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From: Dimitrios Apostolou <[email protected]>
To: David Rowley <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: [email protected]
Subject: Re: SELECT DISTINCT chooses parallel seqscan instead of indexscan on huge table with 1000 partitions
Date: Mon, 13 May 2024 15:52:42 +0200 (CEST)
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvo8yYvqa1+bkW_f5xHX-gmKGYfaGwH+Y_KP-=9TOuF+-g@mail.gmail.com>
References: <[email protected]>
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On Tue, 14 May 2024, David Rowley wrote:

> On Tue, 14 May 2024 at 00:41, Dimitrios Apostolou <[email protected]> wrote:
>>
>> On Sat, 11 May 2024, David Rowley wrote:
>>> It will. It's just that Sorting requires fetching everything from its subnode.
>>
>> Isn't it plain wrong to have a sort step in the plan than? The different
>> partitions contain different value ranges with no overlap, and the last
>> query I posted doesn't even contain an ORDER BY clause, just a DISTINCT
>> clause on an indexed column.
>
> The query does contain an ORDER BY, so if the index is not chosen to
> provide pre-sorted input, then something has to put the results in the
> correct order before the LIMIT is applied.

The last query I tried was:

SELECT DISTINCT workitem_n FROM test_runs_raw LIMIT 10;

See my message at

[1] https://www.postgresql.org/message-id/69077f15-4125-2d63-733f-21ce6eac4f01%40gmx.net

Will re-check things and report back with further debugging info you asked
for later today.


Dimitris

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