Re: Solution to UPDATE or INSERT Problem

Curt Sampson <cjs@cynic.net> writes:
> But why does session 2 hang
> in this instance?

Because the sub-SELECT sees a snapshot of transactions that were already
committed at the start of session 2's transaction.  If session 1 hasn't
committed yet, the EXISTS will return false.  The actual INSERT will
notice the inserted-but-not-yet-committed row, and will block waiting to
see whether it gets committed or not.

            regards, tom lane