Trying to modify a constraint in PostgreSQL
There is no ALTER command for constraints in Postgres. The easiest way to accomplish this is to drop the constraint and re-add it with the desired parameters. Of course any change of the constraint will be run against the current table data.
BEGIN;ALTER TABLE t1 DROP CONSTRAINT ...ALTER TABLE t1 ADD CONSTRAINT ...COMMIT;
According to the correct manual (which is supplied by PostgreSQL, not by Oracle), there is no modify constraint available in the ALTER TABLE statement:
Here is the link to the correct manual:
http://www.postgresql.org/docs/current/static/sql-altertable.html
As of version 9.4, PostgreSQL supports ALTER TABLE ... ALTER CONSTRAINT for foreign keys.
This features will "Allow constraint attributes to be altered,so the default setting of NOT DEFERRABLE can be altered to DEFERRABLE and back." Looking at your question I think that is (kind of) what you have been looking for.
More detailed information and an example can be found here:
http://www.depesz.com/2013/06/30/waiting-for-9-4-alter-table-alter-constraint-for-fks/