pg_opfamily defines operator families.
Each operator family is a collection of operators and associated
support routines that implement the semantics specified for a particular
index access method. Furthermore, the operators in a family are all
“compatible”, in a way that is specified by the access method.
The operator family concept allows cross-data-type operators to be used
with indexes and to be reasoned about using knowledge of access method
Operator families are described at length in Section 37.16.