pg_opclass

The pg_opclass system catalog table defines index access method operator classes. Each operator class defines semantics for index columns of a particular data type and a particular index access method. An operator class essentially specifies that a particular operator family is applicable to a particular indexable column data type. The set of operators from the family that are actually usable with the indexed column are those that accept the column’s data type as their left-hand input.

An operator class’s opcmethod must match the opfmethod of its containing operator family. Also, there must be no more than one pg_opclass row having opcdefault true for any given combination of opcmethod and opcintype.

The PostgreSQL Interfacing Extensions to Indexes documentation describes operator classes at length.

column	type	references	description
oid	oid		Row identifier (hidden attribute; must be explicitly selected)
opcmethod	oid	pg_am.oid	Index access method operator class is for
opcname	name		Name of this operator class
opcnamespace	oid	pg_namespace.oid	Namespace of this operator class
opcowner	oid	pg_authid.oid	Owner of the operator class
opcfamily	oid	pg_opfamily.oid	Operator family containing the operator class
kopcintype	oid	pg_type.oid	Data type that the operator class indexes
opcdefault	boolean		True if this operator class is the default for the data type opcintype
opckeytype	oid	pg_type.oid	Type of data stored in index, or zero if same as opcintype