| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Julien Rouhaud <rjuju123(at)gmail(dot)com> |
| Cc: | David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Antonin Houska <ah(at)cybertec(dot)at>, Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: Ordered Partitioned Table Scans |
| Date: | 2019-03-08 21:52:44 |
| Message-ID: | 16791.1552081964@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Julien Rouhaud <rjuju123(at)gmail(dot)com> writes:
> On Fri, Mar 8, 2019 at 9:15 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I think you should remove all that
>> and restrict this optimization to the case where all the subpaths are
>> natively ordered --- if we have to insert Sorts, it's hardly going to move
>> the needle to worry about simplifying the parent MergeAppend to Append.
> This can be a huge win for queries of the form "ORDER BY partkey LIMIT
> x". Even if the first subpath(s) aren't natively ordered, not all of
> the sorts should actually be performed.
[ shrug... ] We've got no realistic chance of estimating such situations
properly, so I'd have no confidence in a plan choice based on such a
thing. Nor do I believe that this case is all that important.
regards, tom lane
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