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 tables 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 11-1. PostgreSQL System Catalogs
|pg_type||data types (both base and complex)|
|pg_amop||access method operators|
|pg_amproc||access method support functions|
|pg_opclass||access method operator classes|
Figure 11-1 shows the major entities and their relationships in the system catalogs. (Columns that do not refer to other entities are not shown unless they are part of a primary key.) This diagram is more or less incomprehensible until you actually start looking at the contents of the catalogs and see how they relate to each other. For now, the main things to take away from this diagram are as follows:
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 columns used in the queries form foreign keys in other tables.
Many different features (tables, columns, 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.
Note: We use the words procedure and function more or less interchangeably.
Nearly every catalog contains some reference to rows in one or both of these tables. For example, PostgreSQL frequently uses type signatures (e.g., of functions and operators) to identify unique rows of other catalogs.
There are many columns 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 Chapter 17) after we have discussed basic extensions.