public inbox for [email protected]  

help / color / mirror / Atom feed

From: Tom Lane <[email protected]>
To: Vik Fearing <[email protected]>
Cc: Ankit Kumar Pandey <[email protected]>
Cc: David Rowley <[email protected]>
Cc: pghackers <[email protected]>
Subject: Re: Todo: Teach planner to evaluate multiple windows in the optimal order
Date: Wed, 04 Jan 2023 21:48:05 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<CAApHDvp=r1LnEKCmWCYaruMPL-jP4j_sdc8yeFYwaDT1ac5GsQ@mail.gmail.com>
	<[email protected]>
	<CAApHDvq=g2=ny59f1bvwRVvupsgPHK-KjLPBsSL25fVuGZ4idQ@mail.gmail.com>
	<[email protected]>
	<[email protected]>

Vik Fearing <[email protected]> writes:
> On 1/4/23 13:07, Ankit Kumar Pandey wrote:
>> Also, one thing, consider the following query:
>> explain analyze select row_number() over (order by a,b),count(*) over 
>> (order by a) from abcd order by a,b,c;
>> In this case, sorting is done on (a,b) followed by incremental sort on c 
>> at final stage.
>> If we do just one sort: a,b,c at first stage then there won't be need to 
>> do another sort (incremental one).

> This could give incorrect results.

Mmmm ... your counterexample doesn't really prove that.  Yes,
the "rank()" step must consider only two ORDER BY columns while
deciding which rows are peers, but I don't see why it wouldn't
be okay if the rows happened to already be sorted by "c" within
those peer groups.

I don't recall the implementation details well enough to be sure
how hard it would be to keep that straight.

			regards, tom lane

view thread (8+ messages)  latest in thread

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected], [email protected], [email protected]
  Subject: Re: Todo: Teach planner to evaluate multiple windows in the optimal order
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox