The following system-level schemas exist in every database:

1. pg_catalog contains the system catalog tables, built-in data types, functions, and operators. It is always part of the schema search path, even if it is not explicitly named in the search path.

2. information_schema consists of a standardized set of views that contain information about the objects in the database. These views get system information from the system catalog tables in a standardized way.

3. pg_toast stores large objects such as records that exceed the page size. This schema is used internally by the Greenplum Database system. 

4. pg_bitmapindex stores bitmap index objects such as lists of values. This schema is used internally by the Greenplum Database system.

5. pg_aoseg stores append-only table objects. This schema is used internally by the Greenplum Database system.

6. gp_toolkit is an administrative schema that contains external tables, views, and functions that you can access with SQL commands. All database users can access gp_toolkit to view and query the system log files and other system metrics.