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Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Sat Apr 23 14:56:41 EDT 2005
Last updated: Sat Apr 23 14:59:01 EDT 2005
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
......@@ -703,8 +703,7 @@
From psql, type SELECT version();
4.15) Why does my large-object operations get "invalid large obj
descriptor"?
4.15) Why does my large-object operations get "invalid large obj descriptor"?
You need to put BEGIN WORK and COMMIT around any use of a large object
handle, that is, surrounding lo_open ... lo_close.
......@@ -785,8 +784,8 @@
Extending PostgreSQL
5.1) I wrote a user-defined function. When I run it in psql, why does it
dump core?
5.1) I wrote a user-defined function. When I run it in psql, why does it dump
core?
The problem could be a number of things. Try testing your user-defined
function in a stand-alone test program first.
......@@ -804,8 +803,7 @@
table-returning function defined in C can be found in
contrib/tablefunc.
5.4) I have changed a source file. Why does the recompile not see the
change?
5.4) I have changed a source file. Why does the recompile not see the change?
The Makefiles do not have the proper dependencies for include files.
You have to do a make clean and then another make. If you are using
......
doc/src/FAQ/FAQ.html
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......@@ -10,7 +10,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Sat Apr 23 14:56:41 EDT 2005</P>
<P>Last updated: Sat Apr 23 14:59:01 EDT 2005</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)
......@@ -133,7 +133,7 @@
<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 +152,8 @@
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
......@@ -185,7 +185,7 @@
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
......@@ -204,14 +204,14 @@
"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>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
......@@ -230,7 +230,7 @@
"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">
......@@ -240,14 +240,14 @@
"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 8.0.2.</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>
......@@ -272,15 +272,15 @@
<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://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>First, consider the PostgreSQL-specific books mentioned above.
Another one is "Teach Yourself SQL in 21 Days, Second Edition"
......@@ -298,14 +298,14 @@
and at <A href=
"http://sqlcourse.com/">http://sqlcourse.com.</A></P>
<H4><A name="1.11">1.11</A>) How do I join the development
team?</H4>
<H3><A name="1.11">1.11</A>) How do I join the development
team?</H3>
<P>See the <a href="http://www.postgresql.org/docs/faqs.FAQ_DEV.html">
Developer's FAQ</A>.
<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>
......@@ -368,8 +368,8 @@
<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
......@@ -383,8 +383,8 @@
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>
......@@ -396,8 +396,8 @@
<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, see <a href="http://techdocs.postgresql.org/guides/GUITools">
http://techdocs.postgresql.org/guides/GUITools</a> for a
......@@ -408,14 +408,14 @@
<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
......@@ -424,8 +424,8 @@
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>
......@@ -481,15 +481,15 @@
</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>
<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
......@@ -497,8 +497,8 @@
<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 7.4.0 to 7.4.1 does not require a dump and restore.
......@@ -508,7 +508,7 @@
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
......@@ -522,8 +522,8 @@
<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
......@@ -541,9 +541,9 @@
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
......@@ -564,7 +564,7 @@
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>
......@@ -580,8 +580,8 @@
<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>
......@@ -611,8 +611,8 @@ 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>
......@@ -648,8 +648,8 @@ 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 used by every query. Indexes are used only if the
table is larger than a minimum size, and the query selects only a
......@@ -712,14 +712,14 @@ 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
......@@ -740,9 +740,9 @@ table?</TD><TD>unlimited</TD></TR>
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>
......@@ -764,8 +764,8 @@ 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 >
<TR><TH>Type</TH><TH>Internal Name</TH><TH>Notes</TH></TR>
......@@ -801,8 +801,8 @@ 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,
......@@ -826,8 +826,8 @@ length</TD></TR>
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
......@@ -855,23 +855,23 @@ length</TD></TR>
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>TID</SMALL>?</H3>
<P>Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>.
......@@ -893,8 +893,8 @@ length</TD></TR>
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
......@@ -912,13 +912,13 @@ 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>
<H3><A name="4.15">4.15</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H3>
<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE>
around any use of a large object handle, that is, surrounding
......@@ -933,15 +933,15 @@ length</TD></TR>
<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="4.16">4.16</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="4.17">4.17</A>) How do I perform an outer join?</H3>
<P>PostgreSQL supports outer joins using the SQL standard syntax.
Here are two examples:</P>
......@@ -964,8 +964,8 @@ 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="4.18">4.18</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
......@@ -976,16 +976,16 @@ 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="4.19">4.19</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="4.20">4.20</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
......@@ -995,8 +995,8 @@ 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>
<H3><A name="4.21">4.21</A>) What encryption options are available?
</H3>
<UL>
<LI><I>contrib/pgcrypto</I> contains many encryption functions for
use in <SMALL>SQL</SMALL> queries.</LI>
......@@ -1016,21 +1016,21 @@ length</TD></TR>
<H2 align="center">Extending PostgreSQL</H2>
<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>
<H3><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?</H3>
<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>
<H3><A name="5.2">5.2</A>) How can I contribute some nifty new
types and functions to PostgreSQL?</H3>
<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>
<H3><A name="5.3">5.3</A>) How do I write a C function to return a
tuple?</H3>
<P>In versions of PostgreSQL beginning with 7.3, table-returning
functions are fully supported in C, PL/PgSQL, and SQL. See the
......@@ -1038,8 +1038,8 @@ length</TD></TR>
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>
<H3><A name="5.4">5.4</A>) I have changed a source file. Why does
the recompile not see the change?</H3>
<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
......
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