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

On Tue, Aug 14, 2018 at 9:40 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Mon, Aug 13, 2018 at 10:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> As Tom said, it is evident from the plan that the Limit clause is
> pushed in the inner-side of the parallel plan and not all the workers
> compute the same result set for the inner side.
>
>> 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.
>>
>
> Yeah, one idea could be that we detect this in
> max_parallel_hazard_walker during the very first pass it performs on
> query-tree.
>

I have written a patch along those lines. This is still a WIP patch
and it is mainly to demonstrate what I have in mind. There is one
test in select_parallel.sql failing after this patch, but I think that
is expected and we need to adjust that test. Let me know if you see
a flaw in this approach?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com