<para>

As shown in <xref linkend="xindex-btree-support-table"/>, btree defines

one required andthree optional support functions. Thefour

one required andfour optional support functions. Thefive

user-defined methods are:

</para>

<variablelist>

<para>

There are five user-defined methods that an index operator class for

<acronym>SP-GiST</acronym> must provide, andone is optional. All five

<acronym>SP-GiST</acronym> must provide, andtwo are optional. All five

mandatory methods follow the convention of accepting two <type>internal</type>

arguments, the first of which is a pointer to a C struct containing input

values for the support method, while the second argument is a pointer to a

cases, the output struct is initialized to zeroes before calling the

user-defined method. The optional sixth method <function>compress</function>

accepts datum to be indexed as the only argument and returns a value suitable

for physical storage in a leaf tuple.

for physical storage in a leaf tuple. The optional seventh method

<function>options</function> accepts internal pointer to a C struct, where

opclass-specific parameters should be placed, and returns <type>void</type>.

</para>

<para>