# Chapter 52. System Catalogs
Table of Contents
52.23. pg_foreign_data_wrapper
52.31. pg_largeobject_metadata
52.65. pg_available_extensions
52.66. pg_available_extension_versions
52.67. pg_backend_memory_contexts
52.80. pg_replication_origin_status
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 normally SQL commands to do that. (For example, CREATE DATABASE
inserts a row into the pg_database
catalog — and actually creates the database on disk.) There are some exceptions for particularly esoteric operations, but many of those have been made available as SQL commands over time, and so the need for direct manipulation of the system catalogs is ever decreasing.