the
   <xref linkend="sql-create-access-method"/> and
   <xref linkend="sql-drop-access-method"/> SQL commands.
  </para>

  <para>
   An index access method handler function must be declared to accept a
   single argument of type <type>internal</type> and to return the
   pseudo-type <type>index_am_handler</type>.  The argument is a dummy value that
   simply serves to prevent handler functions from being called directly from
   SQL commands.  The result of the function must be a palloc'd struct of
   type <structname>IndexAmRoutine</structname>, which contains everything
   that the core code needs to know to make use of the index access method.
   The <structname>IndexAmRoutine</structname> struct, also called the access
   method's <firstterm>API struct</firstterm>, includes fields specifying assorted
   fixed properties of the access method, such as whether it can support
   multicolumn indexes.  More importantly, it contains pointers to support
   functions for the access method, which do all of the real work to access
   indexes.  These support functions are plain C functions and are not
   visible or callable at the SQL level.  The support functions are described
   in <xref linkend="index-functions"/>.
  </para>

  <para>
   The structure <structname>IndexAmRoutine</structname> is defined thus:
<programlisting>
typedef struct IndexAmRoutine
{
    NodeTag     type;

    /*
     * Total number of strategies (operators) by which we can traverse/search
     * this AM.  Zero if AM does not have a fixed set of strategy assignments.
     */
    uint16      amstrategies;
    /* total number of support functions that this AM uses */
    uint16      amsupport;
    /* opclass options support function number or 0 */
    uint16      amoptsprocnum;
    /* does AM support ORDER BY indexed column's value? */
    bool        amcanorder;
    /* does AM support ORDER BY result of an operator on indexed column? */
    bool        amcanorderbyop;
    /* does AM support hashing using API consistent with the hash AM? */
    bool        amcanhash;
    /* do operators within an opfamily have consistent equality semantics? */
    bool        amconsistentequality;
    /* do operators within an opfamily have consistent ordering semantics? */
    bool        amconsistentordering;
    /* does AM support backward scanning? */
    bool        amcanbackward;
    /* does AM support UNIQUE indexes? */
    bool        amcanunique;
    /* does AM support multi-column indexes? */
    bool        amcanmulticol;
    /* does AM require scans to have a constraint on the first index column? */
    bool        amoptionalkey;
    /* does AM handle ScalarArrayOpExpr quals? */
    bool        amsearcharray;
    /* does AM handle IS NULL/IS NOT NULL quals? */
    bool        amsearchnulls;
    /* can index storage data type differ from column data type? */
    bool        amstorage;
    /* can an index of this type be clustered on? */
    bool        amclusterable;
    /* does AM handle 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