# Extending SQL: An Overview

In the sections that follow, we will discuss how you can extend the PostgreSQL SQL query language by adding:

- functions

- data types

- operators

- aggregates

## How Extensibility Works

PostgreSQL is extensible because its operation is catalog-driven. If you are familiar with standard relational systems, you know that they store information about databases, tables, columns, etc., in what are commonly known as system catalogs. (Some systems call this the data dictionary). The catalogs appear to the user as tables like any other, but the DBMS stores its internal bookkeeping in them. One key difference between PostgreSQL and standard relational systems is that PostgreSQL stores much more information in its catalogs -- not only information about tables and columns, but also information about its types, functions, access methods, and so on. These tables can be modified by the user, and since PostgreSQL bases its internal operation on these tables, this means that PostgreSQL can be extended by users. By comparison, conventional database systems can only be extended by changing hardcoded procedures within the DBMS or by loading modules specially written by the DBMS vendor.

PostgreSQL is also unlike most other data managers in that the server can incorporate user-written code into itself through dynamic loading. That is, the user can specify an object code file (e.g., a shared library) that implements a new type or function and PostgreSQL will load it as required. Code written in SQL is even more trivial to add to the server. This ability to modify its operation on the fly makes PostgreSQL uniquely suited for rapid prototyping of new applications and storage structures.

## The PostgreSQL Type System

The PostgreSQL type system can be broken down in several ways. Types are divided into base types and composite types. Base types are those, like `int4`, that are implemented in a language such as C. They generally correspond to what are often known as abstract data types; PostgreSQL can only operate on such types through methods provided by the user and only understands the behavior of such types to the extent that the user describes them. Composite types are created whenever the user creates a table.

PostgreSQL stores these types in only one way (within the file that stores all rows of a table) but the user can look inside at the attributes of these types from the query language and optimize their retrieval by (for example) defining indexes on the attributes. PostgreSQL base types are further divided into built-in types and user-defined types. Built-in types (like `int4`) are those that are compiled into the system. User-defined types are those created by the user in the manner to be described later.

## About the PostgreSQL System Catalogs

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.)

## PostgreSQL System Catalogs

Catalog Name | Description
pg_database | databases
pg_class | tables
pg_attribute | table columns
pg_index | indexes
pg_proc | procedures/functions
pg_type | data types (both base and complex)
pg_operator | operators
pg_aggregate | aggregate functions
pg_am | access methods
pg_amop | access method operators
pg_amproc | access method support functions
pg_opclass | access method operator classes

### The major PostgreSQL system catalogs

The Developer's Guide gives a more detailed explanation of these catalogs and their columns. However, EXTEND-CATALOGS 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.

  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.