DROP COLLATION removes a previously defined collation. To be able to drop a collation, you must own the collation.
Do not throw an error if the collation does not exist. A notice is issued in this case.
The name of the collation. The collation name can be schema-qualified.
Automatically drop objects that depend on the collation, and in turn all objects that depend on those objects (see 5.13절).
Refuse to drop the collation if any objects depend on it. This is the default.
The DROP COLLATION command conforms to the SQL standard, apart from the IF EXISTS option, which is a PostgreSQL extension.