On 2014-06-09 11:24:22 -0700, Kevin Grittner wrote: > Andres Freund <and...@2ndquadrant.com> wrote: > > On 2014-06-09 09:45:12 -0700, Kevin Grittner wrote: > > > I am not sure, given predicate.c's coding, how > > HEAPTUPLE_DELETE_IN_PROGRESS could cause problems. Could you elaborate, > > since that's the contentious point with Tom? Since 'both in > > progress' > > can only happen if xmin and xmax are the same toplevel xid and you > > resolve subxids to toplevel xids I think it should currently be safe > > either way? > > The only way that it could be a problem is if the DELETE is in a > subtransaction which might get rolled back without rolling back the > INSERT.
The way I understand the code in that case the subxid in xmax would have been resolved the toplevel xid. /* * Find top level xid. Bail out if xid is too early to be a conflict, or * if it's our own xid. */ if (TransactionIdEquals(xid, GetTopTransactionIdIfAny())) return; xid = SubTransGetTopmostTransaction(xid); if (TransactionIdPrecedes(xid, TransactionXmin)) return; if (TransactionIdEquals(xid, GetTopTransactionIdIfAny())) return; That should essentially make that case harmless, right? So it seems the optimization (and pessimization in other cases) of only tracking toplevel xids seems to save the day here? > If we ignore the conflict because we assume the INSERT > will be negated by the DELETE, and that doesn't happen, we would > get false negatives which would compromise correctness. If we > assume that the DELETE might not happen when the DELETE is not in a > separate subtransaction we might get a false positive, which would > only be a performance hit. If we know either is possible and have > a way to check in predicate.c, it's fine to check it there. Given the above I don't think this currently can happen. Am I understand it correctly? If so, it certainly deserves a comment... Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers