Re: Lengthy deletion

"Herouth Maoz" <herouth@unicell.co.il> writes:

I was instructed to delete old records from one of the tables in our production system. The deletion took hours and I had to stop it in mid-operation and reschedule it as a night job. But then I had to do the same when I got up in the morning and it was still running.

I got an interesting clue, though, when I canceled the deletion the second time around. I got the following error message:

Cancel request sent

ERROR:  canceling statement due to user request

CONTEXT:  SQL statement "SELECT 1 FROM ONLY "public"."sent_messages" x WHERE $1 OPERATOR(pg_catalog.=) "subscription_id" FOR SHARE OF x"

Yup, that's a clue all right.  I'll bet a nickel that you don't
have an index on the foreign key's referencing column (ie,
sent_messages.subscription_id).  That means each delete in
the referenced table has to seqscan the referencing table to
see if the delete would result in an FK violation.