Hi, I just ran into this again in another context (see original dicussion, quoted below).
Some time ago, while initially introducting non-default stats target, I set our non-filtering columns to "STATISTICS 10", lower than default, to minimize the size of pg_statistic, which (at least at one point) I perceived to have become bloated and causing issue (partially due to having an excessive number of "daily" granularity partitions, a problem I've since mitigated). The large number of columns with non-default stats target was (I think) causing pg_dump --section=pre-data to take 10+ minutes, which makes pg_upgrade more disruptive than necessary, so now I'm going back and fixing it. [pryzbyj@database ~]$ time sed '/SET STATISTICS 10;$/!d; s//SET STATISTICS -1;/' /srv/cdrperfbackup/ts/2017-10-17/pg_dump-section\=pre-data |psql -1q ts server closed the connection unexpectedly This probably means the server terminated abnormally before or while processing the request. connection to server was lost [pryzbyj@database ~]$ dmesg |tail -n2 Out of memory: Kill process 6725 (postmaster) score 550 or sacrifice child Killed process 6725, UID 26, (postmaster) total-vm:13544792kB, anon-rss:8977764kB, file-rss:8kB So I'm hoping to encourage someone to commit the change contemplated earlier. Thanks in advance. Justin On Tue, Jul 18, 2017 at 07:26:30PM -0400, Tom Lane wrote: > Justin Pryzby <pry...@telsasoft.com> writes: > > I've seen this before while doing SET STATISTICS on a larger number of > > columns > > using xargs, but just came up while doing ADD of a large number of columns. > > Seems to be roughly linear in number of children but superlinear WRT > > columns. > > I think having to do with catalog update / cache invalidation with many > > ALTERs*children*columns? > > I poked into this a bit. The operation is necessarily roughly O(N^2) in > the number of columns, because we rebuild the affected table's relcache > entry after each elementary ADD COLUMN operation, and one of the principal > components of that cost is reading all the pg_attribute entries. However, > that should only be a time cost not a space cost. Eventually I traced the > O(N^2) space consumption to RememberToFreeTupleDescAtEOX, which seems to > have been introduced in Simon's commit e5550d5fe, and which strikes me as > a kluge of the first magnitude. Unless I am missing something, that > function's design concept can fairly be characterized as "let's leak > memory like there's no tomorrow, on the off chance that somebody somewhere > is ignoring basic coding rules". > > I tried ripping that out, and the peak space consumption of your example > (with 20 child tables and 1600 columns) decreased from ~3GB to ~200MB. > Moreover, the system still passes make check-world, so it's not clear > to me what excuse this code has to live. > > It's probably a bit late in the v10 cycle to be taking any risks in > this area, but I'd vote for ripping out RememberToFreeTupleDescAtEOX > as soon as the v11 cycle opens, unless someone can show an example > of non-broken coding that requires it. (And if so, there ought to > be a regression test incorporating that.) -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers