Having introduced the basic extensibility concepts, we can now take a look at how the catalogs are actually laid out. You can skip this section for now, but some later sections will be incomprehensible without the information given here, so mark this page for later reference. All system catalogs have names that begin with pg_. The following classes contain information that may be useful to the end user. (There are many other system catalogs, but there should rarely be a reason to query them directly.)
Table 31-1. Postgres System Catalogs
|pg_proc||procedures (both C and SQL)|
|pg_type||types (both base and complex)|
|pg_aggregate||aggregates and aggregate functions|
|pg_amop||access method operators|
|pg_amproc||access method support functions|
|pg_opclass||access method operator classes|
In several of the sections that follow, we will present various join queries on the system catalogs that display information we need to extend the system. Looking at this diagram should make some of these join queries (which are often three- or four-way joins) more understandable, because you will be able to see that the attributes used in the queries form foreign keys in other classes.
Many different features (classes, attributes, functions, types, access methods, etc.) are tightly integrated in this schema. A simple create command may modify many of these catalogs.
Types and procedures are central to the schema.
Nearly every catalog contains some reference to instances in one or both of these classes. For example, Postgres frequently uses type signatures (e.g., of functions and operators) to identify unique instances of other catalogs.
Note: We use the words procedure and function more or less interchangably.
There are many attributes and relationships that have obvious meanings, but there are many (particularly those that have to do with access methods) that do not. The relationships between pg_am, pg_amop, pg_amproc, pg_operator and pg_opclass are particularly hard to understand and will be described in depth (in the section on interfacing types and operators to indices) after we have discussed basic extensions.