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's to 8.0.X for release.

Browse Source
This commit is contained in:
Bruce Momjian
2005-05-09 17:24:04 +00:00
parent 014fce947d
commit 00283f4b0a
4 changed files with 446 additions and 583 deletions
Download Patch File Download Diff File Expand all files Collapse all files

468
doc/src/FAQ/FAQ.html
View File

@@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Tue Mar 15 17:38:18 EST 2005</P>
<P>Last updated: Mon May 9 13:15:04 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
@@ -40,6 +40,7 @@
<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>) Who controls PostgreSQL?<BR>
<H2 align="center">User Client Questions</H2>
@@ -99,41 +100,29 @@
reused on transaction abort? Why are there gaps in the numbering of
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="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory
<SMALL>CTID</SMALL>?<BR>
<A href="#4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory
exhausted in AllocSetAlloc()"</I>?<BR>
<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I
am running?<BR>
<A href="#4.15">4.15</A>) Why does my large-object operations get
<I>"invalid large obj descriptor"</I>?<BR>
<A href="#4.16">4.16</A>) How do I create a column that will
<A href="#4.15">4.15</A>) How do I create a column that will
default to the current time?<BR>
<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
<A href="#4.16">4.16</A>) How do I perform an outer join?<BR>
<A href="#4.17">4.17</A>) How do I perform queries using multiple
databases?<BR>
<A href="#4.19">4.19</A>) How do I return multiple rows or columns
<A href="#4.18">4.18</A>) How do I return multiple rows or columns
from a function?<BR>
<A href="#4.20">4.20</A>) Why do I get "relation with OID #####
<A href="#4.19">4.19</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL
functions?<BR>
<A href="#4.21">4.21</A>) What encryption options are available?<BR>
<A href="#4.20">4.20</A>) What replication solutions are available?<BR>
<H2 align="center">Extending PostgreSQL</H2>
<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run
it in <I>psql</I>, why does it dump core?<BR>
<A href="#5.2">5.2</A>) How can I contribute some nifty new types
and functions to PostgreSQL?<BR>
<A href="#5.3">5.3</A>) How do I write a C function to return a
tuple?<BR>
<A href="#5.4">5.4</A>) I have changed a source file. Why does the
recompile not see the change?<BR>
<HR>
<H2 align="center">General Questions</H2>
<H4><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H4>
<H3><A name="1.1">1.1</A>) What is PostgreSQL? How is it pronounced?</H3>
<P>PostgreSQL is pronounced <I>Post-Gres-Q-L</I>, also called just
<I>Postgres</I>.</P>
@@ -152,8 +141,8 @@
http://www.postgresql.org/files/documentation/faqs/FAQ_DEV.html</A>
</P>
<H4><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H4>
<H3><A name="1.2">1.2</A>) What is the copyright of
PostgreSQL?</H3>
<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
@@ -185,17 +174,16 @@
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
<H4><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H4>
<H3><A name="1.3">1.3</A>) What platforms does PostgreSQL support?</H3>
<P>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.</P>
<P>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 <a href=
"http://pgfoundry.org/projects/pginstaller">
<P>PostgreSQL also runs natively on Microsoft Windows NT-based operating
systems like Win2000, WinXP, and Win2003. A prepackaged installer is
available at <a href= "http://pgfoundry.org/projects/pginstaller">
http://pgfoundry.org/projects/pginstaller</a>. MSDOS-based versions
of Windows (Win95, Win98, WinMe) can run PostgreSQL using Cygwin.</P>
@@ -205,13 +193,14 @@
"http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F">
http://hobbes.nmsu.edu/cgi-bin/h-search?sh=1&amp;button=Search&amp;key=postgreSQL&amp;stype=all&amp;sort=type&amp;dir=%2F</a>.</p>
<H4><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H4>
<H3><A name="1.4">1.4</A>) Where can I get PostgreSQL?</H3>
<P>The primary anonymous ftp site for PostgreSQL is <A href=
"ftp://ftp.PostgreSQL.org/pub/">ftp://ftp.PostgreSQL.org/pub/</A>.
For mirror sites, see our main web site.</P>
<P>Via web browser, use <a href="http://www.postgresql.org/ftp/">
http://www.postgresql.org/ftp/</a>, and via ftp, use
<A href="ftp://ftp.PostgreSQL.org/pub/">
ftp://ftp.PostgreSQL.org/pub/</A>.</P>
<H4><A name="1.5">1.5</A>) Where can I get support?</H4>
<H3><A name="1.5">1.5</A>) Where can I get support?</H3>
<P>The PostgreSQL community provides assistance to many of its users
via email. The main web site to subscribe to the email lists is
@@ -222,7 +211,7 @@
<P>The major IRC channel is <I>#postgresql</I> on Freenode
(<I>irc.freenode.net</I>). To connect you can use the Unix
program <CODE>irc -c '#postgresql' "$USER" irc.freenode.net</CODE>
or use any of the other popular IRC clients. A Spanish one also exists
or use any other 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>
@@ -230,7 +219,7 @@
"http://techdocs.postgresql.org/companies.php">
http://techdocs.postgresql.org/companies.php</A>.</P>
<H4><A name="1.6">1.6</A>) How do I submit a bug report?</H4>
<H3><A name="1.6">1.6</A>) How do I submit a bug report?</H3>
<P>Visit the PostgreSQL bug form at <A href=
"http://www.postgresql.org/support/submitbug">
@@ -240,13 +229,14 @@
"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>
<H3><A name="1.7">1.7</A>) What is the latest release?</H3>
<P>The latest release of PostgreSQL is version 8.0.1.</P>
<P>The latest release of PostgreSQL is version 8.0.2.</P>
<P>We plan to have major releases every ten to twelve months.</P>
<P>We plan to have a major release every year, with minor releases
every few months.</P>
<H4><A name="1.8">1.8</A>) What documentation is available?</H4>
<H3><A name="1.8">1.8</A>) What documentation is available?</H3>
<P>PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the <I>/doc</I>
@@ -258,8 +248,9 @@
"http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
and <A href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
There is a list of PostgreSQL books available for purchase at <A
href=
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 <A href=
"http://techdocs.postgresql.org/techdocs/bookreviews.php">http://techdocs.PostgreSQL.org/techdocs/bookreviews.php</A>.
There is also a collection of PostgreSQL technical articles at <A
href=
@@ -271,22 +262,25 @@
<P>Our web site contains even more documentation.</P>
<H4><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H4>
<H3><A name="1.9">1.9</A>) How do I find out about known bugs or
missing features?</H3>
<P>PostgreSQL supports an extended subset of <SMALL>SQL</SMALL>-92.
See our <A href="http://developer.PostgreSQL.org/todo.php">TODO</A>
See our <A href="http://www.postgresql.org/docs/faqs.TODO.html">TODO</A>
list for known bugs, missing features, and future plans.</P>
<H4><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H4>
<H3><A name="1.10">1.10</A>) How can I learn
<SMALL>SQL</SMALL>?</H3>
<P>The PostgreSQL book at <A href=
"http://www.postgresql.org/docs/books/awbook.html">http://www.postgresql.org/docs/books/awbook.html</A>
teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
There is a nice tutorial at <A href=
<P>First, consider the PostgreSQL-specific books mentioned above.
Another one is "Teach Yourself SQL in 21 Days, Second Edition"
at <A href=
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A>.
Many of our users like <I>The Practical SQL Handbook</I>,
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<P>There is also a nice tutorial at <A href=
"http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
at <A href=
"http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM">
@@ -294,31 +288,14 @@
and at <A href=
"http://sqlcourse.com/">http://sqlcourse.com.</A></P>
<P>Another one is "Teach Yourself SQL in 21 Days, Second Edition"
at <A href=
"http://members.tripod.com/er4ebus/sql/index.htm">http://members.tripod.com/er4ebus/sql/index.htm</A></P>
<H3><A name="1.11">1.11</A>) How do I join the development
team?</H3>
<P>Many of our users like <I>The Practical SQL Handbook</I>,
Bowman, Judith S., et al., Addison-Wesley. Others like <I>The
Complete Reference SQL</I>, Groff et al., McGraw-Hill.</P>
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A>.
<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 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>
<P>There are about a dozen people who have commit privileges to the
PostgreSQL <SMALL>CVS</SMALL> 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.</P>
<H4><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H4>
<H3><A name="1.12">1.12</A>) How does PostgreSQL compare to other
<SMALL>DBMS</SMALL>s?</H3>
<P>There are several ways of measuring software: features,
performance, reliability, support, and price.</P>
@@ -339,13 +316,7 @@
<DD>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
<I>Features</I> section above. We are built for reliability and
features, and we continue to improve performance in every
release. <BR>
others. Our performance is usually +/-10% compared to other databases.
<BR>
</DD>
@@ -383,13 +354,24 @@
</DD>
</DL>
<H3><A name="1.13">1.13</A>) Who controls PostgreSQL?<BR>
<P>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 <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A> for information on how to get involved in PostgreSQL
development.)</P>
<HR>
<H2 align="center">User Client Questions</H2>
<H4><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H4>
<H3><A name="2.1">2.1</A>) What interfaces are available for
PostgreSQL?</H3>
<P>The PostgreSQL install includes only the <SMALL>C</SMALL> and embedded
<SMALL>C</SMALL> interfaces. All other interfaces are independent projects
@@ -403,8 +385,8 @@
in the <I>Drivers/Interfaces</I> section and via Internet search.
</P>
<H4><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H4>
<H3><A name="2.2">2.2</A>) What tools are available for using
PostgreSQL with Web pages?</H3>
<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>
@@ -416,25 +398,11 @@
<P>For complex cases, many use the Perl and DBD::Pg with CGI.pm or
mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H4>
<H3><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface?</H3>
<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/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
PostgreSQL.</P>
<P>See <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a more
<P>Yes, see <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a
detailed list.</P>
<HR>
@@ -442,14 +410,14 @@
<H2 align="center">Administrative Questions</H2>
<H4><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H4>
<H3><A name="3.1">3.1</A>) How do I install PostgreSQL somewhere
other than <I>/usr/local/pgsql</I>?</H3>
<P>Specify the <I>--prefix</I> option when running
<I>configure</I>.</P>
<H4><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H4>
<H3><A name="3.2">3.2</A>) How do I control connections from other
hosts?</H3>
<P>By default, PostgreSQL only allows connections from the local
machine using Unix domain sockets or TCP/IP connections. Other
@@ -458,8 +426,8 @@
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>
<H3><A name="3.3">3.3</A>) How do I tune the database engine for
better performance?</H3>
<P>There are three major areas for potential performance
improvement:</P>
@@ -515,57 +483,15 @@
</DD>
</DL>
<H4><A name="3.4">3.4</A>) What debugging features are
available?</H4>
<H3><A name="3.4">3.4</A>) What debugging features are
available?</H3>
<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><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>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
<SMALL>SQL</SMALL> statement directly. This is recommended
<B>only</B> 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 <I>postmaster</I>, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.</P>
<P>If <I>postmaster</I> is running, start <I>psql</I> in one
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
You can set breakpoints in the debugger and issue queries from
<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
to delay for <I>n</I> seconds so you can attach to the process with
the debugger, set any breakpoints, and continue through the startup
sequence.</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
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
<H4><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H4>
<H3><A name="3.5">3.5</A>) Why do I get <I>"Sorry, too many
clients"</I> when trying to connect?</H3>
<P>You have reached the default limit is 100 database sessions. You
need to increase the <I>postmaster</I>'s limit on how many
@@ -573,18 +499,18 @@
<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>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H4>
<H3><A name="3.6">3.6</A>) Why do I need to do a dump and restore
to upgrade between major PostgreSQL releases?</H3>
<P>The PostgreSQL team makes only small changes between minor releases,
so upgrading from 7.4 to 7.4.1 does not require a dump and restore.
so upgrading from 7.4.0 to 7.4.1 does not require a dump and restore.
However, major releases (e.g. from 7.3 to 7.4) often change the internal
format of system tables and data files. These changes are often complex,
so we don't maintain backward compatibility for data files. A dump outputs
data in a generic format that can then be loaded in using the new internal
format.</P>
<H4><A name="3.7">3.7</A>) What computer hardware should I use?</H4>
<H3><A name="3.7">3.7</A>) What computer hardware should I use?</H3>
<P>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
@@ -598,8 +524,8 @@
<H2 align="center">Operational Questions</H2>
<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>
<H3><A name="4.1">4.1</A>) How do I <SMALL>SELECT</SMALL> only the
first few rows of a query? A random row?</H3>
<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
@@ -617,9 +543,9 @@
LIMIT 1;
</PRE>
<H4><A name="4.2">4.2</A>) How do I find out what tables, indexes,
<H3><A name="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?</H4>
by <I>psql</I> to display them?</H3>
<P>Use the \dt command to see tables in <I>psql</I>. For a complete list of
commands inside psql you can use \?. Alternatively you can read the source
@@ -640,7 +566,7 @@
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 change a column's data type?</H4>
<H3><A name="4.3">4.3</A>) How do you change a column's data type?</H3>
<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>.</P>
@@ -656,12 +582,12 @@
<P>You might then want to do <I>VACUUM FULL tab</I> to reclaim the
disk space used by the expired rows.</P>
<H4><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H4>
<H3><A name="4.4">4.4</A>) What is the maximum size for a row, a
table, and a database?</H3>
<P>These are the limits:</P>
<BLOCKQUOTE>
<TABLE >
<TABLE>
<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>
@@ -687,8 +613,8 @@ table?</TD><TD>unlimited</TD></TR>
<P>The maximum table size and maximum number of columns can be
quadrupled by increasing the default block size to 32k.</P>
<H4><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H4>
<H3><A name="4.5">4.5</A>) How much database disk space is required
to store data from a typical text file?</H3>
<P>A PostgreSQL database may require up to five times the disk
space to store data from a text file.</P>
@@ -724,14 +650,14 @@ table?</TD><TD>unlimited</TD></TR>
<P><SMALL>NULL</SMALL>s are stored as bitmaps, so they
use very little space.</P>
<H4><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H4>
<H3><A name="4.6">4.6</A>) Why are my queries slow? Why don't they
use my indexes?</H3>
<P>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. </P>
<P>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. </P>
<P>To determine if an index should be used, PostgreSQL must have
statistics about the table. These statistics are collected using
@@ -759,7 +685,7 @@ table?</TD><TD>unlimited</TD></TR>
<P>If you believe the optimizer is incorrect in choosing a
sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and
run tests to see if an index scan is indeed faster.</P>
run query again to see if an index scan is indeed faster.</P>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used in certain circumstances:</P>
@@ -788,14 +714,14 @@ table?</TD><TD>unlimited</TD></TR>
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
evaluating my query?</H4>
<H3><A name="4.7">4.7</A>) How do I see how the query optimizer is
evaluating my query?</H3>
<P>See the <SMALL>EXPLAIN</SMALL> manual page.</P>
<H4><A name="4.8">4.8</A>) How do I perform regular expression
<H3><A name="4.8">4.8</A>) How do I perform regular expression
searches and case-insensitive regular expression searches? How do I
use an index for case-insensitive searches?</H4>
use an index for case-insensitive searches?</H3>
<P>The <I>~</I> operator does regular expression matching, and
<I>~*</I> does case-insensitive regular expression matching. The
@@ -809,19 +735,18 @@ table?</TD><TD>unlimited</TD></TR>
FROM tab
WHERE lower(col) = 'abc';
</PRE>
This will not use an standard index. However, if you create a
expresssion index, it will be used:
<PRE>
CREATE INDEX tabindex ON tab (lower(col));
</PRE>
<H4><A name="4.9">4.9</A>) In a query, how do I detect if a field
<H3><A name="4.9">4.9</A>) In a query, how do I detect if a field
is <SMALL>NULL</SMALL>? How can I sort on whether a field is <SMALL>
NULL</SMALL> or not?</H4>
NULL</SMALL> or not?</H3>
<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS
NOT NULL</SMALL>.</P>
NOT NULL</SMALL>, like this:</P>
<PRE>
SELECT *
@@ -830,7 +755,7 @@ table?</TD><TD>unlimited</TD></TR>
</PRE>
<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL>
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>WHERE</SMALL> clause.
and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause.
Things that are <I>true</I> will sort higher than things that are <I>false</I>,
so the following will put NULL entries at the top of the resulting list:</P>
@@ -840,10 +765,10 @@ table?</TD><TD>unlimited</TD></TR>
ORDER BY (col IS NOT NULL)
</PRE>
<H4><A name="4.10">4.10</A>) What is the difference between the
various character types?</H4>
<H3><A name="4.10">4.10</A>) What is the difference between the
various character types?</H3>
<BLOCKQUOTE>
<TABLE >
<TABLE>
<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>
@@ -863,9 +788,8 @@ length</TD></TR>
<P>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 <SMALL>TOAST</SMALL>, so the space on disk
might also be less than expected.</P>
However, long values are also subject to compression, so the space
on disk might also be less than expected.</P>
<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length
strings and it limits how long a string can be. <SMALL>TEXT</SMALL>
@@ -877,12 +801,11 @@ length</TD></TR>
particularly values that include <SMALL>NULL</SMALL> bytes. All the
types described here have similar performance characteristics.</P>
<H4><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H4>
<H3><A name="4.11.1">4.11.1</A>) How do I create a
serial/auto-incrementing field?</H3>
<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It
auto-creates a sequence. For example,
this:</P>
auto-creates a sequence. For example, this:</P>
<PRE>
CREATE TABLE person (
id SERIAL,
@@ -891,19 +814,20 @@ length</TD></TR>
</PRE>
is automatically translated into this:
<PRE>
CREATE SEQUENCE person_id_seq;
CREATE TABLE person (
id INT4 NOT NULL DEFAULT nextval('person_id_seq'),
name TEXT
name TEXT
);
</PRE>
See the <I>create_sequence</I> manual page for more information
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>
<H3><A name="4.11.2">4.11.2</A>) How do I get the value of a
<SMALL>SERIAL</SMALL> insert?</H3>
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function
@@ -915,39 +839,39 @@ length</TD></TR>
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
</PRE>
You would then also have the new value stored in
<CODE>new_id</CODE> for use in other queries (e.g., as a foreign
key to the <CODE>person</CODE> table). Note that the name of the
automatically created <SMALL>SEQUENCE</SMALL> object will be named
&lt;<I>table</I>&gt;_&lt;<I>serialcolumn</I>&gt;_<I>seq</I>, where
<I>table</I> and <I>serialcolumn</I> are the names of your table
and your <SMALL>SERIAL</SMALL> column, respectively.
You would then also have the new value stored in <CODE>new_id</CODE>
for use in other queries (e.g., as a foreign key to the <CODE>person
</CODE> table). Note that the name of the automatically created
<SMALL>SEQUENCE</SMALL> object will be named &lt;<I>table</I>&gt;_&lt;<I>
serialcolumn</I>&gt;_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I>
are the names of your table and your <SMALL>SERIAL</SMALL> column,
respectively.
<P>Alternatively, you could retrieve the assigned
<SMALL>SERIAL</SMALL> value with the <I>currval()</I> function
<I>after</I> it was inserted by default, e.g.,</P>
<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL>
value with the <I>currval()</I> function <I>after</I> it was inserted by
default, e.g.,</P>
<PRE>
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
<H4><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H4>
<H3><A name="4.11.3">4.11.3</A>) Doesn't <I>currval()</I>
lead to a race condition with other users?</H3>
<P>No. <I>currval()</I> returns the current value assigned by your
session, not by all sessions.</P>
<H4><A name="4.11.4">4.11.4</A>) Why aren't my sequence numbers
<H3><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
my sequence/SERIAL column?</H4>
my sequence/SERIAL column?</H3>
<P>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.</P>
<H4><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4>
<H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>CTID</SMALL>?</H3>
<P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
@@ -964,13 +888,13 @@ length</TD></TR>
<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence
values.</P>
<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows
with block and offset values. T<SMALL>ID</SMALL>s change after rows
<P>C<SMALL>TID</SMALL>s are used to identify specific physical rows
with block and offset values. C<SMALL>TID</SMALL>s change after rows
are modified or reloaded. They are used by index entries to point
to physical rows.</P>
<H4><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
<H3><A name="4.13">4.13</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H3>
<P>You probably have run out of virtual memory on your system,
or your kernel has a low limit for certain resources. Try this
@@ -988,36 +912,20 @@ length</TD></TR>
backend is returning too much data, try it before starting the
client.
<H4><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H4>
<H3><A name="4.14">4.14</A>) How do I tell what PostgreSQL version
I am running?</H3>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<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>
around any use of a large object handle, that is, surrounding
<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P>
<P>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 <I>invalid large obj descriptor</I>. 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.</P>
<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="4.16">4.16</A>) How do I create a column that will
default to the current time?</H4>
<H3><A name="4.15">4.15</A>) How do I create a column that will
default to the current time?</H3>
<P>Use <I>CURRENT_TIMESTAMP</I>:</P>
<PRE>
CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP );
</PRE>
<H4><A name="4.17">4.17</A>) How do I perform an outer join?</H4>
<H3><A name="4.16">4.16</A>) How do I perform an outer join?</H3>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
@@ -1040,8 +948,8 @@ length</TD></TR>
<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins
are called <SMALL>INNER</SMALL> joins.</P>
<H4><A name="4.18">4.18</A>) How do I perform queries using
multiple databases?</H4>
<H3><A name="4.17">4.17</A>) How do I perform queries using
multiple databases?</H3>
<P>There is no way to query a database other than the current one.
Because PostgreSQL loads database-specific system catalogs, it is
@@ -1052,16 +960,16 @@ length</TD></TR>
connections to different databases and merge the results on the
client side.</P>
<H4><A name="4.19">4.19</A>) How do I return multiple rows or
columns from a function?</H4>
<H3><A name="4.18">4.18</A>) How do I return multiple rows or
columns from a function?</H3>
<P>It is easy using set-returning functions,
<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions">
http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>.
<H4><A name="4.20">4.20</A>) Why do I get "relation with OID #####
<H3><A name="4.19">4.19</A>) Why do I get "relation with OID #####
does not exist" errors when accessing temporary tables in PL/PgSQL
functions?</H4>
functions?</H3>
<P>PL/PgSQL caches function scripts, and an unfortunate side effect
is that if a PL/PgSQL function accesses a temporary table, and that
@@ -1071,56 +979,26 @@ length</TD></TR>
<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This
will cause the query to be reparsed every time.</P>
<H4><A name="4.21">4.21</A>) What encryption options are available?
</H4>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
use in <SMALL>SQL</SMALL> queries.</LI>
<LI>To encrypt transmission from the client to the server, the server
must have the <I>ssl</I> option set to <I>true</I> in <I>postgresql.conf,
</I> and an applicable <I>host</I> or <I>hostssl</I> record must exist in
<I>pg_hba.conf</I>, and the client <I>sslmode</I> must not be
<I>disable.</I> (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.)</LI>
<LI>Database user passwords are automatically encrypted when stored in
the system tables.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>
<H3><A name="4.20">4.20</A>) What replication solutions are available?
</H3>
<HR>
<P>Though "replication" is a single term, there are several technologies
for doing replication, with advantages and disadvantages for each.</P>
<H2 align="center">Extending PostgreSQL</H2>
<P>Master/slave replication allows a single master to receive read/write
queries, while slaves can only accept read/<SMALL>SELECT</SMALL>
queries. The most popular freely available master-slave PostgreSQL
replication solution is <A
href="http://gborg.postgresql.org/project/slony1/projdisplay.php">
Slony-I</A>.</P>
<H4><A name="5.1">5.1</A>) I wrote a user-defined function. When I
run it in <I>psql</I>, why does it dump core?</H4>
<P>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.
<A href="http://pgfoundry.org/projects/pgcluster/">Pgcluster</a> is the
most popular such solution freely available for PostgreSQL.</P>
<P>The problem could be a number of things. Try testing your
user-defined function in a stand-alone test program first.</P>
<H4><A name="5.2">5.2</A>) How can I contribute some nifty new
types and functions to PostgreSQL?</H4>
<P>Send your extensions to the <I>pgsql-hackers</I> mailing list,
and they will eventually end up in the <I>contrib/</I>
subdirectory.</P>
<H4><A name="5.3">5.3</A>) How do I write a C function to return a
tuple?</H4>
<P>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
<I>contrib/tablefunc</I>.</P>
<H4><A name="5.4">5.4</A>) I have changed a source file. Why does
the recompile not see the change?</H4>
<P>The <I>Makefiles</I> do not have the proper dependencies for
include files. You have to do a <I>make clean</I> and then another
<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the
<I>--enable-depend</I> option of <I>configure</I> to have the
compiler compute the dependencies automatically.</P>
<P>There are also commercial and hardware-based replication solutions
available supporting a variety of replication models.</P>
</BODY>
</HTML>

57
doc/src/FAQ/FAQ_DEV.html
View File

@@ -13,7 +13,7 @@
<H1>Developer's Frequently Asked Questions (FAQ) for
PostgreSQL</H1>
<P>Last updated: Sun Mar 13 22:07:18 EST 2005</P>
<P>Last updated: Fri May 6 13:47:54 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -44,8 +44,8 @@
<A href="#1.10">1.10</A>) What books are good for developers?<BR>
<A href="#1.11">1.11</A>) What is configure all about?<BR>
<A href="#1.12">1.12</A>) How do I add a new port?<BR>
<A href="#1.13">1.13</A>) Why don't you use threads/raw
devices/async-I/O, &lt;insert your favorite wizz-bang feature
<A href="#1.13">1.13</A>) Why don't you use threads, raw
devices, async-I/O, &lt;insert your favorite wizz-bang feature
here&gt;?<BR>
<A href="#1.14">1.14</A>) How are RPM's packaged?<BR>
<A href="#1.15">1.15</A>) How are CVS branches handled?<BR>
@@ -71,6 +71,8 @@
<I>pfree</I>() to allocate memory?<BR>
<A href="#2.6">2.6</A>) What is ereport()?<BR>
<A href="#2.7">2.7</A>) What is CommandCounterIncrement()?<BR>
<A href="#2.8">2.8</A>) What debugging features are available?<BR>
<BR>
<HR>
@@ -230,6 +232,12 @@
<P>There have been a number of discussions about other testing
frameworks and some developers are exploring these ideas.</P>
<P>Keep in mind the <I>Makefiles</I> do not have the proper
dependencies for include files. You have to do a <I>make clean</I>
and then another <I>make</I>. If you are using <SMALL>GCC</SMALL>
you can use the <I>--enable-depend</I> option of <I>configure</I>
to have the compiler compute the dependencies automatically.</P>
<H3><A name="1.9">1.9</A>) What tools are available for
developers?</H3>
@@ -422,8 +430,8 @@
handling. There is a <I>backend/port</I> directory if you need
special files for your OS.</P>
<H3><A name="1.13">1.13</A>) Why don't you use threads/raw
devices/async-I/O, &lt;insert your favorite wizz-bang feature
<H3><A name="1.13">1.13</A>) Why don't you use threads, raw
devices, async-I/O, &lt;insert your favorite wizz-bang feature
here&gt;?</H3>
<P>There is always a temptation to use the newest operating system
@@ -941,6 +949,45 @@
modified by previous pieces. <I>CommandCounterIncrement()</I>
increments the Command Counter, creating a new part of the
transaction.</P>
<H3><A name="2.8">2.8</A>) What debugging features are
available?</H3>
<P>First, try 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>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 the <I>postmaster</I> is not running, you can actually run the
<I>postgres</I> backend from the command line, and type your
<SMALL>SQL</SMALL> statement directly. This is recommended
<B>only</B> for debugging purposes. If you have compiled with debugging
symbols, you can use a debugger to see what is happening. Because
the backend was not started from <I>postmaster</I>, it is not
running in an identical environment and locking/backend interaction
problems may not be duplicated.</P>
<P>If the <I>postmaster</I> is running, start <I>psql</I> in one
window, then find the <SMALL>PID</SMALL> of the <I>postgres</I>
process used by <I>psql</I> using <CODE>SELECT pg_backend_pid()</CODE>.
Use a debugger to attach to the <I>postgres</I> <SMALL>PID</SMALL>.
You can set breakpoints in the debugger and issue queries from
<I>psql</I>. If you are debugging <I>postgres</I> startup, you can
set PGOPTIONS="-W n", then start <I>psql</I>. This will cause startup
to delay for <I>n</I> seconds so you can attach to the process with
the debugger, set any breakpoints, and continue through the startup
sequence.</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
file will be put in the client's current directory. Linux requires
a compile with <I>-DLINUX_PROFILE</I> for proper profiling.</P>
</BODY>
</HTML>