The procedures described thus far let you define new types, new functions, and new operators. However, we cannot yet define a secondary index (such as a B-tree, R-tree, or hash access method) over a new type or its operators.
Look back at Figure 11-1. The right half shows the catalogs that we must modify in order to tell PostgreSQL how to use a user-defined type and/or user-defined operators with an index (i.e., pg_am, pg_amop, pg_amproc, pg_operator and pg_opclass). Unfortunately, there is no simple command to do this. We will demonstrate how to modify these catalogs through a running example: a new operator class for the B-tree access method that stores and sorts complex numbers in ascending absolute value order.
Read the upstream documentation. For corrections, first check the current version.