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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Mon Jan 31 17:57:02 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 remove a column from a table, or change its 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) What is the meaning of some of the terms used in PostgreSQL?
4.14) Why do I get the error "ERROR: Memory exhausted in
AllocSetAlloc()"?
4.15) How do I tell what PostgreSQL version I am running?
4.16) Why does my large-object operations get "invalid large obj
descriptor"?
4.17) How do I create a column that will default to the current time?
4.18) How do I perform an outer join?
4.19) How do I perform queries using multiple databases?
4.20) How do I return multiple rows or columns from a function?
4.21) Why can't I reliably create/drop temporary tables in PL/PgSQL
functions?
4.22) 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=postgre
SQL&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_co
nf_e.html and
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html.
Hardware Selection
The effect of hardware on performance is detailed in
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/inde
x.html.
3.4) What debugging features are available?
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 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.
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 7.2 to 7.2.1 does not require a dump and restore.
However, major releases (e.g. from 7.2 to 7.3) often change the
internal format of system tables and data files. These changes are
often complex, so we don't maintain backward compatability for data
files. A dump outputs data in a generic format that can then be loaded
in using the new internal format.
In releases where the on-disk format does not change, the pg_upgrade
script can be used to upgrade without a dump/restore. The release
notes mention whether pg_upgrade is available for the release.
3.8) What computer hardware should I use?
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?
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 using 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.
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 remove a column from a table, or change its data type?
DROP COLUMN functionality was added in release 7.3 with ALTER TABLE
DROP COLUMN. In earlier versions, you can do this:
BEGIN;
LOCK TABLE old_table;
SELECT ... -- select all columns but the one you want to remove
INTO TABLE new_table
FROM old_table;
DROP TABLE old_table;
ALTER TABLE new_table RENAME TO old_table;
COMMIT;
Changing the data type of a column can be done easily in 8.0 and later
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 functional indexes, which are described in
section 4.10.
* The default C locale must be used during initdb because it is not
possible to know the next-greater character in a non-C locale. You
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 is 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
functional 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. 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.
4.11.2) 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')");
Finally, you could use the OID returned from the INSERT statement to
look up the default value, though this is probably the least portable
approach, and the oid value will wrap around when it reaches 4
billion. In Perl, using DBI with the DBD::Pg module, the oid value is
made available via $sth->{pg_oid_status} after $sth->execute().
4.11.3) Doesn't currval() lead to a race condition with other users?
No. currval() returns the current value assigned by your backend, not
by all 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?
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) 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://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary
/glossary.html
4.14) Why do I get the error "ERROR: Memory exhausted in AllocSetAlloc()"?
You probably have run out of virtual memory on your system, or your
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.
4.15) How do I tell what PostgreSQL version I am running?
From psql, type SELECT version();
4.16) 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.
4.17) 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 );
4.18) 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
4.19) 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 make simultaneous connections to different
databases and merge the results on the client side.
4.20) How do I return multiple rows or columns from a function?
In 7.3, you can easily return multiple rows or columns from a
function, http://techdocs.postgresql.org/guides/SetReturningFunctions.
4.21) 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.
4.22) 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
version 7.3. In previous versions, you must enable the option
PASSWORD_ENCRYPTION in postgresql.conf.
* 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.
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