On Tue, Sep 6, 2016 at 2:00 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> Yeah, but I've seen actual breakage from exactly this issue on >> customer systems even with the 1GB limit, and when we start allowing >> 100GB it's going to get a whole lot worse. > > While it's not necessarily a bad idea to consider these things, > I think people are greatly overestimating the consequences of the > patch-as-proposed. AFAICS, it does *not* let you tell VACUUM to > eat 100GB of workspace. Note the line right in front of the one > being changed: > > maxtuples = (vac_work_mem * 1024L) / sizeof(ItemPointerData); > maxtuples = Min(maxtuples, INT_MAX); > - maxtuples = Min(maxtuples, MaxAllocSize / sizeof(ItemPointerData)); > + maxtuples = Min(maxtuples, MaxAllocHugeSize / > sizeof(ItemPointerData)); > > Regardless of what vac_work_mem is, we aren't gonna let you have more > than INT_MAX ItemPointers, hence 12GB at the most. So the worst-case > increase from the patch as given is 12X. Maybe that's enough to cause > bad consequences on some systems, but it's not the sort of disaster > Robert posits above.
Hmm, OK. Yes, that is a lot less bad. (I think it's still bad.) > If we think the expected number of dead pointers is so much less than > that, why don't we just decrease LAZY_ALLOC_TUPLES, and take a hit in > extra index vacuum cycles when we're wrong? Because that's really inefficient. Growing the array, even with a stupid approach that copies all of the TIDs every time, is a heck of a lot faster than incurring an extra index vac cycle. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
