Question

I have a table with approx 5 million rows which has a fk constraint referencing the primary key of another table (also approx 5 million rows).

I need to delete about 75000 rows from both tables. I know that if I try doing this with the fk constraint enabled it’s going to take an unacceptable amount of time.

Coming from an Oracle background my first thought was to disable the constraint, do the delete & then reenable the constraint. PostGres appears to let me disable constraint triggers if I am a super user (I’m not, but I am logging in as the user that owns/created the objects) but that doesn’t seem to be quite what I want.

The other option is to drop the constraint and then reinstate it. I’m worried that rebuilding the constraint is going to take ages given the size of my tables.

Any thoughts?

edit: after Billy’s encouragement I’ve tried doing the delete without changing any constraints and it takes in excess of 10 minutes. However, I have discovered that the table from which I’m trying to delete has a self referential foreign key … duplicated (& non indexed).

Final update – I dropped the self referential foreign key, did my delete and added it back in. Billy’s right all round but unfortunately I can’t accept his comment as the answer!

Answer 1

Per previous comments, it should be a problem. That said, there is a command that may be what you’re looking to – it’ll set the constraints to deferred so they’re checked on COMMIT, not on every delete. If you’re doing just one big DELETE of all the rows, it won’t make a difference, but if you’re doing it in pieces, it will.

SET CONSTRAINTS ALL DEFERRED

is what you are looking for in that case. Note that constraints must be marked as DEFERRABLE before they can be deferred. For example:

ALTER TABLE table_name
  ADD CONSTRAINT constraint_uk UNIQUE(column_1, column_2)
  DEFERRABLE INITIALLY IMMEDIATE;

The constraint can then be deferred in a transaction or function as follows:

CREATE OR REPLACE FUNCTION f() RETURNS void AS
$BODY$
BEGIN
  SET CONSTRAINTS ALL DEFERRED;

  -- Code that temporarily violates the constraint...
  -- UPDATE table_name ...
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;