Information schema

In relational databases, the information schema (information_schema) is an ANSI-standard set of read-only views that provide information about all of the tables, views, columns, and procedures in a database. It can be used as a source of the information that some databases make available through non-standard commands, such as:

• the SHOW command of MySQL
• the DESCRIBE command of Oracle's SQL*Plus
• the \d command in psql (PostgreSQL's default command-line program).

 => 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)