Horton

Re: BUG #15324: Non-deterministic behaviour from parallelised sub-query

Andrew Fletcher <andy@prestigedigital.com>

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From: Andrew Fletcher <andy@prestigedigital.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-bugs@lists.postgresql.org
Date: 2018-08-13T16:40:49Z
Lists: pgsql-bugs

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Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Don't allow LIMIT/OFFSET clause within sub-selects to be pushed to workers.

  2. Back-patch "Fix parallel hash join path search."

  3. Prohibit pushing subqueries containing window function calculation to

  4. Fix parallel hash join path search.

Sorry the bug report was unclear.

Its not just that its returning different accounts from the subquery.

In the first repro you can see account 304873 appears twice in the correct
result but only once in the incorrect one.  Even though its the same data,
same query etc.  If account 304873 is selected within the limit of the
subquery then all the results for it should be returned in the outer query.

In the second repro you can see more than 3 accounts in the outer query,
even though the inner one is limited to 3.

Hope that makes it clearer.

Cheers,

Andy


On Mon, Aug 13, 2018 at 5:35 PM, Andres Freund <andres@anarazel.de> wrote:

> Hi,
>
> On 2018-08-13 16:14:03 +0000, PG Bug reporting form wrote:
> > Execute this query (multiple times!)
> >
> > select * from events where account in (select account from events where
> > data->>'page' = 'success.html' limit 3);
>
> Well, the subselect with thelimit going to return different results from
> run to run. Unless you add an ORDER BY there's no guaranteed order in
> which tuples are returned.  So I don't think it's surprising that you're
> getting results that differ between runs.
>
> Greetings,
>
> Andres Freund
>