On Thu, Apr 27, 2017 at 5:22 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> But if we delete many rows from beginning or end of index, it would be >>> very expensive too because we will fetch each dead row and reject it. > >> Yep, and I've seen that turn into a serious problem in production. > > How so? Shouldn't the indexscan go back and mark such tuples dead in > the index, such that they'd be visited this way only once? If that's > not happening, maybe we should try to fix it.
Hmm. Actually, I think the scenario I saw was where there was a large number of tuples at the end of the index that weren't dead yet due to an old snapshot held open. That index was being scanned by lots of short-running queries. Those queries executed just fine, but they took a long to plan because they had to step over all of the dead tuples in the index one by one. That increased planning time - multiplied by the number of times it was incurred - was sufficient to cripple the system. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers