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Documentation of the system catalogs, directed toward PostgreSQL developers

$PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.95 2005/01/05 23:42:03 tgl Exp $

$PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.96 2005/02/13 03:04:15 tgl Exp $

-->

<chapter id="catalogs">

</indexterm>

<para>

The catalog <structname>pg_am</structname> stores information about index access

methods. There is one row for each index access method supported by

the system.

The catalog <structname>pg_am</structname> stores information about index

access methods. There is one row for each index access method supported by

the system. The contents of this catalog are discussed in detail in

<xref linkend="indexam">.

</para>

<table>

</tgroup>

</table>

<para>

An index access method that supports multiple columns (has

<structfield>amcanmulticol</structfield> true) <emphasis>must</>

support indexing null values in columns after the first, because the planner

will assume the index can be used for queries on just the first

column(s). For example, consider an index on (a,b) and a query with

<literal>WHERE a = 4</literal>. The system will assume the index can be used to scan for

rows with <literal>a = 4</literal>, which is wrong if the index omits rows where <literal>b</> is null.

It is, however, OK to omit rows where the first indexed column is null.

(GiST currently does so.)

<structfield>amindexnulls</structfield> should be set true only if the

index access method indexes all rows, including arbitrary combinations of null values.

</para>

</sect1>

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<!-- $PostgreSQL: pgsql/doc/src/sgml/filelist.sgml,v 1.42 2005/02/13 03:04:15 tgl Exp $ -->

<!entity history SYSTEM "history.sgml">

<!entity info SYSTEM "info.sgml">

<!entity catalogs SYSTEM "catalogs.sgml">

<!entity geqo SYSTEM "geqo.sgml">

<!entity gist SYSTEM "gist.sgml">

<!entityindexcostSYSTEM "indexcost.sgml">

<!entityindexamSYSTEM "indexam.sgml">

<!entity nls SYSTEM "nls.sgml">

<!entity plhandler SYSTEM "plhandler.sgml">

<!entity protocol SYSTEM "protocol.sgml">