On Mon, Jan 5, 2015 at 5:53 PM, Peter Geoghegan <p...@heroku.com> wrote: > It's not all that easy to recreate, even with my custom > instrumentation. With fsync=off, it occurs somewhat infrequently with > a custom instrumentation Postgres build: > > pg@hamster:~/jjanes_upsert$ perl count_upsert.pl 8 100000 > [Mon Jan 5 17:31:57 2015] init done at count_upsert.pl line 101. > [Mon Jan 5 17:32:17 2015] child abnormal exit [Mon Jan 5 17:32:17 > 2015] DBD::Pg::db selectall_arrayref failed: ERROR: mismatch in xmin > for (322,264). Initially 4324145, then 4324429 at count_upsert.pl line > 210.\n at count_upsert.pl line 227. > [Mon Jan 5 17:32:49 2015] sum is -800 > [Mon Jan 5 17:32:49 2015] count is 9515 > [Mon Jan 5 17:32:49 2015] normal exit at 1420507969 after 733912 > items processed at count_upsert.pl line 182. > > I think that "super deleting" broken promise tuples undermines how > vacuum interlocking is supposed to work [1].
Hmm. On second thought, I think that the instrumentation I added here can be confused by redirecting line pointers, and that may account for this apparent problem. Still, I would like for us to figure out how it was previously possible for the implementation to update a "super-deleted" tuple (resulting in unusual all-NULLs tuples) since I though that in order for that to happen, other xacts would have to see what was only ever a promise tuple as a fully-fledged tuple -- in order for the logic that pre-checks for conflicts to find a conflict, it would have to find the tuple already committed (so it certainly couldn't be an unresolved promise tuple). My superficial fix [1] was written without fully understanding what the problem was. In any case, I doubt the changes that I made to tqual.c in that fix will be accepted as-is. [1] http://www.postgresql.org/message-id/cam3swztftt_fehet3tu3ykcpcypynnauquz3q+naasnh-60...@mail.gmail.com -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
