<structname>pg_cast</structname> entry has different source and
   target types and a function that takes more than one argument, it
   represents converting from one type to another and applying a length
   coercion in a single step.  When no such entry is available, coercion
   to a type that uses a type modifier involves two steps, one to
   convert between data types and a second to apply the modifier.
  </para>
 </sect1>

 <sect1 id="catalog-pg-class">
  <title><structname>pg_class</structname></title>

  <indexterm zone="catalog-pg-class">
   <primary>pg_class</primary>
  </indexterm>

  <para>
   The catalog <structname>pg_class</structname> describes tables and
   other objects that have columns or are otherwise similar to a
   table.  This includes indexes (but see also <link
   linkend="catalog-pg-index"><structname>pg_index</structname></link>),
   sequences (but see also <link
   linkend="catalog-pg-sequence"><structname>pg_sequence</structname></link>),
   views, materialized views, composite types, and TOAST tables;
   see <structfield>relkind</structfield>.
   Below, when we mean all of these kinds of objects we speak of
   <quote>relations</quote>.  Not all of <structname>pg_class</structname>'s
   columns are meaningful for all relation kinds.
  </para>

  <table>
   <title><structname>pg_class</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>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>oid</structfield> <type>oid</type>
      </para>
      <para>
       Row identifier
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relname</structfield> <type>name</type>
      </para>
      <para>
       Name of the table, index, view, etc.
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relnamespace</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-namespace"><structname>pg_namespace</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the namespace that contains this relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reltype</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       The OID of the data type that corresponds to this table's row type,
       if any; zero for indexes, sequences, and toast tables, which have
       no <structname>pg_type</structname> entry
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>reloftype</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-type"><structname>pg_type</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       For typed tables, the OID of the underlying composite type;
       zero for all other relations
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relowner</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-authid"><structname>pg_authid</structname></link>.<structfield>oid</structfield>)
      </para>
      <para>
       Owner of the relation
      </para></entry>
     </row>

     <row>
      <entry role="catalog_table_entry"><para role="column_definition">
       <structfield>relam</structfield> <type>oid</type>
       (references <link linkend="catalog-pg-am"><structname>pg_am</structname></link>.<structfield>oid</structfield>)