11.8. Operator Classes

An index definition may specify an operator class for each column of an index.

CREATE INDEX name ON table (column opclass [, ...]);

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 point of 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.

There are also some built-in operator classes besides the default ones:

The following query shows all defined operator classes:

SELECT am.amname AS index_method,
       opc.opcname AS opclass_name
    FROM pg_am am, pg_opclass opc
    WHERE opc.opcamid = am.oid
    ORDER BY index_method, opclass_name;

It can be extended to show all the operators included in each class:

SELECT am.amname AS index_method,
       opc.opcname AS opclass_name,
       opr.oid::regoperator AS opclass_operator
    FROM pg_am am, pg_opclass opc, pg_amop amop, pg_operator opr
    WHERE opc.opcamid = am.oid AND
          amop.amopclaid = opc.oid AND
          amop.amopopr = opr.oid
    ORDER BY index_method, opclass_name, opclass_operator;