# Frequently Asked Questions (FAQ) for PostgreSQL

Last updated: Thu Jul 11 12:37:48 EDT 2002

Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)

The most recent version of this document can be viewed at http://www.PostgreSQL.org/docs/faq-english.html.

Platform-specific questions are answered at http://www.PostgreSQL.org/users-lounge/docs/faq.html.

## General Questions

1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright on PostgreSQL?
1.3) What Unix platforms does PostgreSQL run on?
1.4) What non-Unix ports are available?
1.5) Where can I get PostgreSQL?
1.6) Where can I get support?
1.7) What is the latest release?
1.8) What documentation is available?
1.9) How do I find out about known bugs or missing features?
1.10) How can I learn SQL?
1.11) Is PostgreSQL Y2K compliant?
1.12) How do I join the development team?
1.13) How do I submit a bug report?
1.14) How does PostgreSQL compare to other DBMSs?
1.15) How can I financially assist PostgreSQL?

## User Client Questions

2.1) Are there ODBC drivers for PostgreSQL?
2.2) What tools are available for using PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface? A report generator? An embedded query language interface?
2.4) What languages are available to communicate with PostgreSQL?

## Administrative Questions

3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
3.2) When I start postmaster, I get a Bad System Call or core dumped message. Why?
3.3) When I try to start postmaster, I get IpcMemoryCreate errors. Why?
3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors. Why?
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 are the pg_sorttempNNN.NN files in my database directory?
3.10) Why do I need to do a dump and restore to upgrade PostgreSQL?

## Operational Questions

4.1) What is the difference between binary cursors and normal cursors?
4.2) How do I SELECT only the first few rows of a query?
4.3) How do I get a list of tables or other things I can see in psql?
4.4) How do you remove a column from a table?
4.5) 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 typical text file?
4.7) How do I find out what tables, indexes, databases, and users are defined?
4.8) My queries are slow or don't make use of the indexes. Why?
4.9) How do I see how the query optimizer is evaluating my query?
4.10) What is an R-tree index?
4.11) What is the Genetic Query Optimizer?
4.12) How do I perform regular expression searches and case-insensitive regular expression searches? How do I use an index for case-insensitive searches?
4.13) In a query, how do I detect if a field is NULL?
4.14) What is the difference between the various character types?
4.15.1) How do I create a serial/auto-incrementing field?
4.15.2) How do I get the value of a SERIAL insert?
4.15.3) Don't currval() and nextval() lead to a race condition with other users?
4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are there gaps in the numbering of my sequence/SERIAL column?
4.16) What is an OID? What is a TID?
4.17) What is the meaning of some of the terms used in PostgreSQL?
4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
4.19) How do I tell what PostgreSQL version I am running?
4.20) Why does my large-object operations get "invalid large obj descriptor"?
4.21) How do I create a column that will default to the current time?
4.22) Why are my subqueries using `IN` so slow?
4.23) How do I perform an outer join?
4.24) How do I perform queries using multiple databases?
4.25) How do I return multiple rows or columns from a function?
4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL functions?

## Extending PostgreSQL

5.1) I wrote a user-defined function. When I run it in psql, why does it dump core?
5.2) How can I contribute some nifty new types and functions to PostgreSQL?
5.3) How do I write a C function to return a tuple?
5.4) I have changed a source file. Why does the recompile not see the change?

## General Questions

#### 1.1) What is PostgreSQL?

PostgreSQL is pronounced Post-Gres-Q-L.

PostgreSQL is an enhancement of the POSTGRES database management system, a next-generation DBMS research prototype. While PostgreSQL retains the powerful data model and rich data types of POSTGRES, it replaces the 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 Internet developers who all subscribe to the PostgreSQL development mailing list. The current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See below on how to join). This team is now responsible for all development of PostgreSQL.

The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. 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.

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 Data Base Management System

Portions copyright (c) 1996-2002, PostgreSQL Global Development Group Portions Copyright (c) 1994-6 Regents of the University of California

Permission to use, copy, modify, and distribute this software and its documentation for any purpose, without fee, and without a written agreement is hereby granted, provided that the above copyright notice and this paragraph and the following two paragraphs appear in all copies.

IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE SOFTWARE PROVIDED HEREUNDER IS ON AN "AS IS" BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE, SUPPORT, 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 Unix platforms does PostgreSQL run on?

In general, a modern Unix-compatible platform should be able to run PostgreSQL. The platforms that had received explicit testing at the time of release are listed in the installation instructions.

#### 1.4) What non-Unix ports are available?

Client

It is possible to compile the libpq C library, psql, and other interfaces and binaries to run on MS Windows platforms. In this case, the client is running on MS Windows, and communicates via TCP/IP to a server running on one of our supported Unix platforms. A file win31.mak is included in the distribution for making a Win32 libpq library and psql. PostgreSQL also communicates with ODBC clients.

Server

The database server can run on Windows NT and Win2k using Cygwin, the Cygnus Unix/NT porting library. See pgsql/doc/FAQ_MSWIN in the distribution or the MS Windows FAQ on our web site. We have no plan to do a native port to any Microsoft platform.

#### 1.5) Where can I get PostgreSQL?

The primary anonymous ftp site for PostgreSQL is ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main web site.

#### 1.6) Where can I get support?

The main mailing list is: pgsql-general@PostgreSQL.org. It is available for discussion of matters pertaining to PostgreSQL. To subscribe, send mail with the following lines in the body (not the subject line):

        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

There is also an IRC channel on EFNet, channel #PostgreSQL. I use the Unix command `irc -c '#PostgreSQL' "$USER" irc.phoenix.net.`

A list of commercial support companies is available at http://www.postgresql.org/users-lounge/commercial-support.html.

#### 1.7) What is the latest release?

The latest release of PostgreSQL is version 7.2.1.

We plan to have major releases every four months.

#### 1.8) What documentation is available?

Several manuals, manual pages, and some small test examples are included in the distribution. See the /doc directory. You can also browse the manual online at http://www.PostgreSQL.org/users-lounge/docs/.

There are two PostgreSQL books available online at http://www.PostgreSQL.org/docs/awbook.html and http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL books available for purchase at http://www.postgresql.org/books/. There is also a collection of PostgreSQL technical articles at http://techdocs.postgresql.org/.

psql has some nice \d commands to show information about types, operators, functions, aggregates, etc.

Our web site contains even more documentation.

#### 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 for known bugs, missing features, and future plans.

#### 1.10) How can I learn SQL?

The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html teaches SQL. There is another PostgreSQL book at http://www.commandprompt.com/ppbook. There is a nice tutorial at http://www.intermedia.net/support/sql/sqltut.shtm, at http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM, and at http://sqlcourse.com.

Another one is "Teach Yourself SQL in 21 Days, Second Edition" at http://members.tripod.com/er4ebus/sql/index.htm

Many of our users like The Practical SQL Handbook, Bowman, Judith S., et al., Addison-Wesley. Others like The Complete Reference SQL, Groff et al., McGraw-Hill.

#### 1.11) Is PostgreSQL Y2K compliant?

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 documentation on our web site, or in the distribution. Second, subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, submit high quality patches to pgsql-patches.

There are about a dozen people who have commit privileges to the PostgreSQL CVS archive. They each have submitted so many high-quality patches that it was impossible for the existing committers to keep up, and we had confidence that patches they committed were of high quality.

#### 1.13) How do I submit a bug report?

Please visit the PostgreSQL BugTool page, which gives guidelines and directions on how to submit a bug.

Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if there is a more recent PostgreSQL version or patches.

#### 1.14) How does PostgreSQL compare to other DBMSs?

There are several ways of measuring software: features, performance, reliability, support, and price.

Features
  PostgreSQL has most features present in large commercial DBMSs, like transactions, subselects, triggers, views, foreign key referential integrity, and sophisticated locking. We have some features they do not have, like user-defined types, inheritance, rules, and multi-version concurrency control to reduce lock contention.
Performance
  PostgreSQL has performance similar to other commercial and open source databases. it is faster for some things, slower for others. In comparison to MySQL or leaner database systems, we are slower on inserts/updates because of transaction overhead. Of course, MySQL does not have any of the features mentioned in the Features section above. We are built for reliability and features, though we continue to improve performance in every release. There is an interesting Web page comparing PostgreSQL to MySQL at http://openacs.org/why-not-mysql.html
Reliability
  We realize that a DBMS must be reliable, or it is worthless. We strive to release well-tested, stable code that has a minimum of bugs. Each release has at least one month of beta testing, and our release history shows that we can provide stable, solid releases that are ready for production use. We believe we compare favorably to other database software in this area.
Support
  Our mailing list provides a large group of developers and users to help resolve any problems encountered. While we can not guarantee a fix, commercial DBMSs do not always supply a fix either. Direct access to developers, the user community, manuals, and the source code often make PostgreSQL support superior to other DBMSs. There is commercial per-incident support available for those who need it. (See support FAQ item.)
Price
  We are free for all use, both commercial and non-commercial. You can add our code to your product with no limitations, except those outlined in our BSD-style license stated above.

#### 1.15) How can I financially assist PostgreSQL?

PostgreSQL has had a first-class infrastructure since we started six years ago. 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://www.pgsql.com/pg_goodies 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.

## User Client Questions

#### 2.1) Are there ODBC drivers for PostgreSQL?

There are two ODBC drivers available, PsqlODBC and OpenLink ODBC.

PsqlODBC is included in the distribution. More information about it can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/.

OpenLink ODBC can be gotten from http://www.openlinksw.com. It works 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.

See also the ODBC chapter of the Programmer's Guide.

#### 2.2) What tools are available for using PostgreSQL with Web pages?

A nice introduction to Database-backed Web pages can be seen at: http://www.webreview.com

There is also one at http://www.phone.net/home/mwm/hotlist/.

For Web integration, PHP is an excellent interface. It is at http://www.php.net.

For complex cases, many use the Perl interface and CGI.pm.

#### 2.3) Does PostgreSQL have a graphical user interface? A report generator? An embedded query language interface?

We have a nice graphical user interface called pgaccess, which is shipped as part of the distribution. pgaccess also has a report generator. The Web page is http://www.flex.ro/pgaccess

We also include ecpg, which is an embedded SQL query language interface for C.

#### 2.4) What languages are available to communicate with PostgreSQL?

We have:

- C (libpq)
- C++ (libpq++)
- Embedded C (ecpg)
- Java (jdbc)
- Perl (perl5)
- ODBC (odbc)
- Python (PyGreSQL)
- TCL (libpgtcl)
- C Easy API (libpgeasy)
- Embedded HTML (PHP from http://www.php.net)

## Administrative Questions

#### 3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?

Specify the --prefix option when running configure.

#### 3.2) When I start postmaster, I get a Bad System Call or core dumped 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 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 using Unix domain sockets. Other machines will not be able to connect unless you add the -i flag to postmaster, and enable host-based authentication by modifying the file $PGDATA/pg_hba.conf accordingly. This will allow TCP/IP connections.

#### 3.6) How do I tune the database engine for better performance?

Certainly, indexes can speed up queries. The EXPLAIN command 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. You can disable fsync() by starting postmaster with a -o -F option. This will prevent fsync()s from flushing to disk after every transaction.

You can also use the postmaster -B 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 64 buffers.

You can also use the backend -S option to increase the maximum amount of memory used by the backend process for temporary sorts. The -S value is measured in kilobytes, and the default is 512 (i.e. 512K).

You can also use the CLUSTER command to group data in tables to match an index. See the CLUSTER manual page for more details.

#### 3.7) What debugging features are available?

PostgreSQL has several features that report status information that can be valuable for debugging purposes.

First, by running configure with the --enable-cassert option, many assert()s monitor the progress of the backend and halt the program when something unexpected occurs.

Both postmaster and postgres have several debug options available. First, whenever you start postmaster, make sure you send the standard output and error to a log file, like:

        cd /usr/local/pgsql
        ./bin/postmaster >server.log 2>&1 &

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. Postmaster has a -d option that allows even more detailed information to be reported. The -d option takes a number that specifies the debug level. Be warned that high debug level values generate large log files.

If postmaster is not running, you can actually run the postgres backend from the command line, and type your SQL statement directly. This is recommended only for debugging purposes. Note that a newline terminates the query, not a semicolon. If you have compiled with debugging symbols, you can use a debugger to see what is happening. Because the backend was not started from postmaster, it is not running in an identical environment and locking/backend interaction problems may not be duplicated.

If postmaster is running, start psql in one window, then find the PID of the postgres process used by psql. Use a debugger to attach to the postgres PID. You can set breakpoints in the debugger and issue queries from psql. If you are debugging postgres startup, you can set PGOPTIONS="-W n", then start psql. This will cause startup to delay for n seconds so you can attach to the process with the debugger, set any breakpoints, and continue through the startup sequence.

The postgres program has -s, -A, and -t options that can be very useful for debugging and performance measurements.

You can also compile with profiling to see what functions are taking execution time. The backend profile files will be deposited in the pgsql/data/base/dbname directory. The client profile file will be put in the client's current directory. Linux requires a compile with -DLINUX_PROFILE for proper profiling.

#### 3.8) Why do I get "Sorry, too many clients" when trying to connect?

You need to increase postmaster's limit on how many concurrent backend processes it can start.

The default limit is 32 processes. You can increase it by restarting 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.

In PostgreSQL versions prior to 6.5, the maximum number of backends was 64, and changing it required a rebuild after altering the MaxBackendId constant in include/storage/sinvaladt.h.

#### 3.9) What are the pg_sorttempNNN.NN files in my database directory?

They are temporary files generated by the query executor. For example, if a sort needs to be done to satisfy an ORDER BY, and the sort requires more space than the backend's -S parameter allows, then temporary files are created to hold the extra data.

The temporary files should be deleted automatically, but might not if a backend crashes during a sort. If you have no backends running at the time, it is safe to delete the pg_tempNNN.NN files.

#### 3.10) Why do I need to do a dump and restore to upgrade PostgreSQL?

The PostgreSQL team tries very heard to maintain compatability across minor releases. So upgrading from 7.2 to 7.2.1 does not require a dump a restore. However, new features are continuously being adding and sometimes this requires new fields to be added to system tables.

These changes may be across many tables and so maintaining backward compatability would be quite difficult. Thus, restoring from a dump is required to make everything work.

Note that the actual on-disk file format does not change very often, a feature the pg_upgrade script uses quite successfully. There the dump is used create the necessary information in the system tables. The data files are then just copied across. This method is not as guarenteed as the dump/restore method but when it works it can make upgrades very efficient.

## Operational Questions

#### 4.1) What is the difference between binary cursors and normal cursors?

See the DECLARE manual page for a description.

#### 4.2) How do I SELECT only the first few rows of a query?

See the FETCH manual page, or use SELECT ... LIMIT....

The entire query may have to be evaluated, even if you only want the first few rows. Consider a query that has an ORDER BY. If there 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.

#### 4.3) How do I get a list of tables or other things I can see in psql?

You can read the source code for psql in file pgsql/src/bin/psql/describe.c. It contains SQL commands that generate the output for psql's backslash commands. You can also start psql with the -E option so it will print out the queries it uses to execute the commands you give.

#### 4.4) How do you remove a column from a table?

We do not support ALTER TABLE DROP COLUMN, but 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;

#### 4.5) What is the maximum size for a row, a table, and a database?

These are the limits:

        Maximum size for a database?             unlimited (500 GB databases exist)
        Maximum size for a table?                16 TB
        Maximum size for a row?                  1.6TB
        Maximum size for a field?                1 GB
        Maximum number of rows in 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 disk space and memory/swap space. Performance may suffer when these values get unusually large.

The maximum table size of 16 TB does not require large file support from the operating system. Large tables are stored as multiple 1 GB files so file system size limits are not important.

The maximum table size and maximum number of columns can be increased if the default block size is increased to 32k.

#### 4.6) How much database disk space is required to store data from a typical text file?

A PostgreSQL database may require up to five times the disk space to store data from a text file.

As an example, consider a file of 100,000 lines with an integer and text description on each line. Suppose the text string avergages twenty bytes in length. The flat file would be 2.8 MB. The size of the PostgreSQL database file containing this data can be estimated as 6.4 MB:

        36 bytes: each row header (approximate)
        24 bytes: one int field and one text filed
       + 4 bytes: pointer on page to tuple
       ----------------------------------------
        64 bytes per row

       The data page size in PostgreSQL is 8192 bytes (8 KB), so:

       8192 bytes per page
       -------------------   =  128 rows per database page (rounded down)
         64 bytes per row

       100000 data rows
       --------------------  =  782 database pages (rounded up)
          128 rows per page

    782 database pages * 8192 bytes per page  =  6,406,144 bytes (6.4 MB)

Indexes do not require as much overhead, but do contain the data that is being indexed, so they can be large also.

NULLs are stored in bitmaps, so they use very little space.

#### 4.7) How do I find out what tables, indexes, databases, and users are defined?

psql has a variety of backslash commands to show such information. Use \? to see them. There are also system tables beginning with pg_ that describe these too. Also, psql -l will list all databases.

Also try the file pgsql/src/tutorial/syscat.source. It illustrates many of the SELECTs needed to get information from the database system tables.

#### 4.8) My queries are slow or don't make use of the indexes. Why?

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 only a small percentage of the rows in the table. This is because the random disk access caused by an index scan can be slower than a straight read through the table, or sequential scan.

To determine if an index should be used, PostgreSQL must have statistics about the table. These statistics are collected using VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer knows how many rows are in the table, and can better determine if indexes should be used. Statistics are also valuable in determining optimal join order and join methods. Statistics collection should be performed periodically as the contents of the table change.

Indexes are normally not used for ORDER BY or to perform joins. A sequential scan followed by an explicit sort is usually faster than an index scan of a large table.

However, LIMIT combined with ORDER BY often will use an index because only a small portion of the table is returned. In fact, though MAX() and MIN() don't use indexes, it is possible to retrieve such values using an index with ORDER BY and LIMIT:

        SELECT col
        FROM tab
        ORDER BY col [ DESC ]
        LIMIT 1

When using wild-card operators such as LIKE or ~, indexes can only be used in certain circumstances:

- The beginning of the search string must be anchored to the start of the string, i.e.:

- LIKE patterns must not start with %.
- ~ (regular expression) patterns must start with ^.

- The search string can not start with a character class, e.g. [a-e].
- Case-insensitive searches like ILIKE and ~* can not be used. Instead, use functional indexes, which are described later in this FAQ.
- The default C local must be used during initdb.

#### 4.9) How do I see how the query optimizer is evaluating my query?

See the EXPLAIN manual page.

#### 4.10) What is an R-tree index?

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.

#### 4.11) 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.

#### 4.12) How do I perform regular expression searches and case-insensitive regular expression searches? How do I use an index for case-insensitive searches?

The ~ operator does regular expression matching, and ~* does case-insensitive regular expression matching. The case-insensitive variant of LIKE is called ILIKE.

Case-insensitive equality comparisons are normally expressed as:

        SELECT *
        FROM tab
        WHERE lower(col) = 'abc'

This will not use an standard index. However, if you create a functional index, it will be used:

        CREATE INDEX tabindex on tab (lower(col));

#### 4.13) In a query, how do I detect if a field is NULL?

You test the column with IS NULL and IS NOT NULL.

#### 4.14) What is the difference between the various character types?

    Type            Internal Name   Notes
    --------------------------------------------------
    "char"          char            1 character
    CHAR(#)         bpchar          blank padded to the specified fixed length
    VARCHAR(#)      varchar         size specifies maximum length, no padding
    TEXT            text            no specific upper limit on length
    BYTEA           bytea           variable-length byte array (null-byte safe)

You will see the internal name when examining system catalogs and in some error messages.

The last four types above are "varlena" types (i.e., the first four bytes on disk are the length, followed by the data). Thus the actual space used is slightly greater than the declared size. However, these data types are also subject to compression or being stored out-of-line by TOAST, so the space on disk might also be less than expected.

CHAR() is best when storing strings that are usually the same length. VARCHAR() is best when storing variable-length strings but it limits how long a string can be. TEXT is for strings of unlimited length, maximum 1 gigabyte. BYTEA is for storing binary data, particularly values that include NULL bytes.

#### 4.15.1) How do I create a serial/auto-incrementing field?

PostgreSQL supports a SERIAL data type. It auto-creates a sequence and index on the column. For example, this:

        CREATE TABLE person (
            id   SERIAL,
            name TEXT
        );

is automatically translated into this:

        CREATE SEQUENCE person_id_seq;
        CREATE TABLE person (
            id   INT4 NOT NULL DEFAULT nextval('person_id_seq'),
            name TEXT
        );
        CREATE UNIQUE INDEX person_id_key ON person ( id );

See the create_sequence manual page for more information about sequences. You can also use each row's OID field as a unique value. 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.

#### 4.15.2) How do I get the value of a SERIAL insert?

One approach is to retrieve the next SERIAL value from the sequence object with the nextval() function before inserting and then insert it explicitly. Using the example table in 4.15.1, that might look like this in Perl:

        new_id = output of "SELECT nextval('person_id_seq')"
        INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');

You would then also have the new value stored in `new_id` for use in other queries (e.g., as a foreign key to the `person` table). Note that the name of the automatically created SEQUENCE object will be named <table>_<serialcolumn>_seq, where table and serialcolumn are the names of your table and your SERIAL column, respectively.

Alternatively, you could retrieve the assigned SERIAL value with the currval() function after it was inserted by default, e.g.,

        INSERT INTO person (name) VALUES ('Blaise Pascal');
        new_id = output of "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. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid value is made available via $sth->{pg_oid_status} after $sth->execute().

#### 4.15.3) Don't currval() and nextval() lead to a race condition with other users?

No. Currval() returns the current value assigned by your backend, not by all users.

#### 4.15.4) Why aren't my sequence numbers reused on transaction abort? Why are there gaps in the numbering of my sequence/SERIAL column?

To improve concurrency, sequence values are given out to running transactions as needed and are not locked until the transaction completes. This causes gaps in numbering from aborted transactions.

#### 4.16) What is an OID? What is a TID?

OIDs are PostgreSQL's answer to unique row ids. Every row that is created in PostgreSQL gets a unique OID. All OIDs generated during initdb are less than 16384 (from backend/access/transam.h). All user-created OIDs are equal to or greater than this. By default, all these OIDs are unique not only within a table or database, but unique within the entire PostgreSQL installation.

PostgreSQL uses OIDs in its internal system tables to link rows between tables. These OIDs can be used to identify specific user rows and used in joins. It is recommended you use column type OID to store OID values. You can create an index on the OID field for faster 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(old_oid oid, mycol int);
            SELECT old_oid, mycol INTO new FROM old;
            COPY new TO '/tmp/pgtable';
            DELETE FROM new;
            COPY new WITH OIDS FROM '/tmp/pgtable';

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 values. TIDs change after rows are modified or reloaded. They are used by index entries to point to physical rows.

#### 4.17) What is the meaning of some of the terms used in PostgreSQL?

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://www.comptechnews.com/~reaster/dbdesign.html

#### 4.18) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?

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 postmaster:

        ulimit -d 262144
        limit datasize 256m

Depending on your shell, only one of these may succeed, but it will set your process data segment limit much higher and perhaps allow the query to complete. This command applies to the current process, and all subprocesses created after the command is run. If you are having a problem with the SQL client because the backend is returning too much data, try it before starting the client.

#### 4.19) How do I tell what PostgreSQL version I am running?

From psql, type `select version();`

#### 4.20) Why does my large-object operations get "invalid large obj descriptor"?

You need to put `BEGIN WORK` and `COMMIT` around any use of a large object handle, that is, surrounding `lo_open` ... `lo_close.`

Currently PostgreSQL enforces the rule by closing large object handles at transaction commit. So the first attempt to do anything with the handle will draw invalid large obj descriptor. So code that used to work (at least most of the time) will now generate that error message if you fail to use a transaction.

If you are using a client interface like ODBC you may need to set `auto-commit off.`

#### 4.21) How do I create a column that will default to the current time?

Use CURRENT_TIMESTAMP:

    CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );

#### 4.22) Why are my subqueries using `IN` so slow?

Currently, we join subqueries to outer queries by sequentially scanning the result of the subquery for each row of the outer query. A workaround is to replace `IN` with `EXISTS`:

    SELECT *
        FROM tab
        WHERE col1 IN (SELECT col2 FROM TAB2)

to:

    SELECT *
        FROM tab
        WHERE EXISTS (SELECT col2 FROM TAB2 WHERE col1 = col2)

We hope to fix this limitation in a future release.

#### 4.23) How do I perform an outer join?

PostgreSQL supports outer joins using the SQL standard syntax. Here are two examples:

        SELECT *
        FROM t1 LEFT OUTER JOIN t2 ON (t1.col = t2.col);

or

        SELECT *
        FROM t1 LEFT OUTER JOIN t2 USING (col);

These identical queries join t1.col to t2.col, and also return any unjoined rows in t1 (those with no match in t2). A RIGHT join would add unjoined rows of t2. A FULL join would return the matched rows plus all unjoined rows from t1 and t2. The word OUTER is optional and is assumed in LEFT, RIGHT, and FULL joins. Ordinary joins are called INNER joins.

In previous releases, outer joins can be simulated using UNION and NOT 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

#### 4.24) How do I perform queries using multiple databases?

There is no way to query any database except the current one. Because PostgreSQL loads database-specific system catalogs, it is uncertain how a cross-database query should even behave.

Of course, a client can make simultaneous connections to different databases and merge the information that way.

#### 4.25) How do I return multiple rows or columns from a function?

You can return result sets from PL/pgSQL functions using refcursors. See http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html, section 23.7.3.3.

#### 4.26) Why can't I reliably create/drop temporary tables in PL/PgSQL functions?

PL/PgSQL caches function contents, and an unfortunate side effect is that if a PL/PgSQL function accesses a temporary table, and that table is later dropped and recreated, and the function called again, the function will fail because the cached function contents still point to the old temporary table. The solution is to use EXECUTE for temporary table access in PL/PgSQL. This will cause the query to be reparsed every time.

## Extending PostgreSQL

#### 5.1) I wrote a user-defined function. When I run it in psql, why does it dump core?

The problem could be a number of things. Try testing your user-defined function in a stand-alone test program first.

#### 5.2) How can I contribute some nifty new types and functions to PostgreSQL?

Send your extensions to the pgsql-hackers mailing list, and they will eventually end up in the contrib/ subdirectory.

#### 5.3) How do I write a C function to return a tuple?

This requires wizardry so extreme that the authors have never tried it, though in principle it can be done.

#### 5.4) I have changed a source file. Why does the recompile not see the change?

The Makefiles do not have the proper dependencies for include files. You have to do a make clean and then another make. If you are using GCC you can use the --enable-depend option of configure to have the compiler compute the dependencies automatically.