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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Fri Oct 26 14:59:45 EDT 2007
Current maintainer: Bruce Momjian (bruce@momjian.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 most recent 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 submit a patch or join the development team?
1.13) How does PostgreSQL compare to other DBMSs?
1.14) Will PostgreSQL handle recent daylight saving time changes in
various countries?
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 the upgrade process for PostgreSQL?
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 do I
concatenate possible NULLs? 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?
Why is capitalization not preserved?
_________________________________________________________________
General Questions
1.1) What is PostgreSQL? How is it pronounced?
PostgreSQL is pronounced Post-Gres-Q-L, but can also be referred to as
simply Postgres, particularly in conversation. (For those curious
about how to say "PostgreSQL", an audio file is available.)
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/docs/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) 1996-2007, 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 SP4, 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.
1.5) 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 most recent release?
The latest release of PostgreSQL is version 8.2.5.
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), a French one,
(#postgresqlfr), and a Brazilian one, (#postgresql-br). There is also
a PostgreSQL channel on EFNet.
A list of commercial support companies is available at
http://www.postgresql.org/support/professional_support.
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://www.postgresql.org/docs/books/. There is also a collection of
PostgreSQL technical articles at
http://www.postgresql.org/docs/techdocs.
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. Many of
our users also like The Practical SQL Handbook, Bowman, Judith S., et
al., Addison-Wesley. Others like The Complete Reference SQL, Groff et
al., McGraw-Hill.
There are also many nice tutorials available online:
* http://www.intermedia.net/support/sql/sqltut.shtm
* http://sqlcourse.com
* http://www.w3schools.com/sql/default.asp
* http://mysite.verizon.net/Graeme_Birchall/id1.html
1.12) How do I submit a patch or 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.
1.14) Will PostgreSQL handle recent daylight saving time changes in various
countries?
USA daylight saving time changes are included in PostgreSQL release
8.0.[4+], and all later major releases, e.g. 8.1. Canada and Western
Australia changes are included in 8.0.[10+], 8.1.[6+], and all later
major releases. PostgreSQL releases prior to 8.0 use the operating
system's timezone database for daylight saving information.
_________________________________________________________________
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?
There are a large number of GUI Tools that are available for
PostgreSQL from both commercial and open source developers. A detailed
list can be found in the PostgreSQL Community Documentation
_________________________________________________________________
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://www.powerpostgresql.com/PerfList/ and
http://momjian.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.
3.5) Why do I get "Sorry, too many clients" when trying to connect?
You have reached the default limit of 100 database sessions. You need
to increase the server's limit on how many concurrent backend
processes it can start by changing the max_connections value in
postgresql.conf and restarting the server.
3.6) What is the upgrade process for PostgreSQL?
See http://www.postgresql.org/support/versioning for a general
discussion about upgrading, and
http://www.postgresql.org/docs/current/static/install-upgrading.html
for specific instructions.
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? 400 GB
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, row size, and maximum number of columns can be
quadrupled by increasing the default block size to 32k. The maximum
table size can also be increased using table partitioning.
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 function 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 5.2
MB:
24 bytes: each row header (approximate)
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
52 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
------------------- = 158 rows per database page (rounded down)
52 bytes per row
100000 data rows
-------------------- = 633 database pages (rounded up)
158 rows per page
633 database pages * 8192 bytes per page = 5,185,536 bytes (5.2 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. It is also possible to use full
text indexing for word searches.
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 an
expression 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 do I concatenate
possible NULLs? 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 concatentate with possible NULLs, use COALESCE(), like this:
SELECT COALESCE(col1, '') || COALESCE(col2, '')
FROM tab
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
);
Automatically created sequence are named <table>_<serialcolumn>_seq,
where table and serialcolumn are the names of the table and SERIAL
column, respectively. See the create_sequence manual page for more
information about sequences.
4.11.2) How do I get the value of a SERIAL insert?
The simplest way is to retrieve the assigned SERIAL value with
RETURNING. Using the example table in 4.11.1, it would look like this:
INSERT INTO person (name) VALUES ('Blaise Pascal') RETURNING id;
You can also call nextval() and use that value in the INSERT, or call
currval() after the INSERT.
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?
If a table is created WITH OIDS, each row gets a unique a OID. OIDs
are automatically 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 rows 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
the server:
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://www.postgresql.org/docs/techdocs.17.
4.19) Why do I get "relation with OID ##### does not exist" errors when
accessing temporary tables in PL/PgSQL functions?
In PostgreSQL versions < 8.3, 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.
This problem does not occur in PostgreSQL 8.3 and later.
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? Why is
capitalization not preserved?
The most common cause of unrecognized names 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
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