An index definition can specify an operator class for each column of an index.
CREATE INDEXnameONtable(columnopclass[sort options] [, ...]);
   The operator class identifies the operators to be used by the index
   for that column.  For example, a B-tree index on the type int4
   would use the int4_ops class; this operator
   class includes comparison functions for values of type int4.
   In practice the default operator class for the column's data type is
   usually sufficient.  The main reason for having operator classes is
   that for some data types, there could be more than one meaningful
   index behavior.  For example, we might want to sort a complex-number data
   type either by absolute value or by real part.  We could do this by
   defining two operator classes for the data type and then selecting
   the proper class when making an index.  The operator class determines
   the basic sort ordering (which can then be modified by adding sort options
   COLLATE,
   ASC/DESC and/or
   NULLS FIRST/NULLS LAST).
  
There are also some built-in operator classes besides the default ones:
      The operator classes text_pattern_ops,
      varchar_pattern_ops, and
      bpchar_pattern_ops support B-tree indexes on
      the types text, varchar, and
      char respectively.  The
      difference from the default operator classes is that the values
      are compared strictly character by character rather than
      according to the locale-specific collation rules.  This makes
      these operator classes suitable for use by queries involving
      pattern matching expressions (LIKE or POSIX
      regular expressions) when the database does not use the standard
      “C” locale.  As an example, you might index a
      varchar column like this:
CREATE INDEX test_index ON test_table (col varchar_pattern_ops);
      Note that you should also create an index with the default operator
      class if you want queries involving ordinary <,
      <=, >, or >= comparisons
      to use an index.  Such queries cannot use the
      xxx_pattern_opsxxx_pattern_ops
The following query shows all defined operator classes:
SELECT am.amname AS index_method,
       opc.opcname AS opclass_name,
       opc.opcintype::regtype AS indexed_type,
       opc.opcdefault AS is_default
    FROM pg_am am, pg_opclass opc
    WHERE opc.opcmethod = am.oid
    ORDER BY index_method, opclass_name;
An operator class is actually just a subset of a larger structure called an operator family. In cases where several data types have similar behaviors, it is frequently useful to define cross-data-type operators and allow these to work with indexes. To do this, the operator classes for each of the types must be grouped into the same operator family. The cross-type operators are members of the family, but are not associated with any single class within the family.
This expanded version of the previous query shows the operator family each operator class belongs to:
SELECT am.amname AS index_method,
       opc.opcname AS opclass_name,
       opf.opfname AS opfamily_name,
       opc.opcintype::regtype AS indexed_type,
       opc.opcdefault AS is_default
    FROM pg_am am, pg_opclass opc, pg_opfamily opf
    WHERE opc.opcmethod = am.oid AND
          opc.opcfamily = opf.oid
    ORDER BY index_method, opclass_name;
This query shows all defined operator families and all the operators included in each family:
SELECT am.amname AS index_method,
       opf.opfname AS opfamily_name,
       amop.amopopr::regoperator AS opfamily_operator
    FROM pg_am am, pg_opfamily opf, pg_amop amop
    WHERE opf.opfmethod = am.oid AND
          amop.amopfamily = opf.oid
    ORDER BY index_method, opfamily_name, opfamily_operator;