Trying to modify a constraint in PostgreSQL

Question

I have checked the documentation provided by Oracle and found a way to modify a constraint without dropping the table. Problem is, it errors out at modify as it does not recogn

Answer 1

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;

Answer 2

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

Answer 3

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/

Answer 4

ALTER CONSTRAINT would require knowing of foreign key name, which is not always convenient.

Here is function, where you need to know only table and column names. Usage:

select replace_foreign_key('user_rates_posts', 'post_id', 'ON DELETE CASCADE');

Function:

CREATE OR REPLACE FUNCTION 
    replace_foreign_key(f_table VARCHAR, f_column VARCHAR, new_options VARCHAR) 
RETURNS VARCHAR
AS $$
DECLARE constraint_name varchar;
DECLARE reftable varchar;
DECLARE refcolumn varchar;
BEGIN

SELECT tc.constraint_name, ccu.table_name AS foreign_table_name, ccu.column_name AS foreign_column_name 
FROM 
    information_schema.table_constraints AS tc 
    JOIN information_schema.key_column_usage AS kcu
      ON tc.constraint_name = kcu.constraint_name
    JOIN information_schema.constraint_column_usage AS ccu
      ON ccu.constraint_name = tc.constraint_name
WHERE constraint_type = 'FOREIGN KEY' 
   AND tc.table_name= f_table AND kcu.column_name= f_column
INTO constraint_name, reftable, refcolumn;

EXECUTE 'alter table ' || f_table || ' drop constraint ' || constraint_name || 
', ADD CONSTRAINT ' || constraint_name || ' FOREIGN KEY (' || f_column || ') ' ||
' REFERENCES ' || reftable || '(' || refcolumn || ') ' || new_options || ';';

RETURN 'Constraint replaced: ' || constraint_name || ' (' || f_table || '.' || f_column ||
 ' -> ' || reftable || '.' || refcolumn || '); New options: ' || new_options;

END;
$$ LANGUAGE plpgsql;

Be aware: this function won't copy attributes of initial foreign key. It only takes foreign table name / column name, drops current key and replaces with new one.