PostgreSQL 7.1.3 Documentation
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# 7.6. 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 four-byte integers would use the `int4_ops` class; this operator class includes comparison functions for four-byte integers. 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 ordering. 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 operator classes with special purposes:

- The operator classes `box_ops` and `bigbox_ops` both support R-tree indices on the `box` data type. The difference between them is that `bigbox_ops` scales box coordinates down, to avoid floating point exceptions from doing multiplication, addition, and subtraction on very large floating point coordinates. If the field on which your rectangles lie is about 20 000 units square or larger, you should use `bigbox_ops`.

The following query shows all defined operator classes:

    SELECT am.amname AS acc_name,
           opc.opcname AS ops_name,
           opr.oprname AS ops_comp
        FROM pg_am am, pg_amop amop,
             pg_opclass opc, pg_operator opr
        WHERE amop.amopid = am.oid AND
              amop.amopclaid = opc.oid AND
              amop.amopopr = opr.oid
        ORDER BY acc_name, ops_name, ops_comp;

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