database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-09-02 04:21:28 +03:00
Code Activity

Backpatch FAQ changes to 8.0.X.

Browse Source
This commit is contained in:
Bruce Momjian
2005-02-01 02:42:03 +00:00
parent b46fa4ba81
commit 925320fe3d
2 changed files with 385 additions and 691 deletions
Download Patch File Download Diff File Expand all files Collapse all files

609
doc/src/FAQ/FAQ.html
View File

@@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Jan 29 23:44:48 EST 2005</P>
<P>Last updated: Mon Jan 31 21:40:28 EST 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
@@ -27,21 +27,19 @@
<H2 align="center">General Questions</H2>
<A href="#1.1">1.1</A>) What is PostgreSQL? How is it pronounced?<BR>
<A href="#1.2">1.2</A>) What is the copyright on PostgreSQL?<BR>
<A href="#1.2">1.2</A>) What is the copyright of PostgreSQL?<BR>
<A href="#1.3">1.3</A>) What platforms does PostgreSQL support?<BR>
<A href="#1.4">1.4</A>) Where can I get PostgreSQL?<BR>
<A href="#1.5">1.5</A>) Where can I get support?<BR>
<A href="#1.6">1.6</A>) What is the latest release?<BR>
<A href="#1.7">1.7</A>) What documentation is available?<BR>
<A href="#1.8">1.8</A>) How do I find out about known bugs or
<A href="#1.6">1.6</A>) How do I submit a bug report?<BR>
<A href="#1.7">1.7</A>) What is the latest release?<BR>
<A href="#1.8">1.8</A>) What documentation is available?<BR>
<A href="#1.9">1.9</A>) How do I find out about known bugs or
missing features?<BR>
<A href="#1.9">1.9</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.10">1.10</A>) How do I join the development team?<BR>
<A href="#1.11">1.11</A>) How do I submit a bug report?<BR>
<A href="#1.10">1.10</A>) How can I learn <SMALL>SQL</SMALL>?<BR>
<A href="#1.11">1.11</A>) How do I join the development team?<BR>
<A href="#1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?<BR>
<A href="#1.13">1.13</A>) How can I financially assist
PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2>
@@ -76,8 +74,7 @@
<A href="#4.2">4.2</A>) How do I find out what tables, indexes,
databases, and users are defined? How do I see the queries used
by <I>psql</I> to display them?<BR>
<A href="#4.3">4.3</A>) How do you remove a column from a
table, or change its data type?<BR>
<A href="#4.3">4.3</A>) How do you change a column's data type?<BR>
<A href="#4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?<BR>
<A href="#4.5">4.5</A>) How much database disk space is required
@@ -104,24 +101,22 @@
my sequence/SERIAL column?<BR>
<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR>
<A href="#4.13">4.13</A>) What is the meaning of some of the terms
used in PostgreSQL?<BR>
<A href="#4.14">4.14</A>) Why do I get the error <I>"ERROR: Memory
<A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.15">4.15</A>) How do I tell what PostgreSQL version I
<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#4.16">4.16</A>) Why does my large-object operations get
<A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.17">4.17</A>) How do I create a column that will
<A href="#4.16">4.16</A>) How do I create a column that will
default to the current time?<BR>
<A href="#4.18">4.18</A>) How do I perform an outer join?<BR>
<A href="#4.19">4.19</A>) How do I perform queries using multiple
<A href="#4.17">4.17</A>) How do I perform an outer join?<BR>
<A href="#4.18">4.18</A>) How do I perform queries using multiple
databases?<BR>
<A href="#4.20">4.20</A>) How do I return multiple rows or columns
<A href="#4.19">4.19</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.21">4.21</A>) Why can't I reliably create/drop
<A href="#4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?<BR>
<A href="#4.22">4.22</A>) What encryption options are available?<BR>
<A href="#4.21">4.21</A>) What encryption options are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
@@ -140,45 +135,31 @@
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>.</P>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P>
<P>PostgreSQL is an enhancement of the POSTGRES database management
system (and is still sometimes reffered to as simply "Postgres"),
a next-generation <SMALL>DBMS</SMALL> research prototype.
While PostgreSQL retains the powerful data model and rich data
types of POSTGRES, it replaces the PostQuel query language with an
extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
complete source is available.</P>
<P>PostgreSQL is an object-relational database system that has the
features of traditional commercial database systems with
enhancements to be found in next-generation <SMALL>DBMS</SMALL>
systems. PostgreSQL is free and the complete source code is
available.</P>
<P>PostgreSQL development is performed by a team of
developers who all subscribe to the PostgreSQL development mailing
list. The current coordinator is Marc G. Fournier (<A href=
"mailto:scrappy@PostgreSQL.org">scrappy@PostgreSQL.org</A>). (See
section <a href="#1.6">1.6</a> on how to join). This team is now
responsible for all development of PostgreSQL. It is a community
project and is not controlled by any company. To get involved, see
the developer's FAQ at <A href=
<P>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 <A href=
"http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html">
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P>
<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
Many others have contributed to the porting, testing, debugging,
and enhancement of the code. The original Postgres code, from which
PostgreSQL is derived, was the effort of many graduate students,
undergraduate students, and staff programmers working under the
direction of Professor Michael Stonebraker at the University of
California, Berkeley.</P>
<P>The original name of the software at Berkeley was Postgres. When
<SMALL>SQL</SMALL> functionality was added in 1995, its name was
changed to Postgres95. The name was changed at the end of 1996 to
PostgreSQL.</P>
<H4><A name="1.2">1.2</A>) What is the copyright on
<H4><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4>
<P>PostgreSQL is subject to the following COPYRIGHT:</P>
<P>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.</P>
<P>This is the BSD license we use:</P>
<P>PostgreSQL Data Base Management System</P>
@@ -204,10 +185,6 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<P>The above is the BSD license, the classic open-source license.
It has no restrictions on how the source code may be used. We like
it and have no intention of changing it.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<P>In general, any modern Unix-compatible platform should be able to
@@ -236,78 +213,46 @@
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<P>The main mailing list is: <A href=
"mailto:pgsql-general@PostgreSQL.org">pgsql-general@PostgreSQL.org</A>.
It is available for discussion of matters pertaining to PostgreSQL.
To subscribe, send mail with the following lines in the body (not
the subject line):</P>
<PRE>
subscribe
end
</PRE>
<P>to <A href=
"mailto:pgsql-general-request@PostgreSQL.org">pgsql-general-request@PostgreSQL.org</A>.</P>
<P>There is also a digest list available. To subscribe to this
list, send email to: <A href=
"mailto:pgsql-general-digest-request@PostgreSQL.org">pgsql-general-digest-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
Digests are sent out to members of this list whenever the main list
has received around 30k of messages.
<P>The bugs mailing list is available. To subscribe to this list,
send email to <A href=
"mailto:pgsql-bugs-request@PostgreSQL.org">pgsql-bugs-request@PostgreSQL.org</A>
with a body of:</P>
<PRE>
subscribe
end
</PRE>
There is also a developers discussion mailing list available. To
subscribe to this list, send email to <A href=
"mailto:pgsql-hackers-request@PostgreSQL.org">pgsql-hackers-request@PostgreSQL.org</A>
with a body of:
<PRE>
subscribe
end
</PRE>
<P>Additional mailing lists and information about PostgreSQL can be
found via the PostgreSQL WWW home page at:</P>
<BLOCKQUOTE>
<A href="http://www.PostgreSQL.org">http://www.PostgreSQL.org</A>
</BLOCKQUOTE>
<P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is
<a href="http://www.postgresql.org/community/lists/">
http://www.postgresql.org/community/lists/</a>. The <I>general</I>
or <I>bugs</I> lists are a good place to start.
<P>The major IRC channel is <I>#postgresql</I> on Freenode
(<I>irc.freenode.net</I>). To connect you can use the Unix
command <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
or use any of the other popular IRC clients. A Spanish one also exists
on the same network, (<I>#postgresql-es</I>), and a French one,
(<I>#postgresqlfr</I>). There is also a PostgreSQL channel on EFNet.
<P>A list of commercial support companies is available at <A href=
"http://techdocs.postgresql.org/companies.php">http://techdocs.postgresql.org/companies.php</A>.</P>
"http://techdocs.postgresql.org/companies.php">http://techdocs.postg
resql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) What is the latest release?</H4>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
<P>The latest release of PostgreSQL is version 8.0.0.</P>
<P>We plan to have major releases every six to eight months.</P>
<P>We plan to have major releases every ten to twelve months.</P>
<H4><A name="1.7">1.7</A>) What documentation is available?</H4>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<P>Several manuals, manual pages, and some small test examples are
included in the distribution. See the <I>/doc</I> directory. You
can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.</P>
<P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I>
directory. You can also browse the manuals online at <A href=
"http://www.PostgreSQL.org/docs">http://www.PostgreSQL.org/docs</A>.
</P>
<P>There are two PostgreSQL books available online at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
@@ -326,14 +271,14 @@
<P>Our web site contains even more documentation.</P>
<H4><A name="1.8">1.8</A>) How do I find out about known bugs or
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
list for known bugs, missing features, and future plans.</P>
<H4><A name="1.9">1.9</A>) How can I learn
<H4><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4>
<P>The PostgreSQL book at <A href=
@@ -357,12 +302,12 @@
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<H4><A name="1.10">1.10</A>) How do I join the development
<H4><A name="1.11">1.11</A>) How do I join the development
team?</H4>
<P>First, download the latest source and read the PostgreSQL
Developers documentation on our web site, or in the distribution.
Second, subscribe to the <I>pgsql-hackers</I> and
Developers FAQ and documentation on our web site, or in the
distribution. Second, subscribe to the <I>pgsql-hackers</I> and
<I>pgsql-patches</I> mailing lists. Third, submit high quality
patches to pgsql-patches.</P>
@@ -372,16 +317,6 @@
committers to keep up, and we had confidence that patches they
committed were of high quality.</P>
<H4><A name="1.11">1.11</A>) How do I submit a bug report?</H4>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
http://www.postgresql.org/support/submitbug</A>.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
see if there is a more recent PostgreSQL version or patches.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
@@ -448,31 +383,6 @@
</DD>
</DL>
<H4><A name="1.13">1.13</A>) How can I financially assist
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
in 1996. This is all thanks to Marc Fournier, who has created
and managed this infrastructure over the years.</P>
<P>Quality infrastructure is very important to an open-source
project. It prevents disruptions that can greatly delay forward
movement of the project.</P>
<P>Of course, this infrastructure is not cheap. There are a variety
of monthly and one-time expenses that are required to keep it
going. If you or your company has money it can donate to help fund
this effort, please go to <A href="http://store.pgsql.com/shopping/">http://store.pgsql.com/shopping/</A>
and make a donation.</P>
<P>Although the web page mentions PostgreSQL, Inc, the
"contributions" item is solely to support the PostgreSQL project
and does not fund any specific company. If you prefer, you can also
send a check to the contact address.</P>
<P>Also, if you have a success story about PostgreSQL, please email
it to our advocacy list at <a href="mailto:pgsql-advocacy@postgresql.org">
pgsql-advocacy@postgresql.org</a>.</P>
<HR>
@@ -499,27 +409,33 @@
<P>A nice introduction to Database-backed Web pages can be seen at:
<A href="http://www.webreview.com">http://www.webreview.com</A></P>
<P>For Web integration, PHP is an excellent interface. It is at <A
href="http://www.php.net">http://www.php.net</A>.</P>
<P>For Web integration, PHP (<A
href="http://www.php.net">http://www.php.net</A>) is an excellent
interface.</P>
<P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
<P>For complex cases, many use the Perl and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4>
<P>Yes, there are several graphical interfaces to PostgreSQL available.
These include PgAccess <a href="http://www.pgaccess.org">
http://www.pgaccess.org</a>), pgAdmin III (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhdb/
</a>), TORA (<a href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
partly commercial), and Rekall (<a href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin
(<a href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
<P>Yes, there are several graphical interfaces to PostgreSQL
available. These include pgAdmin III (<a
href="http://www.pgadmin.org">http://www.pgadmin.org</a>, PgAccess
<a href="http://www.pgaccess.org"> http://www.pgaccess.org</a>),
RHDB Admin (<a
href="http://sources.redhat.com/rhdb/">http://sources.redhat.com/rhd
b/ </a>), TORA (<a
href="http://www.globecom.net/tora/">http://www.globecom.net/tora/</a>,
partly commercial), and Rekall (<a
href="http://www.rekallrevealed.org/">
http://www.rekallrevealed.org/</a>). There is also PhpPgAdmin (<a
href="http://phppgadmin.sourceforge.net/">
http://phppgadmin.sourceforge.net/ </a>), a web-based interface to
PostgreSQL.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">http://techdocs.postgresql.org/guides/GUITools</a> for a more detailed list.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
detailed list.</P>
<HR>
@@ -538,75 +454,85 @@
<P>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 <B>and</B> enable
host-based authentication by modifying the file
<I>$PGDATA/pg_hba.conf</I> accordingly.</P>
<I>listen_addresses</I> in the <I>postgresql.conf</I> file, enable
host-based authentication by modifying the
<I>$PGDATA/pg_hba.conf</I> file, and restart the server.</P>
<H4><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H4>
<P>Certainly, indexes can speed up queries. The
<SMALL>EXPLAIN ANALYZE</SMALL> command allows you to see how
PostgreSQL is interpreting your query, and which indexes are
being used.</P>
<P>There are three major areas for potential performance
improvement:</P>
<DL>
<DT><B>Query Changes</B></DT>
<P>If you are doing many <SMALL>INSERTs</SMALL>, consider doing
them in a large batch using the <SMALL>COPY</SMALL> command. This
is much faster than individual <SMALL>INSERTS</SMALL>. Second,
statements not in a <SMALL>BEGIN WORK/COMMIT</SMALL> transaction
block are considered to be in their own transaction. Consider
performing several statements in a single transaction block. This
reduces the transaction overhead. Also, consider dropping and
recreating indexes when making large data changes.</P>
<DD>This involves modifying queries to obtain better
performance:
<ul>
<li>Creation of indexes, including expression and partial
indexes</li>
<li>Use of COPY instead of multiple <SMALL>INSERT</SMALL>s</li>
<li>Grouping of multiple statements into a single transaction to
reduce commit overhead</li>
<li>Use of <SMALL>CLUSTER</SMALL> when retrieving many rows from an
index</li>
<li>Use of <SMALL>LIMIT</SMALL> for returning a subset of a query's
output</li>
<li>Use of Prepared queries</li>
<li>Use of <SMALL>ANALYZE</SMALL> to maintain accurate optimizer
statistics</li>
<li>Regular use of <SMALL>VACUUM</SMALL> or <I>pg_autovacuum</I>
<li>Dropping of indexes during large data changes</li>
</ul><BR>
<BR>
</DD>
<P>There are several tuning options in the <a href=
<DT><B>Server Configuration</B></DT>
<DD>A number of <I>postgresql.conf</I> settings affect performance.
For more details, see <a href=
"http://www.postgresql.org/docs/current/static/runtime.html">
Administration Guide/Server Run-time Environment/Run-time Configuration</a>.
You can disable <I>fsync()</I> by using <i>fsync</I> option. This will
prevent <I>fsync()</I>s from flushing to disk after every
transaction.</P>
Administration Guide/Server Run-time Environment/Run-time
Configuration</a> for a full listing, and for commentary see <a
href="http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/annotated_conf_e.html</a>
and <a href="http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html">
http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html</a>.
<BR>
<BR>
</DD>
<P>You can use the <I>shared_buffers</I> option to
increase the number of shared memory buffers used by the backend
processes. If you make this parameter too high, the
<I>postmaster</I> may not start because you have exceeded your
kernel's limit on shared memory space. Each buffer is 8K and the
default is 1000 buffers.</P>
<DT><B>Hardware Selection</B></DT>
<P>You can also use the <I>sort_mem</I> (from PostgreSQL 8.0: <I>work_mem</I>)
options to increase the maximum amount of memory used by the backend
processes for each temporary sort. The default is 1024 (i.e. 1MB).</P>
<P>You can also use the <SMALL>CLUSTER</SMALL> command to group
data in tables to match an index. See the <SMALL>CLUSTER</SMALL>
manual page for more details.</P>
<DD>The effect of hardware on performance is detailed in <a
href="http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html">
http://candle.pha.pa.us/main/writings/pgsql/hw_performance/index.html</a>.
<BR>
<BR>
</DD>
</DL>
<H4><A name="3.4">3.4</A>) What debugging features are
available?</H4>
<P>PostgreSQL has several features that report status information
that can be valuable for debugging purposes.</P>
<P>There are many <CODE>log_*</CODE> server configuration variables
that enable printing of query and process statistics which can be
very useful for debugging and performance measurements.</P>
<P>First, by running <I>configure</I> with the --enable-cassert
<P><B>The following detailed debug instructions are to be used to
provide more detailed information for server developers debugging a
problem.</B></P>
<P>It is also possible to debug the server if it isn't operating
properly. First, by running <I>configure</I> with the --enable-cassert
option, many <I>assert()</I>s monitor the progress of the backend
and halt the program when something unexpected occurs.</P>
<P>Both <I>postmaster</I> and <I>postgres</I> have several debug
options available. First, whenever you start <I>postmaster</I>,
make sure you send the standard output and error to a log file,
like:</P>
<PRE>
cd /usr/local/pgsql
./bin/postmaster &gt;server.log 2&gt;&amp;1 &amp;
</PRE>
<P>This will put a server.log file in the top-level PostgreSQL
directory. This file contains useful information about problems or
errors encountered by the server. <I>Postmaster</I> has a <I>-d</I>
option that allows even more detailed information to be reported.
The <I>-d</I> option takes a number that specifies the debug level.
Be warned that high debug level values generate large log
files.</P>
<P>The <I>postmaster</I> has a <I>-d</I> option that allows even more
detailed information to be reported. The <I>-d</I> option takes a
number that specifies the debug level. Be warned that high debug
level values generate large log files.</P>
<P>If <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
@@ -630,10 +556,6 @@
the debugger, set any breakpoints, and continue through the startup
sequence.</P>
<P>There are several <CODE>log_*</CODE> server configuration variables
that enable printing of process statistics which can be very useful
for debugging and performance measurements.</P>
<P>You can also compile with profiling to see what functions are
taking execution time. The backend profile files will be deposited
in the <I>pgsql/data/base/dbname</I> directory. The client profile
@@ -643,27 +565,11 @@
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<P>You need to increase <I>postmaster</I>'s limit on how many
concurrent backend processes it can start.</P>
<P>The default limit is 32 processes. You can increase it by
restarting <I>postmaster</I> with a suitable <I>-N</I> value or
modifying <I>postgresql.conf</I>.</P>
<P>Note that if you make <I>-N</I> larger than 32, you must also
increase <I>-B</I> beyond its default of 64; <I>-B</I> must be at
least twice <I>-N</I>, 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, <SMALL>SHMMAX;</SMALL> the maximum number
of semaphores, <SMALL>SEMMNS</SMALL> and <SMALL>SEMMNI;</SMALL> the
maximum number of processes, <SMALL>NPROC;</SMALL> the maximum
number of processes per user, <SMALL>MAXUPRC;</SMALL> and the
maximum number of open files, <SMALL>NFILE</SMALL> and
<SMALL>NINODE</SMALL>. The reason that PostgreSQL has a limit on
the number of allowed backend processes is so your system won't run
out of resources.</P>
<P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many
concurrent backend processes it can start by changing the
<I>max_connections</I> value in <I>postgresql.conf</I> and
restarting the <I>postmaster</I>.</P>
<H4><A name="3.6">3.6</A>) What is in the <I>pgsql_tmp</I> directory?</H4>
@@ -681,18 +587,13 @@
to upgrade between major PostgreSQL releases?</H4>
<P>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
so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the internal
format of system tables and data files. These changes are often complex,
so we don't maintain backward compatability for data files. A dump outputs
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.</P>
<P>In releases where the on-disk format does not change, the
<I>pg_upgrade</I> script can be used to upgrade without a dump/restore.
The release notes mention whether <I>pg_upgrade</I> is available for the
release.</P>
<H4><A name="3.8">3.8</A>) What computer hardware should I use?</H4>
<P>Because PC hardware is mostly compatible, people tend to believe that
@@ -710,15 +611,13 @@
<H4><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H4>
<P>See the <SMALL>FETCH</SMALL> manual page, or use
<SMALL>SELECT</SMALL> ... <SMALL>LIMIT</SMALL>....</P>
<P>The entire query may have to be evaluated, even if you only want
the first few rows. Consider using a query that has an <SMALL>ORDER
BY</SMALL>. If there is an index that matches the <SMALL>ORDER
BY</SMALL>, 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.</P>
<P>To retrieve only a few rows, if you know at the number of rows
needed at the time of the <SMALL>SELECT</SMALL> use
<SMALL>LIMIT</SMALL> . If an index matches the <SMALL>ORDER
BY</SMALL> it is possible the entire query does not have to be
executed. If you don't know the number of rows at
<SMALL>SELECT</SMALL> time, use a cursor and
<SMALL>FETCH</SMALL>.</P>
<P>To <SMALL>SELECT</SMALL> a random row, use:
<PRE>
@@ -743,28 +642,15 @@
database.</P>
<P>There are also system tables beginning with <I>pg_</I> that describe
these too. Use <I>psql -l</I> will list all databases.</P>
these too.</P>
<P>Use <I>psql -l</I> will list all databases.</P>
<P>Also try the file <I>pgsql/src/tutorial/syscat.source</I>. It
illustrates many of the <SMALL>SELECT</SMALL>s needed to get
information from the database system tables.</P>
<H4><A name="4.3">4.3</A>) How do you remove a column from a
table, or change its data type?</H4>
<P><SMALL>DROP COLUMN</SMALL> functionality was added in release 7.3
with <SMALL>ALTER TABLE DROP COLUMN</SMALL>. In earlier versions,
you can do this:</P>
<PRE>
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;
</PRE>
<H4><A name="4.3">4.3</A>) How do you change a column's data type?</H4>
<P>Changing the data type of a column can be done easily in 8.0
and later with <SMALL>ALTER TABLE ALTER COLUMN TYPE</SMALL>.
@@ -784,19 +670,25 @@
table, and a database?</H4>
<P>These are the limits:</P>
<PRE>
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
</PRE>
<CENTER>
<TABLE BORDER=1>
<TR><TD>Maximum size for a database?</TD><TD>unlimited (32 TB databases
exist)</TD></TR>
<TR><TD>Maximum size for a table?</TD><TD>32 TB</TD></TR>
<TR><TD>Maximum size for a row?</TD><TD>1.6TB</TD></TR>
<TR><TD>Maximum size for a field?</TD><TD>1 GB</TD></TR>
<TR><TD>Maximum number of rows in a table?</TD><TD>unlimited</TD></TR>
<TR><TD>Maximum number of columns in a table?</TD><TD>250-1600 depending
on column types</TD></TR>
<TR><TD>Maximum number of indexes on a
table?</TD><TD>unlimited</TD></TR>
</TABLE>
</CENTER>
<BR>
Of course, these are not actually unlimited, but limited to
<P>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.
when these values get unusually large.</P>
<P>The maximum table size of 32 TB does not require large file
support from the operating system. Large tables are stored as
@@ -893,10 +785,10 @@
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilize indexes. Instead, use functional
indexes, which are described in section <a href="#4.10">4.10</a>.</LI>
<I>~*</I> do not utilize indexes. Instead, use expression
indexes, which are described in section <a href="#4.8">4.8</a>.</LI>
<LI>The default <I>C</I> locale must be used during
<i>initdb</i> because it is not possible to know the next-greater
<i>initdb</i> because it is not possible to know the next-greatest
character in a non-C locale. You can create a special
<CODE>text_pattern_ops</CODE> index for such cases that work only
for <SMALL>LIKE</SMALL> indexing.
@@ -904,7 +796,7 @@
</UL>
<P>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
types exactly match the index's column types. This was particularly
true of int2, int8, and numeric column indexes.</P>
<H4><A name="4.7">4.7</A>) How do I see how the query optimizer is
@@ -930,7 +822,7 @@
</PRE>
This will not use an standard index. However, if you create a
functional index, it will be used:
expresssion index, it will be used:
<PRE>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
@@ -943,16 +835,20 @@
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
<PRE>
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
</PRE>
<CENTER>
<TABLE BORDER=1>
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
<TR><TD>VARCHAR(n)</TD><TD>varchar</TD><TD>size specifies maximum
length, no padding</TD></TR>
<TR><TD>CHAR(n)</TD><TD>bpchar</TD><TD>blank padded to the specified
fixed length</TD></TR>
<TR><TD>TEXT</TD><TD>text</TD><TD>no specific upper limit on
length</TD></TR>
<TR><TD>BYTEA</TD><TD>bytea</TD><TD>variable-length byte array
(null-byte safe)</TD></TR>
<TR><TD>"char"</TD><TD>char</TD><TD>one character</TD></TR>
</TABLE>
</CENTER>
<P>You will see the internal name when examining system catalogs
and in some error messages.</P>
@@ -996,10 +892,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</PRE>
See the <I>create_sequence</I> manual page for more information
about sequences. You can also use each row's <I>OID</I> field as a
unique value. However, if you need to dump and reload the database,
you need to use <I>pg_dump</I>'s <I>-o</I> option or <SMALL>COPY
WITH OIDS</SMALL> option to preserve the <SMALL>OID</SMALL>s.
about sequences.
<H4><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H4>
@@ -1030,19 +923,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
<P>Finally, you could use the <A href="#4.12"><SMALL>OID</SMALL></A>
returned from the <SMALL>INSERT</SMALL> 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 <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.</P>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4>
<P>No. <I>currval()</I> returns the current value assigned by your
backend, not by all users.</P>
session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
reused on transaction abort? Why are there gaps in the numbering of
@@ -1076,36 +961,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<H4><A name="4.13">4.13</A>) What is the meaning of some of the
terms used in PostgreSQL?</H4>
<P>Some of the source code and older documentation use terms that
have more common usage. Here are some:</P>
<UL>
<LI>table, relation, class</LI>
<LI>row, record, tuple</LI>
<LI>column, field, attribute</LI>
<LI>retrieve, select</LI>
<LI>replace, update</LI>
<LI>append, insert</LI>
<LI><SMALL>OID</SMALL>, serial value</LI>
<LI>portal, cursor</LI>
<LI>range variable, table name, table alias</LI>
</UL>
<P>A list of general database terms can be found at: <A href=
"http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html">http://hea-www.harvard.edu/MST/simul/software/docs/pkgs/pgsql/glossary/glossary.html</A></P>
<H4><A name="4.14">4.14</A>) Why do I get the error <I>"ERROR:
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
<P>You probably have run out of virtual memory on your system,
@@ -1124,12 +980,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
backend is returning too much data, try it before starting the
client.
<H4><A name="4.15">4.15</A>) How do I tell what PostgreSQL version
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.16">4.16</A>) Why does my large-object operations
<H4><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
@@ -1145,15 +1001,15 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>If you are using a client interface like <SMALL>ODBC</SMALL> you
may need to set <CODE>auto-commit off.</CODE></P>
<H4><A name="4.17">4.17</A>) How do I create a column that will
<H4><A name="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
CREATE TABLE test (x int, modtime timestamp DEFAULT CURRENT_TIMESTAMP );
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE>
<H4><A name="4.18">4.18</A>) How do I perform an outer join?</H4>
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
@@ -1176,24 +1032,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P>
<P>In previous releases, outer joins can be simulated using
<SMALL>UNION</SMALL> and <SMALL>NOT IN</SMALL>. For example, when
joining <I>tab1</I> and <I>tab2</I>, the following query does an
<I>outer</I> join of the two tables:<BR>
<BR>
</P>
<PRE>
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
</PRE>
<H4><A name="4.19">4.19</A>) How do I perform queries using
<H4><A name="4.18">4.18</A>) How do I perform queries using
multiple databases?</H4>
<P>There is no way to query a database other than the current one.
@@ -1201,29 +1040,29 @@ BYTEA bytea variable-length byte array (null-byte safe)
uncertain how a cross-database query should even behave.</P>
<P><I>contrib/dblink</I> allows cross-database queries using
function calls. Of course, a client can make simultaneous
function calls. Of course, a client can also make simultaneous
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.20">4.20</A>) How do I return multiple rows or
<H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4>
<P>In 7.3, you can easily return multiple rows or columns from a
function,
<P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a>.
<H4><A name="4.21">4.21</A>) Why can't I reliably create/drop
<H4><A name="4.20">4.20</A>) Why can't I reliably create/drop
temporary tables in PL/PgSQL functions?</H4>
<P>PL/PgSQL caches function contents, and an unfortunate side effect
<P>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
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
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.22">4.22</A>) What encryption options are available?
<H4><A name="4.21">4.21</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
@@ -1236,8 +1075,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
encrypted transport, such as stunnel or ssh, rather than PostgreSQL's
native SSL connections.)
<LI>Database user passwords are automatically encrypted when stored in
version 7.3. In previous versions, you must enable the option
<I>PASSWORD_ENCRYPTION</I> in <I>postgresql.conf</I>.</LI>
the system tables.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
@@ -1277,4 +1115,3 @@ BYTEA bytea variable-length byte array (null-byte safe)
compiler compute the dependencies automatically.</P>
</BODY>
</HTML>