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

On Sun, Sep 2, 2018 at 10:18 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > Yeah, let me summarize the problems which require patches:
> > > (a) Consider the presence of a LIMIT/OFFSET in a sub-select as making
> > > it parallel-unsafe.
> > >
> >
> > As mentioned up-thread, I have considered adding a check in
> > max_parallel_hazard_walker, but it turns out that it will make the
> > whole query parallel-unsafe even if one of the sub-selects has
> > Limit/Offset.  I think the better idea is to detect that during
> > set_rel_consider_parallel.  Attached patch
> > prohibit_parallel_limit_subselect_v2 implements the fix for same.
> >
>
> I was trying this patch on back-branches and found that it doesn't
> apply cleanly beyond PG11, so created separate patches for 10 and 9.6.
>   Further, I found that the test for this patch was not failing for
> 9.6 (without the patch) even though the code doesn't deal with this
> problem.  On further investigation, I found that it is because the
> commit
> 655393a022bd653e2b48dbf20b69236981e35195 has not been backpatched to
> 9.6.  I don't see any reason why we shouldn't backpatch this commit.
> So, I have attached a patch (fix_parallel_hash_path_v1.patch) which we
> can backpatch in 9.6.
>
> Robert, your input will be highly appreciated here especially for the
> back patch (to 9.6) I am proposing?
>

I have rebased the HEAD patch and done some cosmetic changes like
improved the test by giving aliases to table names and modified the
comment a bit, otherwise, the core logic remains the same.  As the
back-branch patches are just the matter of rebasing them, I will do
that before commit.

I am still waiting for input, but if there is none, my plan is to
commit this in a day or two and back-patch it as well.  Along with
this, I would also like to back-patch commit
655393a022bd653e2b48dbf20b69236981e35195 for the reasons mentioned
above.

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