Horton

Re: Ordered Partitioned Table Scans

Julien Rouhaud <rjuju123@gmail.com>

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From: Julien Rouhaud <rjuju123@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <david.rowley@2ndquadrant.com>, Antonin Houska <ah@cybertec.at>, Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2019-03-08T21:20:39Z
Lists: pgsql-hackers 
On Fri, Mar 8, 2019 at 9:15 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> David Rowley <david.rowley@2ndquadrant.com> writes:
> > [ v9-0001-Allow-Append-to-be-used-in-place-of-MergeAppend-f.patch ]
>
> 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.

Commits

  1. Use Append rather than MergeAppend for scanning ordered partitions.