Re: parallelize queries containing initplans

On Wed, Oct 4, 2017 at 12:55 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Wed, Oct 4, 2017 at 3:40 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Tue, Oct 3, 2017 at 7:33 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>
>> Having said all that, I think that this patch only wants to handle the
>> subset of cases (2) and (4) where the relevant InitPlan is attached
>> ABOVE the Gather node -- which seems very reasonable, because
>> evaluating an InitPlan at a level of the plan tree above the level at
>> which it is defined sounds like it might be complex. But I still
>> don't quite see why we need these tests. I mean, if we only allow
>> Param references from a set of safe parameter IDs, and the safe
>> parameter IDs include only those IDs that can be generated in a
>> worker, then won't other InitPlans in the workers anyway be ruled out?
>
> It doesn't happen always. There are cases when the part of required
> conditions are pushed one query level below, so when we check during
> max_parallel_hazard_walker, they look safe, but actually, they are
> not. I will try to explain by example:
>
> postgres=# explain (costs off, verbose) select * from t1 where t1.i in
> ( select 1 + (select max(j) from t3));

If you want to reproduce this case, then you can use the script posted
by Kuntal up thread [1].

[1] - https://www.postgresql.org/message-id/CAGz5QC%2BuHOq78GCika3fbgRyN5zgiDR9Dd1Th5kENF%2BUpnPomQ%40mail.gmail.com

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