.php.net">http://www.php.net</A>.</P> href="http://www.php.net">http://www.php.net</A>) is an excellent
interface.</P>
<P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P> <P>For complex cases, many use the Perl and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user <H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4> interface?</H4>
<P>Yes, there are several graphical interfaces to PostgreSQL available. <P>Yes, there are several graphical interfaces to PostgreSQL
These include PgAccess <a href="http://www.pgaccess.org"> available. These include pgAdmin III (<a
http://www.pgaccess.org</a>), pgAdmin III (<a href="http://www.pgadmin.org">http://www.pgadmin.org</a>, PgAccess
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a <a href="http://www.pgaccess.org"> http://www.pgaccess.org</a>),
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/ RHDB Admin (<a
</a>), TORA (<a href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>, href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhd
partly commercial), and Rekall (<a href="http://www.rekallrevealed.org/"> b/ </a>), TORA (<a
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
(<a href="http://phppgadmin.sourceforge.net/"> partly commercial), and Rekall (<a
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to 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> 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> <P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
detailed list.</P>
<HR> <HR>
@@ -538,75 +454,85 @@
<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
machines will not be able to connect unless you modify machines will not be able to connect unless you modify
listen_addresses in the postgresql.conf <B>and</B> enable <I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
host-based authentication by modifying the file host-based authentication by modifying the
<I>$PGDATA/pg_hba.conf</I> accordingly.</P> <I>$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 <H4><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4> better performance?</H4>
<P>Certainly, indexes can speed up queries. The <P>There are three major areas for potential performance
<SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how improvement:</P>
PostgreSQL is interpreting your query, and which indexes are
being used.</P> <DL>
<DT><B>Query Changes</B></DT>
<P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing <DD>This involves modifying queries to obtain better
them in a large batch using the <SMALL>COPY</SMALL> command. This performance:
is much faster than individual <SMALL>INSERTS</SMALL>. Second, <ul>
statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction <li>Creation of indexes, including expression and partial
block are considered to be in their own transaction. Consider indexes</li>
performing several statements in a single transaction block. This <li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
reduces the transaction overhead. Also, consider dropping and <li>Grouping of multiple statements into a single transaction to
recreating indexes when making large data changes.</P> reduce commit overhead</li>
<li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
index</li>
<li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
output</li>
<li>Use of Prepared queries</li>
<li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
statistics</li>
<li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I>
<li>Dropping of indexes during large data changes</li>
</ul><BR>
<BR>
</DD>
<P>There are several tuning options in the <a href= <DT><B>Server Configuration</B></DT>
<DD>A number of <I>postgresql.conf</I> settings affect performance.
For more details, see <a href=
"http://www.postgresql.org/docs/current/static/runtime.html"> "http://www.postgresql.org/docs/current/static/runtime.html">
Administration Guide/Server Run-time Environment/Run-time Configuration</a>. Administration Guide/Server Run-time Environment/Run-time
You can disable <I>fsync()</I> by using <i>fsync</I> option. This will Configuration</a> for a full listing, and for commentary see <a
prevent <I>fsync()</I>s from flushing to disk after every href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
transaction.</P> http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
<BR>
<BR>
</DD>
<P>You can use the <I>shared_buffers</I> option to <DT><B>Hardware Selection</B></DT>
increase the number of shared memory buffers used by the backend
processes. If you make this parameter too high, the
<I>postmaster</I> may not start because you have exceeded your
kernel's limit on shared memory space. Each buffer is 8K and the
default is 1000 buffers.</P>
<P>You can also use the <I>sort_mem</I> (from PostgreSQL 8.0: <I>work_mem</I>) <DD>The effect of hardware on performance is detailed in <a
options to increase the maximum amount of memory used by the backend href="http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html">
processes for each temporary sort. The default is 1024 (i.e. 1MB).</P> http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html</a>.
<BR>
<P>You can also use the <SMALL>CLUSTER</SMALL> command to group <BR>
data in tables to match an index. See the <SMALL>CLUSTER</SMALL> </DD>
manual page for more details.</P> </DL>
<H4><A name="3.4">3.4</A>) What debugging features are <H4><A name="3.4">3.4</A>) What debugging features are
available?</H4> available?</H4>
<P>PostgreSQL has several features that report status information <P>There are many <CODE>log_*</CODE> server configuration variables
that can be valuable for debugging purposes.</P> that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P>
<P>First, by running <I>configure</I> with the --enable-cassert <P><B>The following detailed debug instructions are to be used to
provide more detailed information for server developers debugging a
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 option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P> and halt the program when something unexpected occurs.</P>
<P>Both <I>postmaster</I> and <I>postgres</I> have several debug <P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
options available. First, whenever you start <I>postmaster</I>, detailed information to be reported. The <I>-d</I> option takes a
make sure you send the standard output and error to a log file, number that specifies the debug level. Be warned that high debug
like:</P> level values generate large log files.</P>
<PRE>
cd /usr/local/pgsql
./bin/postmaster &gt;server.log 2&gt;&amp;1 &amp;
</PRE>
<P>This will put a server.log file in the top-level PostgreSQL
directory. This file contains useful information about problems or
errors encountered by the server. <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 <P>If <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your <I>postgres</I> backend from the command line, and type your
@@ -630,10 +556,6 @@
the debugger, set any breakpoints, and continue through the startup the debugger, set any breakpoints, and continue through the startup
sequence.</P> sequence.</P>
<P>There are several <CODE>log_*</CODE> server configuration variables
that enable printing of process statistics which can be very useful
for debugging and performance measurements.</P>
<P>You can also compile with profiling to see what functions are <P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile in the <I>pgsql/data/base/dbname</I> directory. The client profile
@@ -643,27 +565,11 @@
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many <H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4> clients"</I> when trying to connect?</H4>
<P>You need to increase <I>postmaster</I>'s limit on how many <P>You have reached the default limit is 100 database sessions. You
concurrent backend processes it can start.</P> need to increase the <I>postmaster</I>'s limit on how many
concurrent backend processes it can start by changing the
<P>The default limit is 32 processes. You can increase it by <I>max_connections</I> value in <I>postgresql.conf</I> and
restarting <I>postmaster</I> with a suitable <I>-N</I> value or restarting the <I>postmaster</I>.</P>
modifying <I>postgresql.conf</I>.</P>
<P>Note that if you make <I>-N</I> larger than 32, you must also
increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
least twice <I>-N</I>, and probably should be more than that for
best performance. For large numbers of backend processes, you are
also likely to find that you need to increase various Unix kernel
configuration parameters. Things to check include the maximum size
of shared memory blocks, <SMALL>SHMMAX;</SMALL> the maximum number
of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
maximum number of open files, <SMALL>NFILE</SMALL> and
<SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
the number of allowed backend processes is so your system won't run
out of resources.</P>
<H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4> <H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
@@ -681,18 +587,13 @@
to upgrade between major PostgreSQL releases?</H4> to upgrade between major PostgreSQL releases?</H4>
<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 7.2 to 7.2.1 does not require a dump and restore. so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) 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 compatability 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>
<P>In releases where the on-disk format does not change, the
<I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
The release notes mention whether <I>pg_upgrade</I> is available for the
release.</P>
<H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4> <H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4>
<P>Because PC hardware is mostly compatible, people tend to believe that <P>Because PC hardware is mostly compatible, people tend to believe that
@@ -710,15 +611,13 @@
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the <H4><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?</H4>
<P>See the <SMALL>FETCH</SMALL> manual page, or use <P>To retrieve only a few rows, if you know at the number of rows
<SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P> needed at the time of the <SMALL>SELECT</SMALL> use
<SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
<P>The entire query may have to be evaluated, even if you only want BY</SMALL> it is possible the entire query does not have to be
the first few rows. Consider using a query that has an <SMALL>ORDER executed. If you don't know the number of rows at
BY</SMALL>. If there is an index that matches the <SMALL>ORDER <SMALL>SELECT</SMALL> time, use a cursor and
BY</SMALL>, PostgreSQL may be able to evaluate only the first few <SMALL>FETCH</SMALL>.</P>
records requested, or the entire query may have to be evaluated
until the desired rows have been generated.</P>
<P>To <SMALL>SELECT</SMALL> a random row, use: <P>To <SMALL>SELECT</SMALL> a random row, use:
<PRE> <PRE>
@@ -743,28 +642,15 @@
database.</P> database.</P>
<P>There are also system tables beginning with <I>pg_</I> that describe <P>There are also system tables beginning with <I>pg_</I> that describe
these too. Use <I>psql -l</I> will list all databases.</P> these too.</P>
<P>Use <I>psql -l</I> will list all databases.</P>
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It <P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
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 remove a column from a <H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4>
table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
with <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
you can do this:</P>
<PRE>
BEGIN;
LOCK TABLE old_table;
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;
COMMIT;
</PRE>
<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>. and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.
@@ -784,19 +670,25 @@
table, and a database?</H4> table, and a database?</H4>
<P>These are the limits:</P> <P>These are the limits:</P>
<PRE> <CENTER>
Maximum size for a database? unlimited (32 TB databases exist) <TABLE BORDER=1>
Maximum size for a table? 32 TB <TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
Maximum size for a row? 1.6TB exist)</TD></TR>
Maximum size for a field? 1 GB <TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
Maximum number of rows in a table? unlimited <TR><TD>Maximum size for a row?</TD><TD>1.6TB</TD></TR>
Maximum number of columns in a table? 250-1600 depending on column types <TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
Maximum number of indexes on a table? unlimited <TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
</PRE> <TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
on column types</TD></TR>
<TR><TD>Maximum number of indexes on a
table?</TD><TD>unlimited</TD></TR>
</TABLE>
</CENTER>
<BR>
Of course, these are not actually unlimited, but limited to <P>Of course, these are not actually unlimited, but limited to
available disk space and memory/swap space. Performance may suffer available disk space and memory/swap space. Performance may suffer
when these values get unusually large. when these values get unusually large.</P>
<P>The maximum table size of 32 TB does not require large file <P>The maximum table size of 32 TB does not require large file
support from the operating system. Large tables are stored as support from the operating system. Large tables are stored as
@@ -893,10 +785,10 @@
<LI>The search string can not start with a character class, <LI>The search string can not start with a character class,
e.g. [a-e].</LI> e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and <LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilize indexes. Instead, use functional <I>~*</I> do not utilize indexes. Instead, use expression
indexes, which are described in section <a href="#4.10">4.10</a>.</LI> indexes, which are described in section <a href="#4.8">4.8</a>.</LI>
<LI>The default <I>C</I> locale must be used during <LI>The default <I>C</I> locale must be used during
<i>initdb</i> because it is not possible to know the next-greater <i>initdb</i> because it is not possible to know the next-greatest
character in a non-C locale. You can create a special character in a non-C locale. You can create a special
<CODE>text_pattern_ops</CODE> index for such cases that work only <CODE>text_pattern_ops</CODE> index for such cases that work only
for <SMALL>LIKE</SMALL> indexing. for <SMALL>LIKE</SMALL> indexing.
@@ -904,7 +796,7 @@
</UL> </UL>
<P>In pre-8.0 releases, indexes often can not be used unless the data <P>In pre-8.0 releases, indexes often can not be used unless the data
types exactly match the index's column types. This is 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 <H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
@@ -930,7 +822,7 @@
</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
functional 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>
@@ -943,16 +835,20 @@
<H4><A name="4.10">4.10</A>) What is the difference between the <H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4> various character types?</H4>
<PRE> <CENTER>
Type Internal Name Notes <TABLE BORDER=1>
-------------------------------------------------- <TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
VARCHAR(n) varchar size specifies maximum length, no padding <TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
CHAR(n) bpchar blank padded to the specified fixed length length, no padding</TD></TR>
TEXT text no specific upper limit on length <TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
BYTEA bytea variable-length byte array (null-byte safe) fixed length</TD></TR>
"char" char one character <TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
</PRE> length</TD></TR>
<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
(null-byte safe)</TD></TR>
<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
</TABLE>
</CENTER>
<P>You will see the internal name when examining system catalogs <P>You will see the internal name when examining system catalogs
and in some error messages.</P> and in some error messages.</P>
@@ -996,10 +892,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</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. You can also use each row's <I>OID</I> field as a about sequences.
unique value. However, if you need to dump and reload the database,
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a <H4><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?</H4>
@@ -1030,19 +923,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = execute("SELECT currval('person_id_seq')"); new_id = execute("SELECT currval('person_id_seq')");
</PRE> </PRE>
<P>Finally, you could use the <A href="#4.12"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach,
and the oid value will wrap around when it reaches 4 billion.
In Perl, using DBI with the DBD::Pg module, the oid value is made
available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I> <H4><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?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your <P>No. <I>currval()</I> returns the current value assigned by your
backend, not by all users.</P> session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers <H4><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
@@ -1076,36 +961,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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>) What is the meaning of some of the <H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that
have more common usage. Here are some:</P>
<UL>
<LI>table, relation, class</LI>
<LI>row, record, tuple</LI>
<LI>column, field, attribute</LI>
<LI>retrieve, select</LI>
<LI>replace, update</LI>
<LI>append, insert</LI>
<LI><SMALL>OID</SMALL>, serial value</LI>
<LI>portal, cursor</LI>
<LI>range variable, table name, table alias</LI>
</UL>
<P>A list of general database terms can be found at: <A href=
"http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
<H4><A name="4.14">4.14</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4> Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system, <P>You probably have run out of virtual memory on your system,
@@ -1124,12 +980,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
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="4.15">4.15</A>) How do I tell what PostgreSQL version <H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4> I am running?</H4>
<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="4.16">4.16</A>) Why does my large-object operations <H4><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4> get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE> <P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
@@ -1145,15 +1001,15 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you <P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P> may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.17">4.17</A>) How do I create a column that will <H4><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4> 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="4.18">4.18</A>) How do I perform an outer join?</H4> <H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
<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>
@@ -1176,24 +1032,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<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>
<P>In previous releases, outer joins can be simulated using <H4><A name="4.18">4.18</A>) How do I perform queries using
<SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
joining <I>tab1</I> and <I>tab2</I>, the following query does an
<I>outer</I> join of the two tables:<BR>
<BR>
</P>
<PRE>
SELECT tab1.col1, tab2.col2
FROM tab1, tab2
WHERE tab1.col1 = tab2.col1
UNION ALL
SELECT tab1.col1, NULL
FROM tab1
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1
</PRE>
<H4><A name="4.19">4.19</A>) How do I perform queries using
multiple databases?</H4> multiple databases?</H4>
<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.
@@ -1201,29 +1040,29 @@ BYTEA bytea variable-length byte array (null-byte safe)
uncertain how a cross-database query should even behave.</P> uncertain how a cross-database query should even behave.</P>
<P><I>contrib/dblink</I> allows cross-database queries using <P><I>contrib/dblink</I> allows cross-database queries using
function calls. Of course, a client can make simultaneous function calls. Of course, a client can also make simultaneous
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="4.20">4.20</A>) How do I return multiple rows or <H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4> columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a <P>It is easy using set-returning functions,
function,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions"> <a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>. http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.21">4.21</A>) Why can't I reliably create/drop <H4><A name="4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4> temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, 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
table is later dropped and recreated, and the function called table is later dropped and recreated, and the function called again,
again, the function will fail because the cached function contents the function will fail because the cached function contents still
still point to the old temporary table. The solution is to use point to the old temporary table. The solution is to use
<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="4.22">4.22</A>) What encryption options are available? <H4><A name="4.21">4.21</A>) What encryption options are available?
</H4> </H4>
<UL> <UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for <LI><I>contrib/pgcrypto</I> contains many encryption functions for
@@ -1236,8 +1075,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
encrypted transport, such as stunnel or ssh, rather than PostgreSQL's encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
native SSL connections.) native SSL connections.)
<LI>Database user passwords are automatically encrypted when stored in <LI>Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable the option the system tables.</LI>
<I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
<LI>The server can run using an encrypted file system.</LI> <LI>The server can run using an encrypted file system.</LI>
</UL> </UL>
@@ -1277,4 +1115,3 @@ BYTEA bytea variable-length byte array (null-byte safe)
compiler compute the dependencies automatically.</P> compiler compute the dependencies automatically.</P>
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