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 indexes 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_method,
opc.opcname AS ops_name
FROM pg_am am, pg_opclass opc
WHERE opc.opcamid = am.oid
ORDER BY acc_method, ops_name;
It can be extended to show all the operators included in each class:
SELECT am.amname AS acc_method,
opc.opcname AS ops_name,
opr.oprname AS ops_comp
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 acc_method, ops_name, ops_comp;
