Modify table constraint in PostgreSQL Say I have a constraint 

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);


that needs to be modified so that zipcode length is 6. 

Do I have the drop the constraint zipchk first and then recreate it ?

ALTER TABLE distributors DROP CONSTRAINT zipchk;
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 6);


or is there a MODIFY (like) command : 

ALTER TABLE distributors MODIFY CONSTRAINT zipchk CHECK (char_length(zipcode) = 6);
To sum up the comments:

Like @ypercube hinted, you can do it in a single command, which is cheaper and safer:

ALTER TABLE distributors
  DROP CONSTRAINT zipchk
, ADD  CONSTRAINT zipchk CHECK (length(zipcode) = 6);


ALTER CONSTRAINT in Postgres 9.4 or later (like you found) can only change the "deferability" of a FK constraints. So not what you are looking for. Other than that, there is no "MODIFY (like) command" for constraints (referring to MySQL's MODIFY I assume).

Details in the manual for ALTER TABLE, like @a_horse mentioned.