struct, which the function must fill with result data.
<programlisting>
typedef struct spgConfigIn
{
    Oid         attType;        /* Data type to be indexed */
} spgConfigIn;

typedef struct spgConfigOut
{
    Oid         prefixType;     /* Data type of inner-tuple prefixes */
    Oid         labelType;      /* Data type of inner-tuple node labels */
    Oid         leafType;       /* Data type of leaf-tuple values */
    bool        canReturnData;  /* Opclass can reconstruct original data */
    bool        longValuesOK;   /* Opclass can cope with values &gt; 1 page */
} spgConfigOut;
</programlisting>

      <structfield>attType</structfield> is passed in order to support polymorphic
      index operator classes; for ordinary fixed-data-type operator classes, it
      will always have the same value and so can be ignored.
     </para>

     <para>
      For operator classes that do not use prefixes,
      <structfield>prefixType</structfield> can be set to <literal>VOIDOID</literal>.
      Likewise, for operator classes that do not use node labels,
      <structfield>labelType</structfield> can be set to <literal>VOIDOID</literal>.
      <structfield>canReturnData</structfield> should be set true if the operator class
      is capable of reconstructing the originally-supplied index value.
      <structfield>longValuesOK</structfield> should be set true only when the
      <structfield>attType</structfield> is of variable length and the operator
      class is capable of segmenting long values by repeated suffixing
      (see <xref linkend="spgist-limits"/>).
     </para>

     <para>
      <structfield>leafType</structfield> should match the index storage type
      defined by the operator class's <structfield>opckeytype</structfield>
      catalog entry.
      (Note that <structfield>opckeytype</structfield> can be zero,
      implying the storage type is the same as the operator class's input
      type, which is the most common situation.)
      For reasons of backward compatibility, the <function>config</function>
      method can set <structfield>leafType</structfield> to some other value,
      and that value will be used; but this is deprecated since the index
      contents are then incorrectly identified in the catalogs.
      Also, it's permissible to
      leave <structfield>leafType</structfield> uninitialized (zero);
      that is interpreted as meaning the index storage type derived from
      <structfield>opckeytype</structfield>.
     </para>

     <para>
      When <structfield>attType</structfield>
      and <structfield>leafType</structfield> are different, the optional
      method <function>compress</function> must be provided.
      Method <function>compress</function> is responsible
      for transformation of datums to be indexed from <structfield>attType</structfield>
      to <structfield>leafType</structfield>.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>choose</function></term>
     <listitem>
      <para>
        Chooses a method for inserting a new value into an inner tuple.
      </para>

     <para>
      The <acronym>SQL</acronym> declaration of the function must look like this:
<programlisting>
CREATE FUNCTION my_choose(internal, internal) RETURNS void ...
</programlisting>
      The first argument is a pointer to a <structname>spgChooseIn</structname>
      C struct, containing input data for the function.
      The second argument is a pointer to a <structname>spgChooseOut</structname>
      C struct, which the function must fill with result data.
<programlisting>
typedef struct spgChooseIn
{
    Datum       datum;          /* original datum to be indexed */
    Datum       leafDatum;      /* current datum to be stored at leaf */
    int         level;          /* current level (counting from zero) */

    /* Data from current inner tuple */
    bool        allTheSame;     /* tuple is marked all-the-same? */
    bool        hasPrefix;