On Sat, Oct 11, 2014 at 1:32 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Nick Barnes <nickbarnes01@gmail.com> writes: > I'm looking at the code behind the foreign key checks in ri_triggers.c, and > something's got me a little confused.
> In both cases (FK insert/update checking the PK, and PK update/delete > checking the FK) the check is done with a SELECT ... FOR KEY SHARE.
> This makes perfect sense for PK checks, but in the FK check, it seems > pointless at best; if it actually manages to find something to lock, it > will fail the check and error out moments later. And in any case, I don't > see how the key fields in the FK relation (to which the KEY SHARE lock > applies) are even relevant to the constraint in question.
> What am I missing?
Race conditions.
Example case: you're trying to delete the row for PK 'foo', while concurrently somebody is inserting a row that references foo. With no locking, neither of you will see the other action, hence both will conclude their action is ok and commit. Presto: FK violation.
The point of the FOR SHARE lock (which also goes along with some cute games played with the query's snapshot) is to make sure there aren't uncommitted changes that would result in an FK violation. We could possibly have done it another way but that would just have resulted in two generally-similar mechanisms.
regards, tom lane
I understand why the FK insert needs to lock on the PK row. But why is the PK delete trying to lock the FK row? If it finds one, won't the delete fail anyway? If it doesn't find one, what is there to lock?