can be used for several different index methods (for example, both B-tree
   and hash index methods have operator classes named
   <literal>int4_ops</literal>), but each such class is an independent
   entity and must be defined separately.
  </para>
 </sect2>

 <sect2 id="xindex-strategies">
  <title>Index Method Strategies</title>

  <para>
   The operators associated with an operator class are identified by
   <quote>strategy numbers</quote>, which serve to identify the semantics of
   each operator within the context of its operator class.
   For example, B-trees impose a strict ordering on keys, lesser to greater,
   and so operators like <quote>less than</quote> and <quote>greater than or equal
   to</quote> are interesting with respect to a B-tree.
   Because
   <productname>PostgreSQL</productname> allows the user to define operators,
   <productname>PostgreSQL</productname> cannot look at the name of an operator
   (e.g., <literal>&lt;</literal> or <literal>&gt;=</literal>) and tell what kind of
   comparison it is.  Instead, the index method defines a set of
   <quote>strategies</quote>, which can be thought of as generalized operators.
   Each operator class specifies which actual operator corresponds to each
   strategy for a particular data type and interpretation of the index
   semantics.
  </para>

  <para>
   The B-tree index method defines five strategies, shown in <xref
   linkend="xindex-btree-strat-table"/>.
  </para>

   <table tocentry="1" id="xindex-btree-strat-table">
    <title>B-Tree Strategies</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Operation</entry>
       <entry>Strategy Number</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>less than</entry>
       <entry>1</entry>
      </row>
      <row>
       <entry>less than or equal</entry>
       <entry>2</entry>
      </row>
      <row>
       <entry>equal</entry>
       <entry>3</entry>
      </row>
      <row>
       <entry>greater than or equal</entry>
       <entry>4</entry>
      </row>
      <row>
       <entry>greater than</entry>
       <entry>5</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   Hash indexes support only equality comparisons, and so they use only one
   strategy, shown in <xref linkend="xindex-hash-strat-table"/>.
  </para>

   <table tocentry="1" id="xindex-hash-strat-table">
    <title>Hash Strategies</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Operation</entry>
       <entry>Strategy Number</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>equal</entry>
       <entry>1</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  <para>
   GiST indexes are more flexible: they do not have a fixed set of
   strategies at all.  Instead, the <quote>consistency</quote> support routine
   of each particular GiST operator class interprets the strategy numbers
   however it likes.  As an example, several of the built-in GiST index
   operator classes index two-dimensional geometric objects, providing
   the <quote>R-tree</quote> strategies shown in
   <xref linkend="xindex-rtree-strat-table"/>.  Four of these are true
   two-dimensional tests (overlaps, same, contains, contained by);
   four of them consider only the X direction; and the other four
   provide the same tests in the Y direction.
  </para>

   <table tocentry="1" id="xindex-rtree-strat-table">
    <title>GiST Two-Dimensional <quote>R-tree</quote> Strategies</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Operation</entry>
       <entry>Strategy Number</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>strictly left of</entry>
       <entry>1</entry>
      </row>
      <row>
       <entry>does not extend to right of</entry>
       <entry>2</entry>
      </row>
      <row>
       <entry>overlaps</entry>
       <entry>3</entry>
      </row>
      <row>
       <entry>does not extend to left of</entry>