PostgreSQL 9.4.1 Documentation
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DROP COLLATION

Name

DROP COLLATION -- remove a collation

DROP COLLATION [ IF EXISTS ] name [ CASCADE | RESTRICT ]

DROP COLLATION removes a previously defined collation. To be able to drop a collation, you must own the collation.

IF EXISTS: Do not throw an error if the collation does not exist. A notice is issued in this case.
name: The name of the collation. The collation name can be schema-qualified.
CASCADE: Automatically drop objects that depend on the collation.
RESTRICT: Refuse to drop the collation if any objects depend on it. This is the default.

To drop the collation named german:

DROP COLLATION german;

The DROP COLLATION command conforms to the SQL standard, apart from the IF EXISTS option, which is a PostgreSQL extension.