I am in the process of implementing cascade on delete constraints retroactively on rather large tables so I can cleanly remove deprecated data. The problem is recreating some foreign key constraints on tables of 55 million rows+ was taking much longer than the maintenance window I had, and now I am looking for tricks to speed up the process, hopefully there is something obvious i am overlooking.
BEGIN;
ALTER TABLE ONLY t1 DROP CONSTRAINT fk_t1_t2_id;
ALTER TABLE ONLY t1 ADD CONSTRAINT fk_t1_t2_id FOREIGN KEY(id) REFERENCES t2(id)
ON DELETE CASCADE
DEFERRABLE INITIALLY DEFERRED;
COMMIT;
t1 has 55 million rows
t2 has 72 million rows
I have tried set constraints deferred, immediate, the id column on table 2 is indexed, its the primary key. There may be memory settings to tweak, I was able to get it to run on a faster test server with local storage in about 10 minutes, but it was running for over an hour in our production environment.. We took down the application and I verified it wasnt waiting for an exclusive lock on the table or anything, it was running the alter table command for that duration.
An additional question - is there any way to check how long postgres is estimating an operation will take to complete while it is running?