Re: [HACKERS] Partition-wise aggregation/grouping

On Wed, Mar 21, 2018 at 8:01 AM, Jeevan Chalke
<jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:
>> In the patch as proposed, create_partial_grouping_paths() can get
>> called even if GROUPING_CAN_PARTIAL_AGG is not set. I think that's
>> wrong.
>
> I don't think so. For parallel case, we do check that. And for partitionwise
> aggregation check, it was checked inside can_partitionwise_grouping()
> function and flags were set accordingly. Am I missing something?

Well, one of us is missing something somewhere. If
GROUPING_CAN_PARTIAL_AGG means that we're allowed to do partial
grouping, and if create_partial_grouping_paths() is where partial
grouping happens, then we should only be calling the latter if the
former is set. I mean, how can it make sense to create
partially-grouped paths if we're not allowed to do partial grouping?

> I have tweaked these conditions and posted in a separate patch (0006).
> However, I have merged all your three patches in one (0005).

OK, thanks. I wasn't sure I had understood what was going on, so
thanks for checking it.

Thanks also for keeping 0004-0006 separate here, but I think you can
flatten them into one patch in the next version.

>> I think that if the last test in can_partitionwise_grouping were moved
>> before the previous test, it could be simplified to test only
>> (extra->flags & GROUPING_CAN_PARTIAL_AGG) == 0 and not
>> *perform_partial_partitionwise_aggregation.
>
> I think we can't do this way. If *perform_partial_partitionwise_aggregation
> found to be true then only we need to check whether partial aggregation
> itself is possible or not. If we are going to perform a full partitionwise
> aggregation then test for can_partial_agg is not needed. Have I misread your
> comments?

It seems you're correct, because when I change it the tests fail. I
don't yet understand why.

Basically, the main patch seems to use three Boolean signaling mechanisms:

1. GROUPING_CAN_PARTITIONWISE_AGG
2. is_partial_aggregation
3. perform_partial_partitionwise_aggregation

Stuff I don't understand:

- Why is one of them a Boolean shoved into "flags", even though it's
not static across the whole hierarchy like the other flags, and the
other two are separate Booleans?
- What do they all do, anyway?

--
Robert Haas
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