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3.7.5 The System Catalog Schema

In addition to public and user-created schemas, each database contains a pg_catalog schema, which contains the system tables and all the built-in data types, functions, and operators. pg_catalog is always effectively part of the search path. If it is not named explicitly in the path then it is implicitly searched before searching the path's schemas. This ensures that built-in names will always be findable. However, you may explicitly place pg_catalog at the end of your search path if you prefer to have user-defined names override built-in names.

In PostgreSQL versions before 7.3, table names beginning with pg_ were reserved. This is no longer true: you may create such a table name if you wish, in any non-system schema. However, it's best to continue to avoid such names, to ensure that you won't suffer a conflict if some future version defines a system table named the same as your table. (With the default search path, an unqualified reference to your table name would be resolved as the system table instead.) System tables will continue to follow the convention of having names beginning with pg_, so that they will not conflict with unqualified user-table names so long as users avoid the pg_ prefix.