role="column_definition">
       <structfield>aggsortop</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-operator"><structname>pg_operator</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Associated sort operator (zero if none)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>aggtranstype</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Data type of the aggregate function's internal transition (state) data
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>aggtransspace</structfield> <type>int4</type>
      </para>
      <para>
       Approximate average size (in bytes) of the transition state
       data, or zero to use a default estimate
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>aggmtranstype</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Data type of the aggregate function's internal transition (state)
       data for moving-aggregate mode (zero if none)
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>aggmtransspace</structfield> <type>int4</type>
      </para>
      <para>
       Approximate average size (in bytes) of the transition state data
       for moving-aggregate mode, or zero to use a default estimate
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>agginitval</structfield> <type>text</type>
      </para>
      <para>
       The initial value of the transition state.  This is a text
       field containing the initial value in its external string
       representation.  If this field is null, the transition state
       value starts out null.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>aggminitval</structfield> <type>text</type>
      </para>
      <para>
       The initial value of the transition state for moving-aggregate mode.
       This is a text field containing the initial value in its external
       string representation.  If this field is null, the transition state
       value starts out null.
      </para></entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   New aggregate functions are registered with the <link
   linkend="sql-createaggregate"><command>CREATE AGGREGATE</command></link>
   command.  See <xref linkend="xaggr"/> for more information about
   writing aggregate functions and the meaning of the transition
   functions, etc.
  </para>

 </sect1>


 <sect1 id="catalog-pg-am">
  <title><structname>pg_am</structname></title>

  <indexterm zone="catalog-pg-am">
   <primary>pg_am</primary>
  </indexterm>

  <para>
   The catalog <structname>pg_am</structname> stores information about
   relation access methods.  There is one row for each access method supported
   by the system.
   Currently, only tables and indexes have access methods. The requirements for table
   and index access methods are discussed in detail in <xref linkend="tableam"/> and
   <xref linkend="indexam"/> respectively.
  </para>

  <table>
   <title><structname>pg_am</structname> Columns</title>
   <tgroup cols="1">
    <thead>
     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       Column Type
      </para>
      <para>
       Description
      </para></entry>
     </row>
    </thead>

    <tbody>
     <row>