public inbox for [email protected]  

help / color / mirror / Atom feed

From: Tom Lane <[email protected]>
To: Dimitrios Apostolou <[email protected]>
Cc: [email protected]
Subject: Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions
Date: Fri, 10 May 2024 21:33:38 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>

Dimitrios Apostolou <[email protected]> writes:
> On Fri, 10 May 2024, Tom Lane wrote:
>> I'd say the blame lies with that (probably-default) estimate of
>> just 200 distinct rows.  That means the planner expects to have
>> to read about 5% (10/200) of the tables to get the result, and
>> that's making fast-start plans look bad.

> In any case, even after the planner decides to execute the terrible plan
> with the parallel seqscans, why doesn't it finish right when it finds 10
> distinct values?

That plan can't emit anything at all till it finishes the Sort.

I do kind of wonder why it's producing both a hashagg and a Unique
step --- seems like it should do one or the other.

> Thanks, I'll save the ANALYZE as the last step; I feel it's a good
> opportunity to figure out more details about how postgres works. Plus I
> expect ANALYZE to last a couple of days, so I should first find quiet time
> for that. :-)

It really should not take too long --- it reads a sample, not the
whole table.

			regards, tom lane

view thread (17+ 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]
  Subject: Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions
  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