# Frequently Asked Questions (FAQ) for PostgreSQL

Last updated: Fri Feb 24 23:28:40 EST 2006

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) Who controls PostgreSQL?
1.3) What is the copyright of PostgreSQL?
1.4) What platforms does PostgreSQL support?
1.5) Where can I get PostgreSQL?
1.6) What is the latest release?
1.7) Where can I get support?
1.8) How do I submit a bug report?
1.9) How do I find out about known bugs or missing features?
1.10) What documentation is available?
1.11) How can I learn SQL?
1.12) How do I join the development team?
1.13) 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) Why do I need to do a dump and restore to upgrade PostgreSQL releases?
3.7) 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? How can I sort on whether a field is NULL or not?
4.10) What is the difference between the various character types?
4.11.1) How do I create a serial/auto-incrementing field?
4.11.2) How do I get the value of a SERIAL insert?
4.11.3) Doesn't currval() lead to a race condition with other users?
4.11.4) Why aren't my sequence numbers reused on transaction abort? Why are there gaps in the numbering of my sequence/SERIAL column?
4.12) What is an OID? What is a CTID?
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) How do I create a column that will default to the current time?
4.16) How do I perform an outer join?
4.17) How do I perform queries using multiple databases?
4.18) How do I return multiple rows or columns from a function?
4.19) Why do I get "relation with OID ##### does not exist" errors when accessing temporary tables in PL/PgSQL functions?
4.20) What replication solutions are available?
4.21) Why are my table and column names not recognized in my query?

## General Questions

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

PostgreSQL is pronounced Post-Gres-Q-L, and is also sometimes referred to as just Postgres. An audio file is available in MP3 format for those would like to hear the pronunciation.

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) Who controls PostgreSQL?

If you are looking for a PostgreSQL gatekeeper, central committee, or controlling company, give up --- there isn't one. We do have a core committee and CVS committers, but these groups are more for administrative purposes than control. The project is directed by the community of developers and users, which anyone can join. All you need to do is subscribe to the mailing lists and participate in the discussions. (See the Developer's FAQ for information on how to get involved in PostgreSQL development.)

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

PostgreSQL is distributed under the classic BSD license. Basically, it allows users to do anything they want with the code, including reselling binaries without the source code. The only restriction is that you not hold us legally liable for problems with the software. There is also the requirement that this copyright appear in all copies of the software. Here is the actual BSD license we use:

PostgreSQL Data Base Management System

Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group Portions Copyright (c) 1994-1996 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.

### 1.4) 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.

PostgreSQL also 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.

### 1.5) Where can I get PostgreSQL?

Via web browser, use http://www.postgresql.org/ftp/, and via ftp, use ftp://ftp.PostgreSQL.org/pub/.

### 1.6) What is the latest release?

The latest release of PostgreSQL is version 8.1.3.

We plan to have a major release every year, with minor releases every few months.

### 1.7) 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 other 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.postgresql.org/companies.php.

### 1.8) 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.

Bugs submitted using the bug form or posted to any PostgreSQL mailing list typically generates one of the following replies:

- It is not a bug, and why
- It is a known bug and is already on the TODO list
- The bug has been fixed in the current release
- The bug has been fixed but is not packaged yet in an official release
- A request is made for more detailed information:
  - Operating system
  - PostgreSQL version
  - Reproducible test case
  - Debugging information
  - Debugger backtrace output
- The bug is new. The following might happen:
  - A patch is created and will be included in the next major or minor release
  - The bug cannot be fixed immediately and is added to the TODO list

### 1.9) How do I find out about known bugs or missing features?

PostgreSQL supports an extended subset of SQL:2003. See our TODO list for known bugs, missing features, and future plans.

A feature request usually results in one of the following replies:

- The feature is already on the TODO list
- The feature is not desired because:
  - It duplicates existing functionality that already follows the SQL standard
  - The feature would increase code complexity but add little benefit
  - The feature would be insecure or unreliable
- The new feature is added to the TODO list

PostgreSQL does not use a bug tracking system because we find it more efficient to respond directly to email and keep the TODO list up-to-date. In practice, bugs don't last very long in the software, and bugs that affect a large number of users are fixed rapidly. The only place to find all changes, improvements, and fixes in a PostgreSQL release is to read the CVS log messages. Even the release notes do not list every change made to the software.

### 1.10) 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/books/awbook.html and http://www.commandprompt.com/ppbook/. There are a number of PostgreSQL books available for purchase. One of the most popular ones is by Korry Douglas. A list of book reviews can be found at http://techdocs.PostgreSQL.org/techdocs/bookreviews.php. There is also 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.

### 1.11) How can I learn SQL?

First, consider the PostgreSQL-specific books mentioned above. 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.

There is also 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.

### 1.12) How do I join the development team?

See the Developer's FAQ.

### 1.13) 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. Our performance is usually +/-10% compared to other databases.
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 1.7.)
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 DBD::Pg with CGI.pm or mod_perl.

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

Yes, see http://techdocs.postgresql.org/guides/GUITools for a 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 and http://www.powerpostgresql.com/PerfList/.

### 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.

### 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) 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 7.4.0 to 7.4.1 does not require a dump and restore. However, major releases (e.g. from 7.3 to 7.4) often change the internal 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.

### 3.7) 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.

One limitation is that indexes can not be created on columns longer than about 2,000 characters. Fortunately, such indexes are rarely needed. Uniqueness is best guaranteed by a funtion index of an MD5 hash of the long column, and full text indexing allows for searching of words within the column.

### 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 5.6 MB:

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

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

       8192 bytes per page
       -------------------   =  146 rows per database page (rounded down)
         56 bytes per row

       100000 data rows
       --------------------  =  685 database pages (rounded up)
          146 rows per page

    685 database pages * 8192 bytes per page  =  5,611,520 bytes (5.6 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 used by every query. Indexes are used only 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.

If you believe the optimizer is incorrect in choosing a sequential scan, use `SET enable_seqscan TO 'off'` and run query again 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));

If the above index is created as UNIQUE, though the column can store upper and lowercase characters, it can not have identical values that differ only in case. To force a particular case to be stored in the column, use a CHECK constraint or a trigger.

### 4.9) In a query, how do I detect if a field is NULL? How can I sort on whether a field is NULL or not?

You test the column with IS NULL and IS NOT NULL, like this:

       SELECT *
       FROM tab
       WHERE col IS NULL;

To sort by the NULL status, use the IS NULL and IS NOT NULL modifiers in your ORDER BY clause. Things that are true will sort higher than things that are false, so the following will put NULL entries at the top of the resulting list:

       SELECT *
       FROM tab
       ORDER BY (col 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, long values are also subject to compression, 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 CTID?

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.

CTIDs are used to identify specific physical rows with block and offset values. CTIDs 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) 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 );

### 4.16) 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.

### 4.17) 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.

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

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

.

### 4.19) Why do I get "relation with OID ##### does not exist" errors when accessing 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.

### 4.20) What replication solutions are available?

Though "replication" is a single term, there are several technologies for doing replication, with advantages and disadvantages for each.

Master/slave replication allows a single master to receive read/write queries, while slaves can only accept read/SELECT queries. The most popular freely available master-slave PostgreSQL replication solution is Slony-I.

Multi-master replication allows read/write queries to be sent to multiple replicated computers. This capability also has a severe impact on performance due to the need to synchronize changes between servers. Pgcluster is the most popular such solution freely available for PostgreSQL.

There are also commercial and hardware-based replication solutions available supporting a variety of replication models.

### 4.21) Why are my table and column names not recognized in my query?

The most common cause is the use of double-quotes around table or column names during table creation. When double-quotes are used, table and column names (called identifiers) are stored case-sensitive, meaning you must use double-quotes when referencing the names in a query. Some interfaces, like pgAdmin, automatically double-quote identifiers during table creation. So, for identifiers to be recognized, you must either:

- Avoid double-quoting identifiers when creating tables
- Use only lowercase characters in identifiers
- Double-quote identifiers when referencing them in queries