predicate locks? */
    bool        ampredlocks;
    /* does AM support parallel scan? */
    bool        amcanparallel;
    /* does AM support parallel build? */
    bool        amcanbuildparallel;
    /* does AM support columns included with clause INCLUDE? */
    bool        amcaninclude;
    /* does AM use maintenance_work_mem? */
    bool        amusemaintenanceworkmem;
    /* does AM summarize tuples, with at least all tuples in the block
     * summarized in one summary */
    bool        amsummarizing;
    /* OR of parallel vacuum flags */
    uint8       amparallelvacuumoptions;
    /* type of data stored in index, or InvalidOid if variable */
    Oid         amkeytype;

    /* interface functions */
    ambuild_function ambuild;
    ambuildempty_function ambuildempty;
    aminsert_function aminsert;
    aminsertcleanup_function aminsertcleanup;
    ambulkdelete_function ambulkdelete;
    amvacuumcleanup_function amvacuumcleanup;
    amcanreturn_function amcanreturn;   /* can be NULL */
    amcostestimate_function amcostestimate;
    amgettreeheight_function amgettreeheight;   /* can be NULL */
    amoptions_function amoptions;
    amproperty_function amproperty;     /* can be NULL */
    ambuildphasename_function ambuildphasename;   /* can be NULL */
    amvalidate_function amvalidate;
    amadjustmembers_function amadjustmembers; /* can be NULL */
    ambeginscan_function ambeginscan;
    amrescan_function amrescan;
    amgettuple_function amgettuple;     /* can be NULL */
    amgetbitmap_function amgetbitmap;   /* can be NULL */
    amendscan_function amendscan;
    ammarkpos_function ammarkpos;       /* can be NULL */
    amrestrpos_function amrestrpos;     /* can be NULL */

    /* interface functions to support parallel index scans */
    amestimateparallelscan_function amestimateparallelscan;    /* can be NULL */
    aminitparallelscan_function aminitparallelscan;    /* can be NULL */
    amparallelrescan_function amparallelrescan;    /* can be NULL */

    /* interface functions to support planning */
    amtranslate_strategy_function amtranslatestrategy;  /* can be NULL */
    amtranslate_cmptype_function amtranslatecmptype;    /* can be NULL */
} IndexAmRoutine;
</programlisting>
  </para>

  <para>
   To be useful, an index access method must also have one or more
   <firstterm>operator families</firstterm> and
   <firstterm>operator classes</firstterm> defined in
   <link linkend="catalog-pg-opfamily"><structname>pg_opfamily</structname></link>,
   <link linkend="catalog-pg-opclass"><structname>pg_opclass</structname></link>,
   <link linkend="catalog-pg-amop"><structname>pg_amop</structname></link>, and
   <link linkend="catalog-pg-amproc"><structname>pg_amproc</structname></link>.
   These entries allow the planner
   to determine what kinds of query qualifications can be used with
   indexes of this access method.  Operator families and classes are described
   in <xref linkend="xindex"/>, which is prerequisite material for reading
   this chapter.
  </para>

  <para>
   An individual index is defined by a
   <link linkend="catalog-pg-class"><structname>pg_class</structname></link>
   entry that describes it as a physical relation, plus a
   <link linkend="catalog-pg-index"><structname>pg_index</structname></link>
   entry that shows the logical content of the index &mdash; that is, the set
   of index columns it has and the semantics of those columns, as captured by
   the associated operator classes.  The index columns (key values) can be
   either simple columns of the underlying table or expressions over the table
   rows.  The index access method normally has no interest in where the index
   key values come from (it is always handed precomputed key values) but it
   will be very interested in the operator class information in
   <structname>pg_index</structname>.  Both of these catalog entries can be
   accessed as part of the <structname>Relation</structname> data structure that is
   passed to all