public inbox for [email protected]  

help / color / mirror / Atom feed

From: Peter Hendriks <[email protected]>
To: [email protected]
Subject: Re: Delete from locking ordering differences
Date: Thu, 29 Sep 2022 15:51:31 +0200
Message-ID: <CAFhXkLH2u+xSsRpym3m1s_dXsXvVgn1Lcgd3Brzcq1ASLDdADw@mail.gmail.com> (raw)
In-Reply-To: <CAFhXkLG=izjYB5_YK3x3yjcVLF1SCe7YfBaBDf8U0zFtRscr6w@mail.gmail.com>
References: <CAFhXkLG=izjYB5_YK3x3yjcVLF1SCe7YfBaBDf8U0zFtRscr6w@mail.gmail.com>

Yes, we do not really care about order, but to prevent locking issues.
Multiple transactions may run this query at the same time. It should never
contend because of the skip locked, so maybe we should try this without an
order by too. We can not get this query to fail in test so far, just in
production, wo we are bit hesitant to change now that it is finally
working... Our guess is to why this makes such a difference is that the
delete statement in postgres does not guarantee ordering, so maybe the
optimizer makes different choices than expected. Asking the question here,
so maybe someone with more understanding can explain why we need the CTE.

Op vr 23 sep. 2022 om 09:31 schreef Peter Hendriks <[email protected]>:

> We are wondering if anyone can explain the difference we are having in
> production with the following queries:
>
> DELETE FROM store
> WHERE id IN (
>   SELECT id FROM store
>   FOR UPDATE SKIP LOCKED
>   ORDER BY ID
>   LIMIT 1000
> )
> RETURNING id, payload
>
> This query is sometimes executed with high concurrency, and then can hang
> indefinitely, we assume because of a locking problem that postgresql is not
> detecting as a deadlock.
>
> This alternative query does not have the hanging problem:
>
> WITH store_ids AS (
>   SELECT id FROM store
>   FOR UPDATE SKIP LOCKED
>   ORDER BY ID
>   LIMIT 1000
> )
> DELETE FROM store s
> USING store_ids si
> WHERE s.id = si.id
> RETURNING s.id, s.payload
>
> Can anyone explain why the first query is expected to fail (hang), and the
> second query does not have this problem? We would be interested in more
> understanding on this. Thanks!
>


-- 
Met vriendelijke groet,

Peter Hendriks
Mindloops B.V.

T. +31 (0)6 37 23 26 73
E. [email protected]
KvK: 85061921

view thread (3+ messages)

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]
  Subject: Re: Delete from locking ordering differences
  In-Reply-To: <CAFhXkLH2u+xSsRpym3m1s_dXsXvVgn1Lcgd3Brzcq1ASLDdADw@mail.gmail.com>

* 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