In relational databases, the information schema is an ANSI standard set of read-only views which provide information about all of the tables, views, columns, and procedures in a database. It can be used as a source of the information which some databases make available through non-standard commands, such as the SHOW command of MySQL, the DESCRIBE command of Oracle, and the \d command of PostgreSQL.
=> select count(table_name) from information_schema.tables;
count
-------
99
(1 row)
=> select column_name, data_type, column_default, is_nullable
from information_schema.columns where table_name='alpha';
column_name | data_type | column_default | is_nullable
-------------+-----------+----------------+-------------
foo | integer | | YES
bar | character | | YES
(2 rows)
=> select * from information_schema.information_schema_catalog_name;
catalog_name
--------------
johnd
(1 row)