## Chapter 42. System Catalogs

The system catalogs are the place where a relational database management system stores schema metadata, such as information about tables and columns, and internal bookkeeping information. PostgreSQL's system catalogs are regular tables. You can drop and recreate the tables, add columns, insert and update values, and severely mess up your system that way. Normally, one should not change the system catalogs by hand, there are always SQL commands to do that. (For example, `CREATE DATABASE` inserts a row into the `pg_database` catalog [mdash ] and actually creates the database on disk.) There are some exceptions for particularly esoteric operations, such as adding index access methods.

## 42.1. Overview

Table 42.1, “System Catalogs” lists the system catalogs. More detailed documentation of each catalog follows below.

Most system catalogs are copied from the template database during database creation and are thereafter database-specific. A few catalogs are physically shared across all databases in a cluster; these are noted in the descriptions of the individual catalogs.

Table 42.1. System Catalogs

Catalog Name | Purpose
--- | ---
`pg_aggregate` | aggregate functions
`pg_am` | index access methods
`pg_amop` | access method operators
`pg_amproc` | access method support procedures
`pg_attrdef` | column default values
`pg_attribute` | table columns (“attributes”)
`pg_authid` | authorization identifiers (roles)
`pg_auth_members` | authorization identifier membership relationships
`pg_autovacuum` | per-relation autovacuum configuration parameters
`pg_cast` | casts (data type conversions)
`pg_class` | tables, indexes, sequences, views (“relations”)
`pg_constraint` | check constraints, unique constraints, primary key constraints, foreign key constraints
`pg_conversion` | encoding conversion information
`pg_database` | databases within this database cluster
`pg_depend` | dependencies between database objects
`pg_description` | descriptions or comments on database objects
`pg_index` | additional index information
`pg_inherits` | table inheritance hierarchy
`pg_language` | languages for writing functions
`pg_largeobject` | large objects
`pg_listener` | asynchronous notification support
`pg_namespace` | schemas
`pg_opclass` | index access method operator classes
`pg_operator` | operators
`pg_pltemplate` | template data for procedural languages
`pg_proc` | functions and procedures
`pg_rewrite` | query rewrite rules
`pg_shdepend` | dependencies on shared objects
`pg_statistic` | planner statistics
`pg_tablespace` | tablespaces within this database cluster
`pg_trigger` | triggers
`pg_type` | data types