# Frequently Asked Questions (FAQ) for PostgreSQL

Last updated: Mon Jan 31 21:40:28 EST 2005

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

The most recent version of this document can be viewed at http://www.postgresql.org/files/documentation/faqs/FAQ.html.

Platform-specific questions are answered at http://www.postgresql.org/docs/faq/.

## General Questions

1.1) What is PostgreSQL? How is it pronounced?
1.2) What is the copyright of PostgreSQL?
1.3) What platforms does PostgreSQL support?
1.4) Where can I get PostgreSQL?
1.5) Where can I get support?
1.6) How do I submit a bug report?
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) How do I join the development team?
1.12) How does PostgreSQL compare to other DBMSs?

## User Client Questions

2.1) What interfaces are available for PostgreSQL?
2.2) What tools are available for using PostgreSQL with Web pages?
2.3) Does PostgreSQL have a graphical user interface?

## Administrative Questions

3.1) How do I install PostgreSQL somewhere other than /usr/local/pgsql?
3.2) How do I control connections from other hosts?
3.3) How do I tune the database engine for better performance?
3.4) What debugging features are available?
3.5) Why do I get "Sorry, too many clients" when trying to connect?
3.6) What is in the pgsql_tmp directory?
3.7) Why do I need to do a dump and restore to upgrade PostgreSQL releases?
3.8) What computer hardware should I use?

## Operational Questions

4.1) How do I SELECT only the first few rows of a query? A random row?
4.2) How do I find out what tables, indexes, databases, and users are defined? How do I see the queries used by psql to display them?
4.3) How do you change a column's data type?
4.4) What is the maximum size for a row, a table, and a database?
4.5) How much database disk space is required to store data from a typical text file?
4.6) Why are my queries slow? Why don't they use my indexes?
4.7) How do I see how the query optimizer is evaluating my query?
4.8) How do I perform regular expression searches and case-insensitive regular expression searches? How do I use an index for case-insensitive searches?
4.9) In a query, how do I detect if a field is NULL?
4.10) What is the difference between the various character types?
4.11.0) How do I create a serial/auto-incrementing field?
4.11.1) How do I get the value of a SERIAL insert?
4.11.2) Doesn't currval() lead to a race condition with other users?
4.11.3) Why aren't my sequence numbers reused on transaction abort? Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a TID?
4.13) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
4.14) How do I tell what PostgreSQL version I am running?
4.15) Why does my large-object operations get "invalid large obj descriptor"?
4.16) How do I create a column that will default to the current time?
4.17) How do I perform an outer join?
4.18) How do I perform queries using multiple databases?
4.19) How do I return multiple rows or columns from a function?
4.20) Why can't I reliably create/drop temporary tables in PL/PgSQL functions?
4.21) What encryption options are available?

## Extending PostgreSQL

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

## General Questions

#### 1.1) What is PostgreSQL? How is it pronounced?

PostgreSQL is pronounced Post-Gres-Q-L, also called just Postgres.

PostgreSQL is an object-relational database system that has the features of traditional commercial database systems with enhancements to be found in next-generation DBMS systems. PostgreSQL is free and the complete source code is available.

PostgreSQL development is performed by a team of mostly volunteer developers spread throughout the world and communicating via the Internet. It is a community project and is not controlled by any company. To get involved, see the developer's FAQ at http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html

#### 1.2) What is the copyright of PostgreSQL?

PostgreSQL is distributed under the classic BSD license. It has no restrictions on how the source code can be used. We like it and have no intention of changing it.

This is the BSD license we use:

PostgreSQL Data Base Management System

Portions Copyright (c) 1996-2005, 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.

#### 1.3) What platforms does PostgreSQL support?

In general, any 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.

Starting with version 8.0, PostgreSQL now runs natively on Microsoft Windows NT-based operating systems like Win2000, WinXP, and Win2003. A prepackaged installer is available at http://pgfoundry.org/projects/pginstaller. MSDOS-based versions of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.

There is also a Novell Netware 6 port at http://forge.novell.com, and an OS/2 (eComStation) version at http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&button=Search&key=postgreSQL&stype=all&sort=type&dir=%2F.

#### 1.4) 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.

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

The PostgreSQL community provides assistance to many of its users via email. The main web site to subscribe to the email lists is http://www.postgresql.org/community/lists/. The general or bugs lists are a good place to start.

The major IRC channel is #postgresql on Freenode (irc.freenode.net). To connect you can use the Unix program `irc -c '#postgresql' "$USER" irc.freenode.net` or use any of the other popular IRC clients. A Spanish one also exists on the same network, (#postgresql-es), and a French one, (#postgresqlfr). There is also a PostgreSQL channel on EFNet.

A list of commercial support companies is available at http://techdocs.postg resql.org/companies.php.

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

Visit the PostgreSQL bug form at http://www.postgresql.org/support/submitbug.

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

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

The latest release of PostgreSQL is version 8.0.0.

We plan to have major releases every ten to twelve months.

#### 1.8) What documentation is available?

PostgreSQL includes extensive documentation, including a large manual, manual pages, and some test examples. See the /doc directory. You can also browse the manuals online at http://www.PostgreSQL.org/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://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also a collection of PostgreSQL technical articles at http://techdocs.PostgreSQL.org/.

The command line client program psql has some \d commands to show information about types, operators, functions, aggregates, etc. - use \? to display the available commands.

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) How do I join the development team?

First, download the latest source and read the PostgreSQL Developers FAQ and documentation on our web site, or in the distribution. Second, subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, submit high quality patches to pgsql-patches.

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

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

There are several ways of measuring software: features, performance, 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's performance is comparable 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 faster for multiple users, complex queries, and a read/write query load. MySQL is faster for simple SELECT queries done by a few users. Of course, MySQL does not have most of the features mentioned in the Features section above. We are built for reliability and features, and we continue to improve performance in every release.
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 lists provide contact with a large group of developers and users to help resolve any problems encountered. While we cannot 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 FAQ section 1.5.)
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.

## User Client Questions

#### 2.1) What interfaces are available for PostgreSQL?

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.

Some programming languages like PHP include an interface to PostgreSQL. Interfaces for languages like Perl, TCL, Python, and many others are available at http://gborg.postgresql.org in the Drivers/Interfaces section and via Internet search.

#### 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

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

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

#### 2.3) Does PostgreSQL have a graphical user interface?

Yes, there are several graphical interfaces to PostgreSQL available. These include pgAdmin III (http://www.pgadmin.org, PgAccess http://www.pgaccess.org), RHDB Admin (http://sources.redhat.com/rhd b/ ), TORA (http://www.globecom.net/tora/, partly commercial), and Rekall ( http://www.rekallrevealed.org/). There is also PhpPgAdmin ( http://phppgadmin.sourceforge.net/ ), a web-based interface to PostgreSQL.

See http://techdocs.postgresql.org/guides/GUITools for a more detailed list.

## Administrative Questions

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

Specify the --prefix option when running configure.

#### 3.2) How do I control connections from other hosts?

By default, PostgreSQL only allows connections from the local machine using Unix domain sockets or TCP/IP connections. Other machines will not be able to connect unless you modify listen_addresses in the postgresql.conf file, enable host-based authentication by modifying the $PGDATA/pg_hba.conf file, and restart the server.

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

There are three major areas for potential performance improvement:

Query Changes
  This involves modifying queries to obtain better performance:
  - Creation of indexes, including expression and partial indexes
  - Use of COPY instead of multiple INSERTs
  - Grouping of multiple statements into a single transaction to reduce commit overhead
  - Use of CLUSTER when retrieving many rows from an index
  - Use of LIMIT for returning a subset of a query's output
  - Use of Prepared queries
  - Use of ANALYZE to maintain accurate optimizer statistics
  - Regular use of VACUUM or pg_autovacuum
  - Dropping of indexes during large data changes
Server Configuration
  A number of postgresql.conf settings affect performance. For more details, see Administration Guide/Server Run-time Environment/Run-time Configuration for a full listing, and for commentary see http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html and http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
Hardware Selection
  The effect of hardware on performance is detailed in http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html.

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

There are many `log_*` server configuration variables that enable printing of query and process statistics which can be very useful for debugging and performance measurements.

The following detailed debug instructions are to be used to provide more detailed information for server developers debugging a problem.

It is also possible to debug the server if it isn't operating properly. First, by running configure with the --enable-cassert option, many assert()s monitor the progress of the backend and halt the program when something unexpected occurs.

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

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

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

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

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

You have reached the default limit is 100 database sessions. You need to increase the postmaster's limit on how many concurrent backend processes it can start by changing the max_connections value in postgresql.conf and restarting the postmaster.

#### 3.6) What is in the pgsql_tmp directory?

This directory contains 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 here to hold the extra data.

The temporary files are usually deleted automatically, but might remain if a backend crashes during a sort. A stop and restart of the postmaster will remove files from those directories.

#### 3.7) Why do I need to do a dump and restore to upgrade between major PostgreSQL releases?

The PostgreSQL team makes only small changes between minor releases, so upgrading from 7.4 to 7.4.1 does not require a dump and restore. However, major releases (e.g. from 7.3 to 7.4) often change the internal format of system tables and data files. These changes are often complex, so we don't maintain backward compatibility for data files. A dump outputs data in a generic format that can then be loaded in using the new internal format.

#### 3.8) What computer hardware should I use?

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 quality motherboards are more reliable and have better performance than less expensive hardware. PostgreSQL will run on almost any hardware, but if reliability and performance are important it is wise to research your hardware options thoroughly. Our email lists can be used to discuss hardware options and tradeoffs.

## Operational Questions

#### 4.1) How do I SELECT only the first few rows of a query? A random row?

To retrieve only a few rows, if you know at the number of rows needed at the time of the SELECT use LIMIT . If an index matches the ORDER BY it is possible the entire query does not have to be executed. If you don't know the number of rows at SELECT time, use a cursor and FETCH.

To SELECT a random row, use:

        SELECT col
        FROM tab
        ORDER BY random()
        LIMIT 1;

#### 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?

Use the \dt command to see tables in psql. For a complete list of commands inside psql you can use \?. Alternatively you can read the 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. PostgreSQL also provides an SQL compliant INFORMATION SCHEMA interface you can query to get information about the database.

There are also system tables beginning with pg_ that describe these too.

Use 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.

#### 4.3) How do you change a column's data type?

Changing the data type of a column can be done easily in 8.0 and later with ALTER TABLE ALTER COLUMN TYPE.

In earlier releases, do this:

        BEGIN;
        ALTER TABLE tab ADD COLUMN new_col new_data_type;
        UPDATE tab SET new_col = CAST(old_col AS new_data_type);
        ALTER TABLE tab DROP COLUMN old_col;
        COMMIT;

You might then want to do VACUUM FULL tab to reclaim the disk space used by the expired rows.

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

These are the limits:

Maximum size for a database? | unlimited (32 TB databases exist)
Maximum size for a table? | 32 TB
Maximum size for a 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 32 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 quadrupled by increasing the default block size to 32k.

#### 4.5) 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:

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

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

       8192 bytes per page
       -------------------   =  136 rows per database page (rounded down)
         60 bytes per row

       100000 data rows
       --------------------  =  735 database pages (rounded up)
          128 rows per page

    735 database pages * 8192 bytes per page  =  6,021,120 bytes (6 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 as bitmaps, so they use very little space.

#### 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 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;

If you believe the optimizer is incorrect in choosing a sequential scan, use `SET enable_seqscan TO 'off'` and run tests to see if an index scan is indeed faster.

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 such as ILIKE and ~* do not utilize indexes. Instead, use expression indexes, which are described in section 4.8.
- The default C locale must be used during initdb because it is not possible to know the next-greatest character in a non-C locale. You can create a special `text_pattern_ops` index for such cases that work only for LIKE indexing.

In pre-8.0 releases, indexes often can not be used unless the data types exactly match the index's column types. This was particularly true of int2, int8, and numeric column indexes.

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

See the EXPLAIN manual page.

#### 4.8) 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 expresssion index, it will be used:

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

#### 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.

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

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

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

The first four types above are "varlena" types (i.e., the first four bytes on disk are the length, followed by the data). Thus the actual 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.

VARCHAR(n) is best when storing variable-length strings and it limits how long a string can be. TEXT is for strings of unlimited length, with a maximum of one gigabyte.

CHAR(n) is for storing strings that are all the same length. CHAR(n) pads with blanks to the specified length, while VARCHAR(n) only stores the characters supplied. BYTEA is for storing binary data, particularly values that include NULL bytes. All the types described here have similar performance characteristics.

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

PostgreSQL supports a SERIAL data type. It auto-creates a sequence. 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
        );

See the create_sequence manual page for more information about sequences.

#### 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 object with the nextval() function before inserting and then insert it explicitly. Using the example table in 4.11.1, an example in a pseudo-language would look like this:

        new_id = execute("SELECT nextval('person_id_seq')");
        execute("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.,

        execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
        new_id = execute("SELECT currval('person_id_seq')");

#### 4.11.3) Doesn't currval() lead to a race condition with other users?

No. currval() returns the current value assigned by your session, not by all sessions.

#### 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?

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.

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

Every row that is created in PostgreSQL gets a unique OID unless created WITHOUT OIDS. OIDs are autotomatically assigned unique 4-byte integers that are unique across the entire installation. However, they overflow at 4 billion, and then the OIDs start being duplicated. PostgreSQL uses OIDs to link its internal system tables together.

To uniquely number columns in user tables, it is best to use SERIAL rather than OIDs because SERIAL sequences are unique only within a single table. and are therefore less likely to overflow. SERIAL8 is available for storing eight-byte sequence values.

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.

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

You probably have run out of virtual memory on your system, or your 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.

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

From psql, type `SELECT version();`

#### 4.15) 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.`

#### 4.16) 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 );

#### 4.17) 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.

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

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

contrib/dblink allows cross-database queries using function calls. Of course, a client can also make simultaneous connections to different databases and merge the results on the client side.

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

It is easy using set-returning functions, http://techdocs.postgresql.org/guides/SetReturningFunctions.

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

PL/PgSQL caches function scripts, 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.

#### 4.21) What encryption options are available?

- contrib/pgcrypto contains many encryption functions for use in SQL queries.
- To encrypt transmission from the client to the server, the server must have the ssl option set to true in postgresql.conf, and an applicable host or hostssl record must exist in pg_hba.conf, and the client sslmode must not be disable. (Note that it is also possible to use a third-party encrypted transport, such as stunnel or ssh, rather than PostgreSQL's native SSL connections.)
- Database user passwords are automatically encrypted when stored in the system tables.
- The server can run using an encrypted file system.

## Extending PostgreSQL

#### 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?

In versions of PostgreSQL beginning with 7.3, table-returning functions are fully supported in C, PL/PgSQL, and SQL. See the Programmer's Guide for more information. An example of a table-returning function defined in C can be found in contrib/tablefunc.

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

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