| From: | Noah Misch <noah(at)leadboat(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Greg Stark <stark(at)mit(dot)edu>, Jim Nasby <Jim(dot)Nasby(at)BlueTreble(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Josh Berkus <josh(at)agliodbs(dot)com>, Greg Sabino Mullane <greg(at)turnstep(dot)com>, Marti Raudsepp <marti(at)juffo(dot)org>, Tomas Vondra <tv(at)fuzzy(dot)cz>, Atri Sharma <atri(dot)jiit(at)gmail(dot)com>, Svenne Krap <svenne(at)krap(dot)dk> |
| Subject: | Re: Final Patch for GROUPING SETS |
| Date: | 2015-05-14 06:32:04 |
| Message-ID: | 20150514063204.GA3707771@tornado.leadboat.com |
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| Lists: | pgsql-hackers |
On Thu, May 14, 2015 at 07:50:31AM +0200, Andres Freund wrote:
> I still believe that the general approach of chaining vs. a union or CTE
> is correct due to the efficiency arguments upthread. My problem is
> that, unless I very much misunderstand something, the current
> implementation can end up requiring roughly #sets * #input of additional
> space for the "sidechannel tuplestore" in some bad cases. That happens
> if you group by a couple clauses that each lead to a high number of
> groups.
Correct.
> Andrew, is that a structure you could live with, or not?
>
> Others, what do you think?
Andrew and I discussed that very structure upthread:
http://www.postgresql.org/message-id/20141231085845.GA2148306@tornado.leadboat.com
http://www.postgresql.org/message-id/87d26zd9k8.fsf@news-spur.riddles.org.uk
http://www.postgresql.org/message-id/20141231210553.GB2159277@tornado.leadboat.com
I still believe the words I wrote in my two messages cited.
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