| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Marko Tiikkaja <marko(at)joh(dot)to> |
| Cc: | Andres Freund <andres(at)anarazel(dot)de>, andy(at)prestigedigital(dot)com, pgsql-bugs(at)lists(dot)postgresql(dot)org |
| Subject: | Re: BUG #15324: Non-deterministic behaviour from parallelised sub-query |
| Date: | 2018-08-13 17:26:45 |
| Message-ID: | 21038.1534181205@sss.pgh.pa.us |
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| Lists: | pgsql-bugs |
Marko Tiikkaja <marko(at)joh(dot)to> writes:
> On Mon, Aug 13, 2018 at 7:35 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> 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.
> While this is true, that's missing the point.
Yeah, I agree. I think probably what's happening is that the sub-select
is getting pushed down to the parallel workers and they are not all
computing the same set of sub-select results, leading to inconsistent
answers at the top level.
Likely, we need to treat the presence of a LIMIT/OFFSET in a sub-select
as making it parallel-unsafe, for exactly the reason that that makes
its results non-deterministic.
regards, tom lane
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