Chapter 43. 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 — and actually
creates the database on disk.) There are some exceptions for
particularly esoteric operations, such as adding index access methods.