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;