Commit dad0e20f authored by Bruce Momjian's avatar Bruce Momjian

Mention "PostgreSQL"'s hashes as slower/similar to btree.

parent 1415a838
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.1 2002/06/13 05:15:22 momjian Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.2 2002/06/21 19:06:44 momjian Exp $
--> -->
<chapter id="diskusage"> <chapter id="diskusage">
...@@ -22,10 +22,12 @@ $Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.1 2002/06/13 05:15:22 mo ...@@ -22,10 +22,12 @@ $Header: /cvsroot/pgsql/doc/src/sgml/diskusage.sgml,v 1.1 2002/06/13 05:15:22 mo
</para> </para>
<para> <para>
You can monitor disk space from two places; from inside You can monitor disk space from three places: from
<application>psql</> and from the command line using <application>psql</> using <command>VACUUM</> information, from
<application>contrib/oid2name</>. Using <application>psql</> you can <application>psql</> using <application>contrib/dbsize</>, and from
issue queries to see the disk usage for any table: the command line using <application>contrib/oid2name</>. Using
<application>psql</> on a recently vacuumed (or analyzed) database,
you can issue queries to see the disk usage of any table:
<programlisting> <programlisting>
play=# SELECT relfilenode, relpages play=# SELECT relfilenode, relpages
play-# FROM pg_class play-# FROM pg_class
...@@ -38,10 +40,10 @@ play-# WHERE relname = 'customer'; ...@@ -38,10 +40,10 @@ play-# WHERE relname = 'customer';
</para> </para>
<para> <para>
Each page is typically 8 kilobytes. <literal>relpages</> is only Each page is typically 8 kilobytes. (Remember, <literal>relpages</>
updated by <command>VACUUM</> and <command>ANALYZE</>. To show the is only updated by <command>VACUUM</> and <command>ANALYZE</>.) To
space used by <acronym>TOAST</> tables, use a query based on the heap show the space used by <acronym>TOAST</> tables, use a query based on
relfilenode: the heap relfilenode shown above:
<programlisting> <programlisting>
play=# SELECT relname, relpages play=# SELECT relname, relpages
play-# FROM pg_class play-# FROM pg_class
......
<!-- $Header: /cvsroot/pgsql/doc/src/sgml/indices.sgml,v 1.33 2002/06/21 16:52:00 momjian Exp $ --> <!-- $Header: /cvsroot/pgsql/doc/src/sgml/indices.sgml,v 1.34 2002/06/21 19:06:44 momjian Exp $ -->
<chapter id="indexes"> <chapter id="indexes">
<title id="indexes-title">Indexes</title> <title id="indexes-title">Indexes</title>
...@@ -181,10 +181,11 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> ...@@ -181,10 +181,11 @@ CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable>
</synopsis> </synopsis>
<note> <note>
<para> <para>
Testing has shown hash indexes to be similar or slower than btree Testing has shown PostgreSQL's hash indexes to be similar or slower
indexes, and the index size and build time for hash indexes is much than btree indexes, and the index size and build time for hash
worse. Hash indexes also suffer poor performance under high indexes is much worse. Hash indexes also suffer poor performance
concurrency. For these reasons, hash index use is discouraged. under high concurrency. For these reasons, hash index use is
discouraged.
</para> </para>
</note> </note>
</para> </para>
......
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_index.sgml,v 1.33 2002/06/21 16:52:00 momjian Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_index.sgml,v 1.34 2002/06/21 19:06:44 momjian Exp $
PostgreSQL documentation PostgreSQL documentation
--> -->
...@@ -330,10 +330,11 @@ ERROR: Cannot create index: 'index_name' already exists. ...@@ -330,10 +330,11 @@ ERROR: Cannot create index: 'index_name' already exists.
the <literal>=</literal> operator. the <literal>=</literal> operator.
</para> </para>
<para> <para>
Testing has shown hash indexes to be similar or slower than btree Testing has shown PostgreSQL's hash indexes to be similar or slower
indexes, and the index size and build time for hash indexes is much than btree indexes, and the index size and build time for hash
worse. Hash indexes also suffer poor performance under high indexes is much worse. Hash indexes also suffer poor performance
concurrency. For these reasons, hash index use is discouraged. under high concurrency. For these reasons, hash index use is
discouraged.
</para> </para>
<para> <para>
......
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