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 operator classes text_pattern_ops, varchar_pattern_ops, bpchar_pattern_ops, and name_pattern_ops support B-tree indexes on the types text, varchar, char, and name, respectively. The difference from the ordinary 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) if the server 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);
If you do use the C locale, you may instead create an index with the default operator class, and it will still be useful for pattern-matching queries. Also note that you should create an index with the default operator class if you want queries involving ordinary comparisons to use an index. Such queries cannot use the xxx_pattern_ops operator classes. It is allowed to create multiple indexes on the same column with different operator classes.
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.oprname 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;