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Backpatch FAQ changes to 8.0.X.

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@@ -1,7 +1,7 @@
Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Wed Jan 19 14:45:22 EST 2005 Last updated: Sat Jan 29 23:20:03 EST 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
@@ -16,85 +16,72 @@
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 on PostgreSQL?
1.3) What Unix platforms does PostgreSQL run on? 1.3) What platforms does PostgreSQL support?
1.4) What non-Unix ports are available? 1.4) Where can I get PostgreSQL?
1.5) Where can I get PostgreSQL? 1.5) Where can I get support?
1.6) Where can I get support? 1.6) What is the latest release?
1.7) What is the latest release? 1.7) What documentation is available?
1.8) What documentation is available? 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? 1.9) How can I learn SQL?
1.10) How can I learn SQL? 1.10) How do I join the development team?
1.11) Is PostgreSQL Y2K compliant? 1.11) How do I submit a bug report?
1.12) How do I join the development team? 1.12) How does PostgreSQL compare to other DBMSs?
1.13) How do I submit a bug report? 1.13) How can I financially assist PostgreSQL?
1.14) How does PostgreSQL compare to other DBMSs?
1.15) How can I financially assist PostgreSQL?
User Client Questions User Client Questions
2.1) Are there ODBC drivers for PostgreSQL? 2.1) What interfaces are available for PostgreSQL?
2.2) What tools are available for using PostgreSQL with Web pages? 2.2) What tools are available for using PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface? 2.3) Does PostgreSQL have a graphical user interface?
2.4) What languages are available to communicate with PostgreSQL?
Administrative Questions Administrative Questions
3.1) How do I install PostgreSQL somewhere other than 3.1) How do I install PostgreSQL somewhere other than
/usr/local/pgsql? /usr/local/pgsql?
3.2) When I start postmaster, I get a Bad System Call or core dumped 3.2) How do I control connections from other hosts?
message. Why? 3.3) How do I tune the database engine for better performance?
3.3) When I try to start postmaster, I get IpcMemoryCreate errors. 3.4) What debugging features are available?
Why? 3.5) Why do I get "Sorry, too many clients" when trying to connect?
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. 3.6) What is in the pgsql_tmp directory?
Why? 3.7) Why do I need to do a dump and restore to upgrade PostgreSQL
3.5) How do I control connections from other hosts?
3.6) How do I tune the database engine for better performance?
3.7) What debugging features are available?
3.8) Why do I get "Sorry, too many clients" when trying to connect?
3.9) What is in the pgsql_tmp directory?
3.10) Why do I need to do a dump and restore to upgrade PostgreSQL
releases? releases?
3.11) What computer hardware should I use? 3.8) What computer hardware should I use?
Operational Questions Operational Questions
4.1) What is the difference between binary cursors and normal cursors? 4.1) How do I SELECT only the first few rows of a query? A random row?
4.2) 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
4.3) 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?
4.4) How do you remove a column from a table, or change its data type? 4.3) How do you remove a column from a table, or change its data type?
4.5) 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?
4.6) 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?
4.7) My queries are slow or don't make use of the indexes. Why? 4.6) Why are my queries slow? Why don't they use my indexes?
4.8) 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?
4.9) What is an R-tree index? 4.8) How do I perform regular expression searches and case-insensitive
4.10) What is the Genetic Query Optimizer? regular expression searches? How do I use an index for
4.11) How do I perform regular expression searches and case-insensitive searches?
case-insensitive regular expression searches? How do I use an index 4.9) In a query, how do I detect if a field is NULL?
for case-insensitive searches? 4.10) What is the difference between the various character types?
4.12) In a query, how do I detect if a field is NULL? 4.11.0) How do I create a serial/auto-incrementing field?
4.13) What is the difference between the various character types? 4.11.1) How do I get the value of a SERIAL insert?
4.14.0) How do I create a serial/auto-incrementing field? 4.11.2) Doesn't currval() lead to a race condition with other users?
4.14.1) How do I get the value of a SERIAL insert? 4.11.3) Why aren't my sequence numbers reused on transaction abort?
4.14.2) Doesn't currval() lead to a race condition with other users?
4.14.3) 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?
4.15) What is an OID? What is a TID? 4.12) What is an OID? What is a TID?
4.16) What is the meaning of some of the terms used in PostgreSQL? 4.13) What is the meaning of some of the terms used in PostgreSQL?
4.17) Why do I get the error "ERROR: Memory exhausted in 4.14) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"? AllocSetAlloc()"?
4.18) How do I tell what PostgreSQL version I am running? 4.15) How do I tell what PostgreSQL version I am running?
4.19) Why does my large-object operations get "invalid large obj 4.16) Why does my large-object operations get "invalid large obj
descriptor"? descriptor"?
4.20) How do I create a column that will default to the current time? 4.17) How do I create a column that will default to the current time?
4.21) Why are my subqueries using IN so slow? 4.18) How do I perform an outer join?
4.22) How do I perform an outer join? 4.19) How do I perform queries using multiple databases?
4.23) How do I perform queries using multiple databases? 4.20) How do I return multiple rows or columns from a function?
4.24) 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
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions? functions?
4.26) What encryption options are available? 4.22) What encryption options are available?
Extending PostgreSQL Extending PostgreSQL
@@ -172,14 +159,12 @@
no restrictions on how the source code may be used. We like it and no restrictions on how the source code may be used. We like it and
have no intention of changing it. have no intention of changing it.
1.3) What Unix platforms does PostgreSQL run on? 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.
1.4) What non-Unix ports are available?
Starting with version 8.0, PostgreSQL now runs natively on Microsoft Starting with version 8.0, PostgreSQL now runs natively on Microsoft
Windows NT-based operating systems like Win2000, WinXP, and Win2003. A Windows NT-based operating systems like Win2000, WinXP, and Win2003. A
prepackaged installer is available at prepackaged installer is available at
@@ -191,12 +176,12 @@
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.
1.5) Where can I get PostgreSQL? 1.4) Where can I get PostgreSQL?
The primary anonymous ftp site for PostgreSQL is The primary anonymous ftp site for PostgreSQL is
ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site. ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.
1.6) Where can I get support? 1.5) Where can I get support?
The main mailing list is: pgsql-general@PostgreSQL.org. It is The main mailing list is: pgsql-general@PostgreSQL.org. It is
available for discussion of matters pertaining to PostgreSQL. To available for discussion of matters pertaining to PostgreSQL. To
@@ -241,13 +226,13 @@
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.
1.7) What is the latest release? 1.6) 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 six to eight months.
1.8) What documentation is available? 1.7) What documentation is available?
Several manuals, manual pages, and some small test examples are Several manuals, manual pages, and some small test examples are
included in the distribution. See the /doc directory. You can also included in the distribution. See the /doc directory. You can also
@@ -267,12 +252,12 @@
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? 1.8) 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? 1.9) 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
@@ -288,11 +273,7 @@
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.
1.11) Is PostgreSQL Y2K compliant? 1.10) How do I join the development team?
Yes, we easily handle dates past the year 2000 AD, and before 2000 BC.
1.12) 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, documentation on our web site, or in the distribution. Second,
@@ -305,7 +286,7 @@
and we had confidence that patches they committed were of high and we had confidence that patches they committed were of high
quality. quality.
1.13) How do I submit a bug report? 1.11) 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.
@@ -313,7 +294,7 @@
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 or patches. there is a more recent PostgreSQL version or patches.
1.14) 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,
reliability, support, and price. reliability, support, and price.
@@ -353,14 +334,14 @@
community, manuals, and the source code often make PostgreSQL community, manuals, and the source code often make PostgreSQL
support superior to other DBMSs. There is commercial support superior to other DBMSs. There is commercial
per-incident support available for those who need it. (See FAQ per-incident support available for those who need it. (See FAQ
section 1.6.) section 1.5.)
Price Price
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.
1.15) How can I financially assist PostgreSQL? 1.13) How can I financially assist PostgreSQL?
PostgreSQL has had a first-class infrastructure since we started in PostgreSQL has had a first-class infrastructure since we started in
1996. This is all thanks to Marc Fournier, who has created and managed 1996. This is all thanks to Marc Fournier, who has created and managed
@@ -386,21 +367,17 @@
User Client Questions User Client Questions
2.1) Are there ODBC drivers for PostgreSQL? 2.1) What interfaces are available for PostgreSQL?
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC. The PostgreSQL install includes only the C and embedded C interfaces.
All other interfaces are independent projects that are downloaded
separately; being separate allows them to have their own release
schedule and development teams.
You can download PsqlODBC from Some programming languages like PHP include an interface to
http://gborg.postgresql.org/project/psqlodbc/projdisplay.php. PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many
others are available at http://gborg.postgresql.org in the
OpenLink ODBC can be gotten from http://www.openlinksw.com. It works Drivers/Interfaces section and via Internet search.
with their standard ODBC client software so you'll have PostgreSQL
ODBC available on every client platform they support (Win, Mac, Unix,
VMS).
They will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be
available. Please send questions to postgres95@openlink.co.uk.
2.2) What tools are available for using PostgreSQL with Web pages? 2.2) What tools are available for using PostgreSQL with Web pages?
@@ -424,21 +401,6 @@
See http://techdocs.postgresql.org/guides/GUITools for a more detailed See http://techdocs.postgresql.org/guides/GUITools for a more detailed
list. list.
2.4) What languages are able to communicate with PostgreSQL?
Most popular programming languages contain an interface to PostgreSQL.
Check your programming language's list of extension modules.
The following interfaces are included in the PostgreSQL distribution:
* C (libpq)
* Embedded C (ecpg)
* Java (jdbc)
* Python (PyGreSQL)
* TCL (libpgtcl)
Additional interfaces are available at http://gborg.postgresql.org in
the Drivers/Interfaces section.
_________________________________________________________________ _________________________________________________________________
Administrative Questions Administrative Questions
@@ -447,43 +409,7 @@
Specify the --prefix option when running configure. Specify the --prefix option when running configure.
3.2) When I start postmaster, I get a Bad System Call or core dumped 3.2) How do I control connections from other hosts?
message. Why?
It could be a variety of problems, but first check to see that you
have System V extensions installed in your kernel. PostgreSQL requires
kernel support for shared memory and semaphores.
3.3) When I try to start postmaster, I get IpcMemoryCreate errors. Why?
You either do not have shared memory configured properly in your
kernel or you need to enlarge the shared memory available in the
kernel. The exact amount you need depends on your architecture and how
many buffers and backend processes you configure for postmaster. For
most systems, with default numbers of buffers and processes, you need
a minimum of ~1 MB. See the PostgreSQL Administrator's Guide/Server
Run-time Environment/Managing Kernel Resources section for more
detailed information about shared memory and semaphores.
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. Why?
If the error message is IpcSemaphoreCreate: semget failed (No space
left on device) then your kernel is not configured with enough
semaphores. Postgres needs one semaphore per potential backend
process. A temporary solution is to start postmaster with a smaller
limit on the number of backend processes. Use -N with a parameter less
than the default of 32. A more permanent solution is to increase your
kernel's SEMMNS and SEMMNI parameters.
Inoperative semaphores can also cause crashes during heavy database
access.
If the error message is something else, you might not have semaphore
support configured in your kernel at all. See the PostgreSQL
Administrator's Guide for more detailed information about shared
memory and semaphores.
3.5) 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
@@ -491,7 +417,7 @@
postgresql.conf and enable host-based authentication by modifying the postgresql.conf and enable host-based authentication by modifying the
file $PGDATA/pg_hba.conf accordingly. file $PGDATA/pg_hba.conf accordingly.
3.6) 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 Certainly, indexes can speed up queries. The EXPLAIN ANALYZE command
allows you to see how PostgreSQL is interpreting your query, and which allows you to see how PostgreSQL is interpreting your query, and which
@@ -523,7 +449,7 @@
You can also use the CLUSTER command to group data in tables to match You can also use the CLUSTER command to group data in tables to match
an index. See the CLUSTER manual page for more details. an index. See the CLUSTER manual page for more details.
3.7) What debugging features are available? 3.4) What debugging features are available?
PostgreSQL has several features that report status information that PostgreSQL has several features that report status information that
can be valuable for debugging purposes. can be valuable for debugging purposes.
@@ -578,7 +504,7 @@ log_*
in the client's current directory. Linux requires a compile with in the client's current directory. Linux requires a compile with
-DLINUX_PROFILE for proper profiling. -DLINUX_PROFILE for proper profiling.
3.8) 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 need to increase postmaster's limit on how many concurrent backend
processes it can start. processes it can start.
@@ -598,7 +524,7 @@ log_*
that PostgreSQL has a limit on the number of allowed backend processes that PostgreSQL has a limit on the number of allowed backend processes
is so your system won't run out of resources. is so your system won't run out of resources.
3.9) What is in the pgsql_tmp directory? 3.6) What is in the pgsql_tmp directory?
This directory contains temporary files generated by the query This directory contains temporary files generated by the query
executor. For example, if a sort needs to be done to satisfy an ORDER executor. For example, if a sort needs to be done to satisfy an ORDER
@@ -609,7 +535,7 @@ log_*
remain if a backend crashes during a sort. A stop and restart of the remain if a backend crashes during a sort. A stop and restart of the
postmaster will remove files from those directories. postmaster will remove files from those directories.
3.10) Why do I need to do a dump and restore to upgrade between major 3.7) 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,
@@ -624,7 +550,7 @@ log_*
script can be used to upgrade without a dump/restore. The release script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release. notes mention whether pg_upgrade is available for the release.
3.11) 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
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
@@ -637,11 +563,7 @@ log_*
Operational Questions Operational Questions
4.1) What is the difference between binary cursors and normal cursors? 4.1) How do I SELECT only the first few rows of a query? A random row?
See the DECLARE manual page for a description.
4.2) How do I SELECT only the first few rows of a query? A random row?
See the FETCH manual page, or use SELECT ... LIMIT.... See the FETCH manual page, or use SELECT ... LIMIT....
@@ -657,7 +579,7 @@ log_*
ORDER BY random() ORDER BY random()
LIMIT 1; LIMIT 1;
4.3) 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
@@ -676,7 +598,7 @@ log_*
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.
4.4) How do you remove a column from a table, or change its data type? 4.3) How do you remove a column from a table, or change its data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can do this: DROP COLUMN. In earlier versions, you can do this:
@@ -702,7 +624,7 @@ log_*
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.
4.5) 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)
@@ -724,7 +646,7 @@ log_*
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.
4.6) 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
@@ -758,7 +680,7 @@ log_*
NULLs are stored as bitmaps, so they use very little space. NULLs are stored as bitmaps, so they use very little space.
4.7) My queries are slow or don't make use of the indexes. Why? 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 automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query selects used if the table is larger than a minimum size, and the query selects
@@ -800,7 +722,7 @@ log_*
[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 functional indexes, which are described in
section 4.11. section 4.10.
* 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-greater character in a non-C locale. You
can create a special can create a special
@@ -813,40 +735,11 @@ LIKE
types exactly match the index's column types. This is particularly types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes. true of int2, int8, and numeric column indexes.
4.8) 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.
4.9) What is an R-tree index? 4.8) How do I perform regular expression searches and case-insensitive
An R-tree index is used for indexing spatial data. A hash index can't
handle range searches. A B-tree index only handles range searches in a
single dimension. R-trees can handle multi-dimensional data. For
example, if an R-tree index can be built on an attribute of type
point, the system can more efficiently answer queries such as "select
all points within a bounding rectangle."
The canonical paper that describes the original R-tree design is:
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt of
Data, 45-57.
You can also find this paper in Stonebraker's "Readings in Database
Systems".
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
be extended to handle higher number of dimensions. In practice,
extending R-trees requires a bit of work and we don't currently have
any documentation on how to do it.
4.10) What is the Genetic Query Optimizer?
The GEQO module speeds query optimization when joining many tables by
means of a Genetic Algorithm (GA). It allows the handling of large
join queries through nonexhaustive search.
4.11) 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?
@@ -863,11 +756,11 @@ LIKE
functional index, it will be used: functional index, it will be used:
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
4.12) 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?
You test the column with IS NULL and IS NOT NULL. You test the column with IS NULL and IS NOT NULL.
4.13) 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
-------------------------------------------------- --------------------------------------------------
@@ -895,7 +788,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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.
4.14.1) 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:
@@ -916,11 +809,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
However, if you need to dump and reload the database, you need to use 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. pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs.
4.14.2) 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 4.14.1, an example in a explicitly. Using the example table in 4.11.1, an example in a
pseudo-language would look like this: pseudo-language would look like this:
new_id = execute("SELECT nextval('person_id_seq')"); new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')"); execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
@@ -942,52 +835,36 @@ BYTEA bytea variable-length byte array (null-byte safe)
billion. In Perl, using DBI with the DBD::Pg module, the oid value is billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute(). made available via $sth->{pg_oid_status} after $sth->execute().
4.14.3) 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 backend, not
by all users. by all users.
4.14.4) 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.
4.15) What is an OID? What is a TID? 4.12) What is an OID? What is a TID?
OIDs are PostgreSQL's answer to unique row ids. Every row that is Every row that is created in PostgreSQL gets a unique OID unless
created in PostgreSQL gets a unique OID. All OIDs generated during created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte
initdb are less than 16384 (from include/access/transam.h). All integers that are unique across the entire installation. However, they
user-created OIDs are equal to or greater than this. By default, all overflow at 4 billion, and then the OIDs start being duplicated.
these OIDs are unique not only within a table or database, but unique PostgreSQL uses OIDs to link its internal system tables together.
within the entire PostgreSQL installation.
PostgreSQL uses OIDs in its internal system tables to link rows To uniquely number columns in user tables, it is best to use SERIAL
between tables. These OIDs can be used to identify specific user rows rather than OIDs because SERIAL sequences are unique only within a
and used in joins. It is recommended you use column type OID to store single table. and are therefore less likely to overflow. SERIAL8 is
OID values. You can create an index on the OID field for faster available for storing eight-byte sequence values.
access.
OIDs are assigned to all new rows from a central area that is used by
all databases. If you want to change the OID to something else, or if
you want to make a copy of the table, with the original OIDs, there is
no reason you can't do it:
CREATE TABLE new_table(mycol int);
SELECT oid AS old_oid, mycol INTO tmp_table FROM old_table;
COPY tmp_table TO '/tmp/pgtable';
COPY new_table WITH OIDS FROM '/tmp/pgtable';
DROP TABLE tmp_table;
OIDs are stored as 4-byte integers, and will overflow at 4 billion. No
one has reported this ever happening, and we plan to have the limit
removed before anyone does.
TIDs are used to identify specific physical rows with block and offset TIDs are used to identify specific physical rows with block and offset
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.
4.16) What is the meaning of some of the terms used in PostgreSQL? 4.13) What is the meaning of some of the terms used in PostgreSQL?
Some of the source code and older documentation use terms that have Some of the source code and older documentation use terms that have
more common usage. Here are some: more common usage. Here are some:
@@ -1005,7 +882,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html /glossary.html
4.17) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"? 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
@@ -1020,11 +897,11 @@ 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.
4.18) How do I tell what PostgreSQL version I am running? 4.15) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version(); From psql, type SELECT version();
4.19) Why does my large-object operations get "invalid large obj 4.16) 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
@@ -1039,33 +916,12 @@ 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.
4.20) How do I create a column that will default to the current time? 4.17) 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 );
4.21) Why are my subqueries using IN so slow? 4.18) How do I perform an outer join?
In versions prior to 7.4, subqueries were joined to outer queries by
sequentially scanning the result of the subquery for each row of the
outer query. If the subquery returns only a few rows and the outer
query returns many rows, IN is fastest. To speed up other queries,
replace IN with EXISTS:
SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
to:
SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
For this to be fast, subcol should be an indexed column.
In version 7.4 and later, IN actually uses the same sophisticated join
techniques as normal queries, and is prefered to using EXISTS.
4.22) 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:
@@ -1095,7 +951,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2) WHERE tab1.col1 NOT IN (SELECT tab2.col1 FROM tab2)
ORDER BY col1 ORDER BY col1
4.23) How do I perform queries using multiple databases? 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
@@ -1105,12 +961,12 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
course, a client can make simultaneous connections to different course, a client can make simultaneous connections to different
databases and merge the results on the client side. databases and merge the results on the client side.
4.24) How do I return multiple rows or columns from a function? 4.20) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions. function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.25) Why can't I reliably create/drop temporary tables in PL/PgSQL 4.21) 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 contents, and an unfortunate side effect is
@@ -1121,7 +977,7 @@ CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
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.
4.26) What encryption options are available? 4.22) 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.

416
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: Wed Jan 19 14:45:22 EST 2005</P> <P>Last updated: Sat Jan 29 23:20:03 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>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -28,117 +28,100 @@
<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 on PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What Unix platforms does PostgreSQL run <A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
on?<BR> <A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.4">1.4</A>) What non-Unix ports are available?<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 PostgreSQL?<BR> <A href="#1.6">1.6</A>) What is the latest release?<BR>
<A href="#1.6">1.6</A>) Where can I get support?<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="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR> <A href="#1.9">1.9</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.11">1.11</A>) Is PostgreSQL Y2K compliant?<BR> <A href="#1.10">1.10</A>) How do I join the development team?<BR>
<A href="#1.12">1.12</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="#1.13">1.13</A>) How do I submit a bug report?<BR> <A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<A href="#1.14">1.14</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR> <SMALL>DBMS</SMALL>s?<BR>
<A href="#1.15">1.15</A>) How can I financially assist <A href="#1.13">1.13</A>) How can I financially assist
PostgreSQL?<BR> PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
<A href="#2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers for <A href="#2.1">2.1</A>) What interfaces are available for
PostgreSQL?<BR> PostgreSQL?<BR>
<A href="#2.2">2.2</A>) What tools are available for using <A href="#2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?<BR> PostgreSQL with Web pages?<BR>
<A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user <A href="#2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?<BR> interface?<BR>
<A href="#2.4">2.4</A>) What languages are available to
communicate with PostgreSQL?<BR>
<H2 align="center">Administrative Questions</H2> <H2 align="center">Administrative Questions</H2>
<A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other <A href="#3.1">3.1</A>) How do I install PostgreSQL somewhere other
than <I>/usr/local/pgsql</I>?<BR> than <I>/usr/local/pgsql</I>?<BR>
<A href="#3.2">3.2</A>) When I start <I>postmaster</I>, I get a <A href="#3.2">3.2</A>) How do I control connections from other
<I>Bad System Call</I> or core dumped message. Why?<BR>
<A href="#3.3">3.3</A>) When I try to start <I>postmaster</I>, I
get <I>IpcMemoryCreate</I> errors. Why?<BR>
<A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
<A href="#3.5">3.5</A>) How do I control connections from other
hosts?<BR> hosts?<BR>
<A href="#3.6">3.6</A>) How do I tune the database engine for <A href="#3.3">3.3</A>) How do I tune the database engine for
better performance?<BR> better performance?<BR>
<A href="#3.7">3.7</A>) What debugging features are available?<BR> <A href="#3.4">3.4</A>) What debugging features are available?<BR>
<A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many <A href="#3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?<BR> clients"</I> when trying to connect?<BR>
<A href="#3.9">3.9</A>) What is in the <I>pgsql_tmp</I> <A href="#3.6">3.6</A>) What is in the <I>pgsql_tmp</I>
directory?<BR> directory?<BR>
<A href="#3.10">3.10</A>) Why do I need to do a dump and restore <A href="#3.7">3.7</A>) Why do I need to do a dump and restore
to upgrade PostgreSQL releases?<BR> to upgrade PostgreSQL releases?<BR>
<A href="#3.11">3.11</A>) What computer hardware should I use?<BR> <A href="#3.8">3.8</A>) What computer hardware should I use?<BR>
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<A href="#4.1">4.1</A>) What is the difference between binary <A href="#4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
cursors and normal cursors?<BR>
<A href="#4.2">4.2</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?<BR> first few rows of a query? A random row?<BR>
<A href="#4.3">4.3</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="#4.4">4.4</A>) How do you remove a column from a <A href="#4.3">4.3</A>) How do you remove a column from a
table, or change its data type?<BR> table, or change its data type?<BR>
<A href="#4.5">4.5</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="#4.6">4.6</A>) How much database disk space is required <A href="#4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?<BR> to store data from a typical text file?<BR>
<A href="#4.7">4.7</A>) My queries are slow or don't make use of <A href="#4.6">4.6</A>) Why are my queries slow? Why don't they
the indexes. Why?<BR> use my indexes?<BR>
<A href="#4.8">4.8</A>) How do I see how the query optimizer is <A href="#4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?<BR> evaluating my query?<BR>
<A href="#4.9">4.9</A>) What is an R-tree index?<BR> <A href="#4.8">4.8</A>) How do I perform regular expression
<A href="#4.10">4.10</A>) What is the Genetic Query Optimizer?<BR>
<A href="#4.11">4.11</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?<BR> use an index for case-insensitive searches?<BR>
<A href="#4.12">4.12</A>) In a query, how do I detect if a field <A href="#4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?<BR> is <SMALL>NULL</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the difference between the <A href="#4.10">4.10</A>) What is the difference between the
various character types?<BR> various character types?<BR>
<A href="#4.14.0">4.14.0</A>) How do I create a <A href="#4.11.0">4.11.0</A>) How do I create a
serial/auto-incrementing field?<BR> serial/auto-incrementing field?<BR>
<A href="#4.14.1">4.14.1</A>) How do I get the value of a <A href="#4.11.1">4.11.1</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?<BR> <SMALL>SERIAL</SMALL> insert?<BR>
<A href="#4.14.2">4.14.2</A>) Doesn't <I>currval()</I> <A href="#4.11.2">4.11.2</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?<BR> lead to a race condition with other users?<BR>
<A href="#4.14.3">4.14.3</A>) Why aren't my sequence numbers <A href="#4.11.3">4.11.3</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?<BR> my sequence/SERIAL column?<BR>
<A href="#4.15">4.15</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="#4.16">4.16</A>) What is the meaning of some of the terms <A href="#4.13">4.13</A>) What is the meaning of some of the terms
used in PostgreSQL?<BR> used in PostgreSQL?<BR>
<A href="#4.17">4.17</A>) Why do I get the error <I>"ERROR: Memory <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="#4.18">4.18</A>) How do I tell what PostgreSQL version I <A href="#4.15">4.15</A>) How do I tell what PostgreSQL version I
am running?<BR> am running?<BR>
<A href="#4.19">4.19</A>) Why does my large-object operations get <A href="#4.16">4.16</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR> <I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.20">4.20</A>) How do I create a column that will <A href="#4.17">4.17</A>) How do I create a column that will
default to the current time?<BR> default to the current time?<BR>
<A href="#4.21">4.21</A>) Why are my subqueries using <A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
<CODE><SMALL>IN</SMALL></CODE> so slow?<BR> <A href="#4.19">4.19</A>) How do I perform queries using multiple
<A href="#4.22">4.22</A>) How do I perform an outer join?<BR>
<A href="#4.23">4.23</A>) How do I perform queries using multiple
databases?<BR> databases?<BR>
<A href="#4.24">4.24</A>) How do I return multiple rows or columns <A href="#4.20">4.20</A>) How do I return multiple rows or columns
from a function?<BR> from a function?<BR>
<A href="#4.25">4.25</A>) Why can't I reliably create/drop <A href="#4.21">4.21</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR> temporary tables in PL/PgSQL functions?<BR>
<A href="#4.26">4.26</A>) What encryption options are available?<BR> <A href="#4.22">4.22</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2> <H2 align="center">Extending PostgreSQL</H2>
@@ -225,16 +208,13 @@
It has no restrictions on how the source code may be used. We like It has no restrictions on how the source code may be used. We like
it and have no intention of changing it.</P> it and have no intention of changing it.</P>
<H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run <H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
on?</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
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>
<H4><A name="1.4">1.4</A>) What non-Unix ports are available?</H4>
<P>Starting with version 8.0, PostgreSQL now runs natively on <P>Starting with version 8.0, PostgreSQL now runs natively on
Microsoft Windows NT-based operating systems like Win2000, WinXP, Microsoft Windows NT-based operating systems like Win2000, WinXP,
and Win2003. A prepackaged installer is available at <a href= and Win2003. A prepackaged installer is available at <a href=
@@ -248,13 +228,13 @@
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F"> "http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p> http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F</a>.</p>
<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4> <H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4>
<P>The primary anonymous ftp site for PostgreSQL is <A href= <P>The primary anonymous ftp site for PostgreSQL is <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>. "ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A>.
For mirror sites, see our main web site.</P> For mirror sites, see our main web site.</P>
<H4><A name="1.6">1.6</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 main mailing list is: <A href=
"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>. "mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
@@ -316,13 +296,13 @@
<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.postgresql.org/companies.php</A>.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4> <H4><A name="1.6">1.6</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 six to eight months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4> <H4><A name="1.7">1.7</A>) What documentation is available?</H4>
<P>Several manuals, manual pages, and some small test examples are <P>Several manuals, manual pages, and some small test examples are
included in the distribution. See the <I>/doc</I> directory. You included in the distribution. See the <I>/doc</I> directory. You
@@ -346,14 +326,14 @@
<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 <H4><A name="1.8">1.8</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="1.10">1.10</A>) How can I learn <H4><A name="1.9">1.9</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=
@@ -377,12 +357,7 @@
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="1.11">1.11</A>) Is PostgreSQL Y2K compliant?</H4> <H4><A name="1.10">1.10</A>) How do I join the development
<P>Yes, we easily handle dates past the year 2000 AD, and before
2000 BC.</P>
<H4><A name="1.12">1.12</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
@@ -397,7 +372,7 @@
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="1.13">1.13</A>) How do I submit a bug report?</H4> <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= <P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug"> "http://www.postgresql.org/support/submitbug">
@@ -407,7 +382,7 @@
"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 or patches.</P> see if there is a more recent PostgreSQL version or patches.</P>
<H4><A name="1.14">1.14</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>
<P>There are several ways of measuring software: features, <P>There are several ways of measuring software: features,
@@ -460,7 +435,7 @@
community, manuals, and the source code often make PostgreSQL community, manuals, and the source code often make PostgreSQL
support superior to other <SMALL>DBMS</SMALL>s. There is support superior to other <SMALL>DBMS</SMALL>s. There is
commercial per-incident support available for those who need it. commercial per-incident support available for those who need it.
(See <A href="#1.6">FAQ section 1.6</A>.)<BR> (See <A href="#1.5">FAQ section 1.5</A>.)<BR>
<BR> <BR>
</DD> </DD>
@@ -473,7 +448,7 @@
</DD> </DD>
</DL> </DL>
<H4><A name="1.15">1.15</A>) How can I financially assist <H4><A name="1.13">1.13</A>) How can I financially assist
PostgreSQL?</H4> PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started <P>PostgreSQL has had a first-class infrastructure since we started
@@ -503,26 +478,20 @@
<H2 align="center">User Client Questions</H2> <H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) Are there <SMALL>ODBC</SMALL> drivers <H4><A name="2.1">2.1</A>) What interfaces are available for
for PostgreSQL?</H4> PostgreSQL?</H4>
<P>There are two <SMALL>ODBC</SMALL> drivers available, PsqlODBC <P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
and OpenLink <SMALL>ODBC</SMALL>.</P> <SMALL>C</SMALL> interfaces. All other interfaces are independent projects
that are downloaded separately; being separate allows them to have their
own release schedule and development teams.</P>
<P>You can download PsqlODBC from <A href= <P>Some programming languages like <SMALL>PHP</SMALL> include an
"http://gborg.postgresql.org/project/psqlodbc/projdisplay.php"> interface to PostgreSQL. Interfaces for languages like Perl,
http://gborg.postgresql.org/project/psqlodbc/projdisplay.php</A>.</P> <SMALL>TCL</SMALL>, Python, and many others are available at
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
<P>OpenLink <SMALL>ODBC</SMALL> can be gotten from <A href= in the <I>Drivers/Interfaces</I> section and via Internet search.
"http://www.openlinksw.com/">http://www.openlinksw.com</A>. It </P>
works with their standard <SMALL>ODBC</SMALL> client software so
you'll have PostgreSQL <SMALL>ODBC</SMALL> available on every
client platform they support (Win, Mac, Unix, VMS).</P>
<P>They will probably be selling this product to people who need
commercial-quality support, but a freeware version will always be
available. Please send questions to <A href=
"mailto:postgres95@openlink.co.uk">postgres95@openlink.co.uk</A>.</P>
<H4><A name="2.2">2.2</A>) What tools are available for using <H4><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4> PostgreSQL with Web pages?</H4>
@@ -552,34 +521,9 @@
<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>
<H4><A name="2.4">2.4</A>) What languages are able to communicate with
PostgreSQL?</H4>
<P>Most popular programming languages contain an interface to
PostgreSQL. Check your programming language's list of extension
modules.</P>
<P>The following interfaces are included in the PostgreSQL
distribution:</P>
<UL>
<LI>C (libpq)</LI>
<LI>Embedded C (ecpg)</LI>
<LI>Java (jdbc)</LI>
<LI>Python (PyGreSQL)</LI>
<LI>TCL (libpgtcl)</LI>
</UL>
<P>Additional interfaces are available at
<a href="http://gborg.postgresql.org">http://gborg.postgresql.org</A>
in the <I>Drivers/Interfaces</I> section.
</P>
<HR> <HR>
<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 <H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
@@ -588,47 +532,7 @@
<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>) When I start <I>postmaster</I>, I get a <H4><A name="3.2">3.2</A>) How do I control connections from other
<I>Bad System Call</I> or core dumped message. Why?</H4>
<P>It could be a variety of problems, but first check to see that
you have System V extensions installed in your kernel. PostgreSQL
requires kernel support for shared memory and semaphores.</P>
<H4><A name="3.3">3.3</A>) When I try to start <I>postmaster</I>, I
get <I>IpcMemoryCreate</I> errors. Why?</H4>
<P>You either do not have shared memory configured properly in your
kernel or you need to enlarge the shared memory available in the
kernel. The exact amount you need depends on your architecture and
how many buffers and backend processes you configure for
<I>postmaster</I>. For most systems, with default numbers of
buffers and processes, you need a minimum of ~1 MB. See the <A
href="http://www.postgresql.org/docs/current/static/kernel-resources.html">PostgreSQL
Administrator's Guide/Server Run-time Environment/Managing Kernel Resources</A>
section for more detailed information about shared memory and semaphores.</P>
<H4><A name="3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?</H4>
<P>If the error message is <I>IpcSemaphoreCreate: semget failed (No
space left on device)</I> then your kernel is not configured with
enough semaphores. Postgres needs one semaphore per potential
backend process. A temporary solution is to start <I>postmaster</I>
with a smaller limit on the number of backend processes. Use
<I>-N</I> with a parameter less than the default of 32. A more
permanent solution is to increase your kernel's
<SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI</SMALL> parameters.</P>
<P>Inoperative semaphores can also cause crashes during heavy
database access.</P>
<P>If the error message is something else, you might not have
semaphore support configured in your kernel at all. See the
PostgreSQL Administrator's Guide for more detailed information
about shared memory and semaphores.</P>
<H4><A name="3.5">3.5</A>) How do I control connections from other
hosts?</H4> hosts?</H4>
<P>By default, PostgreSQL only allows connections from the local <P>By default, PostgreSQL only allows connections from the local
@@ -638,7 +542,7 @@
host-based authentication by modifying the file host-based authentication by modifying the file
<I>$PGDATA/pg_hba.conf</I> accordingly.</P> <I>$PGDATA/pg_hba.conf</I> accordingly.</P>
<H4><A name="3.6">3.6</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>Certainly, indexes can speed up queries. The
@@ -677,7 +581,7 @@
data in tables to match an index. See the <SMALL>CLUSTER</SMALL> data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
manual page for more details.</P> manual page for more details.</P>
<H4><A name="3.7">3.7</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>PostgreSQL has several features that report status information
@@ -735,7 +639,7 @@
file will be put in the client's current directory. Linux requires file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P> a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
<H4><A name="3.8">3.8</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 need to increase <I>postmaster</I>'s limit on how many
@@ -760,7 +664,7 @@
the number of allowed backend processes is so your system won't run the number of allowed backend processes is so your system won't run
out of resources.</P> out of resources.</P>
<H4><A name="3.9">3.9</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>
<P>This directory contains temporary files generated by the query <P>This directory contains temporary files generated by the query
executor. For example, if a sort needs to be done to satisfy an executor. For example, if a sort needs to be done to satisfy an
@@ -772,7 +676,7 @@
remain if a backend crashes during a sort. A stop and restart of the remain if a backend crashes during a sort. A stop and restart of the
<I>postmaster</I> will remove files from those directories.</P> <I>postmaster</I> will remove files from those directories.</P>
<H4><A name="3.10">3.10</A>) Why do I need to do a dump and restore <H4><A name="3.7">3.7</A>) Why do I need to do a dump and restore
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,
@@ -788,7 +692,7 @@
The release notes mention whether <I>pg_upgrade</I> is available for the The release notes mention whether <I>pg_upgrade</I> is available for the
release.</P> release.</P>
<H4><A name="3.11">3.11</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
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
@@ -802,13 +706,7 @@
<H2 align="center">Operational Questions</H2> <H2 align="center">Operational Questions</H2>
<H4><A name="4.1">4.1</A>) What is the difference between binary <H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
cursors and normal cursors?</H4>
<P>See the <SMALL>DECLARE</SMALL> manual page for a
description.</P>
<H4><A name="4.2">4.2</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>See the <SMALL>FETCH</SMALL> manual page, or use
@@ -829,7 +727,7 @@
LIMIT 1; LIMIT 1;
</PRE> </PRE>
<H4><A name="4.3">4.3</A>) How do I find out what tables, indexes, <H4><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?</H4>
@@ -850,7 +748,7 @@
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="4.4">4.4</A>) How do you remove a column from a <H4><A name="4.3">4.3</A>) How do you remove a column from a
table, or change its data type?</H4> table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3 <P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
@@ -881,7 +779,7 @@
<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="4.5">4.5</A>) What is the maximum size for a row, a <H4><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?</H4>
<P>These are the limits:</P> <P>These are the limits:</P>
@@ -907,7 +805,7 @@
<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="4.6">4.6</A>) How much database disk space is required <H4><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?</H4>
<P>A PostgreSQL database may require up to five times the disk <P>A PostgreSQL database may require up to five times the disk
@@ -944,8 +842,8 @@
<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="4.7">4.7</A>) My queries are slow or don't make use of <H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
the indexes. Why?</H4> use my indexes?</H4>
<P>Indexes are not automatically used by every query. Indexes are only <P>Indexes are not automatically used by every query. Indexes are only
used if the table is larger than a minimum size, and the query used if the table is larger than a minimum size, and the query
@@ -995,7 +893,7 @@
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 functional
indexes, which are described in section <a href="#4.11">4.11</a>.</LI> indexes, which are described in section <a href="#4.10">4.10</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-greater
character in a non-C locale. You can create a special character in a non-C locale. You can create a special
@@ -1008,45 +906,12 @@
types exactly match the index's column types. This is particularly types exactly match the index's column types. This is particularly
true of int2, int8, and numeric column indexes.</P> true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.8">4.8</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
evaluating my query?</H4> evaluating my query?</H4>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P> <P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.9">4.9</A>) What is an R-tree index?</H4> <H4><A name="4.8">4.8</A>) How do I perform regular expression
<P>An R-tree index is used for indexing spatial data. A hash index
can't handle range searches. A B-tree index only handles range
searches in a single dimension. R-trees can handle
multi-dimensional data. For example, if an R-tree index can be
built on an attribute of type <I>point</I>, the system can more
efficiently answer queries such as "select all points within a
bounding rectangle."</P>
<P>The canonical paper that describes the original R-tree design
is:</P>
<P>Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
Searching." Proceedings of the 1984 ACM SIGMOD Int'l Conf on Mgmt
of Data, 45-57.</P>
<P>You can also find this paper in Stonebraker's "Readings in
Database Systems".</P>
<P>Built-in R-trees can handle polygons and boxes. In theory,
R-trees can be extended to handle higher number of dimensions. In
practice, extending R-trees requires a bit of work and we don't
currently have any documentation on how to do it.</P>
<H4><A name="4.10">4.10</A>) What is the Genetic Query
Optimizer?</H4>
<P>The <SMALL>GEQO</SMALL> module speeds query optimization when
joining many tables by means of a Genetic Algorithm (GA). It allows
the handling of large join queries through nonexhaustive
search.</P>
<H4><A name="4.11">4.11</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?</H4>
@@ -1069,13 +934,13 @@
CREATE INDEX tabindex ON tab (lower(col)); CREATE INDEX tabindex ON tab (lower(col));
</PRE> </PRE>
<H4><A name="4.12">4.12</A>) In a query, how do I detect if a field <H4><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>?</H4> is <SMALL>NULL</SMALL>?</H4>
<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>.</P>
<H4><A name="4.13">4.13</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> <PRE>
Type Internal Name Notes Type Internal Name Notes
@@ -1107,7 +972,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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="4.14.1">4.14.1</A>) How do I create a <H4><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4> serial/auto-incrementing field?</H4>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It <P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
@@ -1135,13 +1000,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY 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. WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
<H4><A name="4.14.2">4.14.2</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>
<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
<I>before</I> inserting and then insert it explicitly. Using the <I>before</I> inserting and then insert it explicitly. Using the
example table in <A href="#4.14.1">4.14.1</A>, an example in a example table in <A href="#4.11.1">4.11.1</A>, an example in a
pseudo-language would look like this:</P> pseudo-language would look like this:</P>
<PRE> <PRE>
new_id = execute("SELECT nextval('person_id_seq')"); new_id = execute("SELECT nextval('person_id_seq')");
@@ -1164,7 +1029,7 @@ 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="#4.15"><SMALL>OID</SMALL></A> <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 returned from the <SMALL>INSERT</SMALL> statement to look up the
default value, though this is probably the least portable approach, default value, though this is probably the least portable approach,
and the oid value will wrap around when it reaches 4 billion. and the oid value will wrap around when it reaches 4 billion.
@@ -1172,13 +1037,13 @@ BYTEA bytea variable-length byte array (null-byte safe)
available via <I>$sth-&gt;{pg_oid_status}</I> after available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P> <I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.14.3">4.14.3</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> backend, not by all users.</P>
<H4><A name="4.14.4">4.14.4</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
my sequence/SERIAL column?</H4> my sequence/SERIAL column?</H4>
@@ -1187,47 +1052,30 @@ BYTEA bytea variable-length byte array (null-byte safe)
completes. This causes gaps in numbering from aborted completes. This causes gaps in numbering from aborted
transactions.</P> transactions.</P>
<H4><A name="4.15">4.15</A>) What is an <SMALL>OID</SMALL>? What is <H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4> a <SMALL>TID</SMALL>?</H4>
<P><SMALL>OID</SMALL>s are PostgreSQL's answer to unique row ids. <P>Every row that is created in PostgreSQL gets a unique
Every row that is created in PostgreSQL gets a unique <SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
<SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during O<SMALL>ID</SMALL>s are autotomatically assigned unique 4-byte
<I>initdb</I> are less than 16384 (from integers that are unique across the entire installation. However,
<I>include/access/transam.h</I>). All user-created they overflow at 4 billion, and then the O<SMALL>ID</SMALL>s start
<SMALL>OID</SMALL>s are equal to or greater than this. By default, being duplicated. PostgreSQL uses <SMALL>OID</SMALL>s to link its
all these <SMALL>OID</SMALL>s are unique not only within a table or internal system tables together.</P>
database, but unique within the entire PostgreSQL installation.</P>
<P>PostgreSQL uses <SMALL>OID</SMALL>s in its internal system <P>To uniquely number columns in user tables, it is best to use
tables to link rows between tables. These <SMALL>OID</SMALL>s can <SMALL>SERIAL</> rather than O<SMALL>ID</SMALL>s because
be used to identify specific user rows and used in joins. It is <SMALL>SERIAL<SMALL> sequences are unique only within a single
recommended you use column type <SMALL>OID</SMALL> to store table. and are therefore less likely to overflow.
<SMALL>OID</SMALL> values. You can create an index on the <SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
<SMALL>OID</SMALL> field for faster access.</P> values.</P>
<P>O<SMALL>ID</SMALL>s are assigned to all new rows from a central
area that is used by all databases. If you want to change the
<SMALL>OID</SMALL> to something else, or if you want to make a copy
of the table, with the original <SMALL>OID</SMALL>s, there is no
reason you can't do it:</P>
<PRE>
CREATE TABLE new_table(mycol int);
SELECT oid AS old_oid, mycol INTO tmp_table FROM old_table;
COPY tmp_table TO '/tmp/pgtable';
COPY new_table WITH OIDS FROM '/tmp/pgtable';
DROP TABLE tmp_table;
</PRE>
<P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
overflow at 4 billion. No one has reported this ever happening, and
we plan to have the limit removed before anyone does.</P>
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows <P>T<SMALL>ID</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. T<SMALL>ID</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="4.16">4.16</A>) What is the meaning of some of the <H4><A name="4.13">4.13</A>) What is the meaning of some of the
terms used in PostgreSQL?</H4> terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that <P>Some of the source code and older documentation use terms that
@@ -1256,7 +1104,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>A list of general database terms can be found at: <A href= <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> "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="4.17">4.17</A>) Why do I get the error <I>"ERROR: <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,
@@ -1275,12 +1123,12 @@ 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="4.18">4.18</A>) How do I tell what PostgreSQL version <H4><A name="4.15">4.15</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="4.19">4.19</A>) Why does my large-object operations <H4><A name="4.16">4.16</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>
@@ -1296,7 +1144,7 @@ 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="4.20">4.20</A>) How do I create a column that will <H4><A name="4.17">4.17</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>
@@ -1305,31 +1153,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</CODE> </CODE>
</PRE> </PRE>
<H4><A name="4.21">4.21</A>) Why are my subqueries using <H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
<CODE><SMALL>IN</SMALL></CODE> so slow?</H4>
<P>In versions prior to 7.4, subqueries were joined to outer queries
by sequentially scanning the result of the subquery for each row of
the outer query. If the subquery returns only a few rows and the outer
query returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P>
<PRE> SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab);
</PRE>
to:
<PRE> SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
<P>In version 7.4 and later, <CODE>IN</CODE> actually uses the same
sophisticated join techniques as normal queries, and is prefered
to using <CODE>EXISTS</CODE>.
<H4><A name="4.22">4.22</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>
@@ -1369,7 +1193,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
ORDER BY col1 ORDER BY col1
</PRE> </PRE>
<H4><A name="4.23">4.23</A>) How do I perform queries using <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.
@@ -1381,7 +1205,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
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="4.24">4.24</A>) How do I return multiple rows or <H4><A name="4.20">4.20</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>In 7.3, you can easily return multiple rows or columns from a
@@ -1389,7 +1213,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<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="4.25">4.25</A>) Why can't I reliably create/drop <H4><A name="4.21">4.21</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 contents, 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
@@ -1399,7 +1223,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<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="4.26">4.26</A>) What encryption options are available? <H4><A name="4.22">4.22</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