The catalog pg_opclass 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. Note that there can be multiple operator classes for a given data type/access method combination, thus supporting multiple behaviors.
Operator classes are described at length in Section 33.14.
Table 43-24. pg_opclass Columns
|Index access method operator class is for
|Name of this operator class
|Namespace of this operator class
|Owner of the operator class
|Data type that the operator class indexes
|True if this operator class is the default for opcintype
|Type of data stored in index, or zero if same as opcintype
The majority of the information defining an operator class is actually not in its pg_opclass row, but in the associated rows in pg_amop and pg_amproc. Those rows are considered to be part of the operator class definition — this is not unlike the way that a relation is defined by a single pg_class row plus associated rows in pg_attribute and other tables.