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FAQ updates from Ian Barwick.

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@ -14,7 +14,7 @@
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Fri Oct 11 13:54:56 EDT 2002</P>
<P>Last updated: Sun Oct 13 22:49:56 EDT 2002</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -22,15 +22,14 @@
<P>The most recent version of this document can be viewed at <A
href=
"http://www.Postgresql.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
"http://www.PostgreSQL.org/docs/faq-english.html">http://www.PostgreSQL.org/docs/faq-english.html</A>.</P>
<P>Platform-specific questions are answered at <A href=
"http://www.PostgreSQL.org/users-lounge/docs/faq.html">http://www.PostgreSQL.org/users-lounge/docs/faq.html</A>.</P>
<HR>
<H2 align="center">General Questions</H2>
<A href="#1.1">1.1</A>) What is PostgreSQL? How is it
pronounced?<BR>
<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="#1.3">1.3</A>) What Unix platforms does PostgreSQL run
on?<BR>
@ -161,7 +160,7 @@
<H2 align="center">General Questions</H2>
<H4><A name="1.1">1.1</A>) What is PostgreSQL?</H4>
<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>
@ -172,12 +171,12 @@
extended subset of <SMALL>SQL</SMALL>. PostgreSQL is free and the
complete source is available.</P>
<P>PostgreSQL development is performed by a team of Internet
<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
below on how to join). This team is now responsible for all
development of PostgreSQL.</P>
section <a href="#1.6">1.6</a> on how to join). This team is now
responsible for all development of PostgreSQL.</P>
<P>The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen.
Many others have contributed to the porting, testing, debugging,
@ -239,10 +238,10 @@
<P><STRONG>Client</STRONG></P>
<P>It is possible to compile the <I>libpq</I> C library, psql, and
other interfaces and binaries to run on MS Windows platforms. In
this case, the client is running on MS Windows, and communicates
other interfaces and client applications to run on MS Windows platforms.
In this case, the client is running on MS Windows, and communicates
via TCP/IP to a server running on one of our supported Unix
platforms. A file <I>win31.mak</I> is included in the distribution
platforms. A file <I>win32.mak</I> is included in the distribution
for making a Win32 <I>libpq</I> library and <I>psql</I>. PostgreSQL
also communicates with <SMALL>ODBC</SMALL> clients.</P>
@ -250,10 +249,11 @@
<P>The database server can run on Windows NT and Win2k using
Cygwin, the Cygnus Unix/NT porting library. See
<I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the <A href=
"http://www.postgresql.org/docs/faq-mswin.html">MS Windows FAQ</A>
on our web site. We have no plan to do a native port to any
Microsoft platform.</P>
<I>pgsql/doc/FAQ_MSWIN</I> in the distribution or the MS Windows FAQ
at <A href="http://www.PostgreSQL.org/docs/faq-mswin.html">http://www.PostgreSQL.org/docs/faq-mswin.html</A>.</P>
<p>A native port to some Microsoft platforms is currently being worked
upon.</p>
<H4><A name="1.5">1.5</A>) Where can I get PostgreSQL?</H4>
@ -318,7 +318,7 @@
'#PostgreSQL' "$USER" irc.phoenix.net.</CODE></P>
<P>A list of commercial support companies is available at <A href=
"http://www.postgresql.org/users-lounge/commercial-support.html">http://www.postgresql.org/users-lounge/commercial-support.html</A>.</P>
"http://www.PostgreSQL.org/users-lounge/commercial-support.html">http://www.PostgreSQL.org/users-lounge/commercial-support.html</A>.</P>
<H4><A name="1.7">1.7</A>) What is the latest release?</H4>
@ -339,10 +339,10 @@
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
There is a list of PostgreSQL books available for purchase at <A
href=
"http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
"http://www.ca.PostgreSQL.org/books/">http://www.ca.PostgreSQL.org/books/</A>.
There is also a collection of PostgreSQL technical articles at <A
href=
"http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
"http://techdocs.PostgreSQL.org/">http://techdocs.PostgreSQL.org/</A>.</P>
<P><I>psql</I> has some nice \d commands to show information about
types, operators, functions, aggregates, etc.</P>
@ -402,10 +402,10 @@
<H4><A name="1.13">1.13</A>) How do I submit a bug report?</H4>
<P>Please visit the <A href=
"http://www.postgresql.org/bugs/bugs.php">PostgreSQL BugTool</A>
page, which gives guidelines and directions on how to submit a
bug.</P>
<P>Please visit the PostgreSQL BugTool page at <A href=
"http://www.PostgreSQL.org/bugs/bugs.php">http://www.PostgreSQL.org/bugs/bugs.php</A>,
which gives guidelines and directions on how to submit a
bug report.</P>
<P>Also check out our ftp site <A href=
"ftp://ftp.PostgreSQL.org/pub">ftp://ftp.PostgreSQL.org/pub</A> to
@ -460,14 +460,14 @@
<DT><B>Support</B></DT>
<DD>Our mailing list provides a large group of developers and
users to help resolve any problems encountered. While we can not
<DD>Our mailing lists provide contact with a large group of developers
and users to help resolve any problems encountered. While we cannot
guarantee a fix, commercial <SMALL>DBMS</SMALL>s do not always
supply a fix either. Direct access to developers, the user
community, manuals, and the source code often make PostgreSQL
support superior to other <SMALL>DBMS</SMALL>s. There is
commercial per-incident support available for those who need it.
(See <A href="#1.6">support FAQ item</A>.)<BR>
(See <A href="#1.6">FAQ section 1.6</A>.)<BR>
<BR>
</DD>
@ -484,7 +484,7 @@
PostgreSQL?</H4>
<P>PostgreSQL has had a first-class infrastructure since we started
six years ago. This is all thanks to Marc Fournier, who has created
in 1994. 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
@ -494,9 +494,7 @@
<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=
"https://store.pgsql.com/shopping/index.php?id=1">
https://store.pgsql.com/shopping/index.php?id=1</A>
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
@ -538,28 +536,23 @@
<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>There is also one at <A href=
"http://www.phone.net/home/mwm/hotlist/">http://www.phone.net/home/mwm/hotlist/.</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 complex cases, many use the Perl interface and CGI.pm.</P>
<P>For complex cases, many use the Perl interface and CGI.pm or mod_perl.</P>
<H4><A name="2.3">2.3</A>) Does PostgreSQL have a graphical user
interface? A report generator? An embedded query language
interface?</H4>
<P>We have a nice graphical user interface called <I>pgaccess</I>,
which is shipped as part of the distribution. <I>pgaccess</I> also
has a report generator. The Web page is <A href=
"http://www.flex.ro/pgaccess">http://www.flex.ro/pgaccess</A></P>
<P>We have a nice graphical user interface called PgAccess, which is
shipped as part of the distribution. PgAccess also has a report
generator. The Web page is <A href="http://www.pgaccess.org/">http://www.pgaccess.org/</A>.</P>
<P>We also include <I>ecpg</I>, which is an embedded SQL query
language interface for C.</P>
<H4><A name="2.4">2.4</A>) What languages are available to
communicate with PostgreSQL?</H4>
<H4><A name="2.4">2.4</A>) What languages are able to communicate with PostgreSQL?</H4>
<P>We have:</P>
@ -572,7 +565,7 @@
<LI>Java (jdbc)</LI>
<LI>Perl (perl5)</LI>
<LI>Perl (DBD::Pg)</LI>
<LI>ODBC (odbc)</LI>
@ -582,12 +575,10 @@
<LI>C Easy API (libpgeasy)</LI>
<LI>Embedded <SMALL>HTML</SMALL> (<A href=
"http://www.php.net">PHP from http://www.php.net</A>)</LI>
<LI>PHP ('pg_' functions, Pear::DB)</LI>
</UL>
<P>Additional interfaces are available at <a
href="http://www.postgresql.org/interfaces.html">
http://www.postgresql.org/interfaces.html.</A>
<P>Additional interfaces are available at
<a href="http://www.PostgreSQL.org/interfaces.html">http://www.PostgreSQL.org/interfaces.html</A>.
</P>
<HR>
@ -774,7 +765,7 @@
the MaxBackendId constant in
<I>include/storage/sinvaladt.h</I>.</P>
<H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I>
<H4><A name="3.9">3.9</A>) What are the <I>pg_tempNNN.NN</I>
files in my database directory?</H4>
<P>They are temporary files generated by the query executor. For
@ -793,16 +784,16 @@
<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 often change the internal format of system
tables and data files. These changes are often complex, so we don't
maintain backward compatability for data files. A dump outputs data
in a generic format that can then be loaded in using the new internal
format.
However, major releases (e.g. from 7.2 to 7.3) often change the internal
format of system tables and data files. These changes are often complex,
so we don't maintain backward compatability for data files. A dump outputs
data in a generic format that can then be loaded in using the new internal
format.</P>
<P>In releases where the on-disk format does not change, the
<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.
release.</p>
<HR>
@ -821,7 +812,7 @@
<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 a query that has an <SMALL>ORDER
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
@ -840,8 +831,8 @@
<H4><A name="4.4">4.4</A>) How do you remove a column from a
table?</H4>
<P>We do not support <SMALL>ALTER TABLE DROP COLUMN,</SMALL> but do
this:</P>
<P>Prior to version 7.3, <SMALL>ALTER TABLE DROP COLUMN</SMALL> is not supported.
You can do this instead:</P>
<PRE>
BEGIN;
LOCK TABLE old_table;
@ -892,7 +883,7 @@
be estimated as 6.4 MB:</P>
<PRE>
36 bytes: each row header (approximate)
24 bytes: one int field and one text filed
24 bytes: one int field and one text field
+ 4 bytes: pointer on page to tuple
----------------------------------------
64 bytes per row
@ -957,7 +948,7 @@
SELECT col
FROM tab
ORDER BY col [ DESC ]
LIMIT 1
LIMIT 1;
</PRE>
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
@ -972,10 +963,10 @@
</UL>
<LI>The search string can not start with a character class,
e.g. [a-e].</LI>
<LI>Case-insensitive searches like <SMALL>ILIKE</SMALL> and
<I>~*</I> can not be used. Instead, use functional
indexes, which are described later in this FAQ.</LI>
<LI>The default <I>C</I> local must be used during
<LI>Case-insensitive searches such as <SMALL>ILIKE</SMALL> and
<I>~*</I> do not utilise indexes. Instead, use functional
indexes, which are described in section <a href="#4.12">4.12</a>.</LI>
<LI>The default <I>C</I> locale must be used during
<i>initdb.</i></LI>
</UL>
<P>
@ -1032,13 +1023,13 @@
<PRE>
SELECT *
FROM tab
WHERE lower(col) = 'abc'
WHERE LOWER(col) = 'abc';
</PRE>
This will not use an standard index. However, if you create a
functional index, it will be used:
<PRE>
CREATE INDEX tabindex on tab (lower(col));
CREATE INDEX tabindex ON tab (LOWER(col));
</PRE>
<H4><A name="4.13">4.13</A>) In a query, how do I detect if a field
@ -1053,8 +1044,8 @@
Type Internal Name Notes
--------------------------------------------------
"char" char 1 character
CHAR(#) bpchar blank padded to the specified fixed length
VARCHAR(#) varchar size specifies maximum length, no padding
CHAR(n) bpchar blank padded to the specified fixed length
VARCHAR(n) varchar size specifies maximum length, no padding
TEXT text no specific upper limit on length
BYTEA bytea variable-length byte array (null-byte safe)
</PRE>
@ -1069,8 +1060,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk
might also be less than expected.</P>
<P><SMALL>CHAR()</SMALL> is best when storing strings that are
usually the same length. <SMALL>VARCHAR()</SMALL> is best when
<P><SMALL>CHAR(n)</SMALL> is best when storing strings that are
usually the same length. <SMALL>VARCHAR(n)</SMALL> is best when
storing variable-length strings but it limits how long a string can
be. <SMALL>TEXT</SMALL> is for strings of unlimited length, maximum
1 gigabyte. <SMALL>BYTEA</SMALL> is for storing binary data,
@ -1111,11 +1102,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>One approach is to retrieve the next <SMALL>SERIAL</SMALL> value
from the sequence object with the <I>nextval()</I> function
<I>before</I> inserting and then insert it explicitly. Using the
example table in <A href="#4.15.1">4.15.1</A>, that might look like
this in Perl:</P>
example table in <A href="#4.15.1">4.15.1</A>, an example in a
pseudo-language would look like this:</P>
<PRE>
new_id = output of "SELECT nextval('person_id_seq')"
INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal');
new_id = execute("SELECT nextval('person_id_seq')");
execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')");
</PRE>
You would then also have the new value stored in
@ -1124,14 +1115,14 @@ BYTEA bytea variable-length byte array (null-byte safe)
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.
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>
<PRE>
INSERT INTO person (name) VALUES ('Blaise Pascal');
new_id = output of "SELECT currval('person_id_seq')";
execute("INSERT INTO person (name) VALUES ('Blaise Pascal')");
new_id = execute("SELECT currval('person_id_seq')");
</PRE>
Finally, you could use the <A href="#4.16"><SMALL>OID</SMALL></A>
@ -1139,12 +1130,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
default value, though this is probably the least portable approach.
In Perl, using DBI with Edmund Mergl's DBD::Pg module, the oid
value is made available via <I>$sth-&gt;{pg_oid_status}</I> after
<I>$sth-&gt;execute()</I>.
<I>$sth-&gt;execute()</I>.
<H4><A name="4.15.3">4.15.3</A>) Don't <I>currval()</I> and
<I>nextval()</I> lead to a race condition with other users?</H4>
<P>No. Currval() returns the current value assigned by your
<P>No. <i>currval</i>() returns the current value assigned by your
backend, not by all users.</P>
<H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
@ -1163,7 +1154,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
Every row that is created in PostgreSQL gets a unique
<SMALL>OID</SMALL>. All <SMALL>OID</SMALL>s generated during
<I>initdb</I> are less than 16384 (from
<I>backend/access/transam.h</I>). All user-created
<I>include/access/transam.h</I>). All user-created
<SMALL>OID</SMALL>s are equal to or greater than this. By default,
all these <SMALL>OID</SMALL>s are unique not only within a table or
database, but unique within the entire PostgreSQL installation.</P>
@ -1186,12 +1177,11 @@ BYTEA bytea variable-length byte array (null-byte safe)
COPY new TO '/tmp/pgtable';
DELETE FROM new;
COPY new WITH OIDS FROM '/tmp/pgtable';
</PRE>
<!--
CREATE TABLE new_table (mycol int);
INSERT INTO new_table (oid, mycol) SELECT oid, mycol FROM old_table;
-->
</PRE>
<P>O<SMALL>ID</SMALL>s are stored as 4-byte integers, and will
overflow at 4 billion. No one has reported this ever happening, and
we plan to have the limit removed before anyone does.</P>
@ -1228,7 +1218,7 @@ BYTEA bytea variable-length byte array (null-byte safe)
</UL>
<P>A list of general database terms can be found at: <A href=
"http://www.comptechnews.com/~reaster/dbdesign.html">http://www.comptechnews.com/~reaster/dbdesign.html</A></P>
"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="4.18">4.18</A>) Why do I get the error <I>"ERROR:
Memory exhausted in AllocSetAlloc()"</I>?</H4>
@ -1247,12 +1237,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
and all subprocesses created after the command is run. If you are
having a problem with the <SMALL>SQL</SMALL> client because the
backend is returning too much data, try it before starting the
client.
client.
<H4><A name="4.19">4.19</A>) How do I tell what PostgreSQL version
I am running?</H4>
<P>From <I>psql</I>, type <CODE>select version();</CODE></P>
<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P>
<H4><A name="4.20">4.20</A>) Why does my large-object operations
get <I>"invalid large obj descriptor"</I>?</H4>
@ -1288,22 +1278,18 @@ BYTEA bytea variable-length byte array (null-byte safe)
returns many rows, <CODE><SMALL>IN</SMALL></CODE> is fastest. To
speed up other queries, replace <CODE>IN</CODE> with
<CODE>EXISTS</CODE>:</P>
<PRE>
<CODE>SELECT *
<PRE> SELECT *
FROM tab
WHERE col IN (SELECT subcol FROM subtab)
</CODE>
WHERE col IN (SELECT subcol FROM subtab);
</PRE>
to:
<PRE>
<CODE>SELECT *
to:
<PRE> SELECT *
FROM tab
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col)
</CODE>
WHERE EXISTS (SELECT subcol FROM subtab WHERE subcol = col);
</PRE>
For this to be fast, <CODE>subcol</CODE> should be an indexed column.
We hope to fix this limitation in a future release.
We hope to fix this limitation in a future release.
<H4><A name="4.23">4.23</A>) How do I perform an outer join?</H4>
@ -1362,8 +1348,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>You can return result sets from PL/pgSQL functions using
<I>refcursors</I>. See <A href=
"http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html">
http://developer.postgresql.org/docs/postgres/plpgsql-cursors.html,</A>
"http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html">
http://www.PostgreSQL.org/idocs/index.php?plpgsql-cursors.html</A>,
section 23.7.3.3.</P>
<H4><A name="4.26">4.26</A>) Why can't I reliably create/drop
@ -1381,13 +1367,12 @@ BYTEA bytea variable-length byte array (null-byte safe)
<P>There are several master/slave replication options available.
These allow only the master to make database changes and the slave
can only do database reads. The bottom of <a
href="http://gborg.postgresql.org/genpage?replication_research">
http://gborg.postgresql.org/genpage?replication_research</a> lists
href="http://gborg.PostgreSQL.org/genpage?replication_research">
http://gborg.PostgreSQL.org/genpage?replication_research</a> lists
them. A multi-master replication solution is being worked on at <a
href="http://gborg.postgresql.org/project/pgreplication/projdisplay.
php">http://gborg.postgresql.org/project/pgreplication/projdisplay.php</a>.</P>
href="http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php">http://gborg.PostgreSQL.org/project/pgreplication/projdisplay.php</a>.</P>
<H4><A name="4.27">4.27</A>) What encryption options are available?
<H4><A name="4.28">4.28</A>) What encryption options are available?
</H4>
<UL>
<LI><I>/contrib/pgcrypto</I> contains many encryption functions for
@ -1395,8 +1380,8 @@ BYTEA bytea variable-length byte array (null-byte safe)
<LI>The only way to encrypt transmission from the client to the
server is by using <I>hostssl</I> in <I>pg_hba.conf</I>.</LI>
<LI>Database user passwords are automatically encrypted when stored
in version 7.3. In previous versions, you must enable
<I>password_encryption</I> in <I>postgresql.conf</I>.</LI>
in version 7.3. In previous versions, you must enable the option
<i>PASSWORD_ENCRYPTION</i> in <i>postgresql.conf</i>.</LI>
<LI>The server can run using an encrypted file system.</LI>
</UL>