Re: Tracking down deadlocks - Mailing list pgsql-general

Ben,

My personal solution is to patch the postgres sources so the foreign key
check does NOT lock the parent record. This has the disadvantage that in
some situations the foreign key semantics are not guarrantied, but for
my application works just fine. I think there was a patch posted to do
exactly this, but I didn't use that one. Note that the postgres
regression test was still running through fine after disabling the
parent record locking, so the basic FK functionality is not affected by
that.
Now this patch worked for me, but I won't post it, cause it's just too
dangerous, and I only have it for one postgres version and won't
maintain it. If you want to take your data at danger, patch for yourself
;-)
Hint: take a look at
src/backend/utils/adt/ri_triggers.c
in the postgres sources, that file contains the foreign key logic.

Cheers,
Csaba.

On Wed, 2004-06-16 at 20:11, Ben wrote:
> So is everybody simply accepting the chance of deadlocks, thanks to
> their foreign keys? Given what I know about why this problem exists, it
> doesn't seem to have an easy solution.... but from my naive perspective
> it seems like something that we shouldn't have to just live with,
> either.
>
> On Jun 16, 2004, at 10:50 AM, Joshua D. Drake wrote:
>
> > Hello,
> >
> > Deferred checks can greatly reduce the deadlock chance because of the
> > timing of the foreign key check. I won't say it can eliminate them,
> > and I don't think anyone here would suggest that you don't use Foreign
> > keys.
> >
> > Sincerely,
> >
> > Joshua D. Drake
> >
> > Ben wrote:
> >> Thanks for the quick reply (and summary!).
> >> According to the messages I've found on the list, basically the
> >> answer seems to be, "don't do this." On the other hand, pretty much
> >> every message on the subject is pre-7.4. There is some mention of
> >> using deferred foreign keys to reduce the chance for a deadlock, but
> >> nothing says doing that actually eliminates the chance.
> >> Is this just a known limitation? In this particular instance, I
> >> probably could get rid of my foreign keys and if things go bad it
> >> wouldn't hurt anything.... but I make heavy use of foreign keys
> >> throughout the rest of my schema, which are useful for the programs
> >> that aren't doing data mining. I wouldn't want to get rid of those
> >> foreign keys.
> >> On Jun 16, 2004, at 8:54 AM, Csaba Nagy wrote:
> >>> Hi Ben,
> >>>
> >>> Check this mailing list for "foreign keys" and "deadlock".
> >>> Short info:
> >>> Postgres exclusively locks the referenced records of a foreign key
> >>> relationship when the child record is updated, so multiple runs (in
> >>> different transactions) of one insert query could cause deadlock if
> >>> they
> >>> update rows which reference the same parent keys in reverse order.
> >>> Check your foreign keys...
> >>>
> >>> HTH,
> >>> Csaba.
> >>>
> >>> On Wed, 2004-06-16 at 17:33, Ben wrote:
> >>>
> >>>> I'm doing a bunch of data mining against a postgres database and
> >>>> have
> >>>> run into an interesting problem with deadlocks. The problem is,
> >>>> postgres is detecting them and then wacking the offending process,
> >>>> and
> >>>> I can't figure out what's causing them. I have a ton of select
> >>>> queries
> >>>> (but none for update), and then a single query to insert into a
> >>>> table.
> >>>> Nothing selects from that table. So where could the deadlock be?
> >>>>
> >>>> pg_stat_activity has a column named current_query, which would seem
> >>>> useful in tracking this down, but it's not being populated.
> >>>>
> >>>> Oh, I'm running 7.4.2.
> >>>>
> >>>>
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