Re: [HACKERS] advanced partition matching algorithm for partition-wise join

Hi Tomas and Ashutosh,

On Thu, Apr 2, 2020 at 1:51 AM Ashutosh Bapat
<ashutosh.bapat@2ndquadrant.com> wrote:
> On Thu, 26 Mar 2020 at 05:47, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:

>> three more comments after eye-balling the code for a bit longer.
>>
>> 1) The patch probably needs to tweak config.sgml which says this about
>> the enable_partitionwise_join GUC:
>>
>>    .. Partitionwise join currently applies only when the join conditions
>>    include all the partition keys, which must be of the same data type
>>    and have exactly matching sets of child partitions. ..
>
>
> Done. Actually this wasn't updated when partition pruning was introduced, which could cause a partitionwise join to be not used even when those conditions were met. Similarly when a query involved whole row reference. It's hard to explain all the conditions under which partitionwise join technique will be used. But I have tried to keep it easy to understand.
>
>>
>>
>> Which is probably incorrect, because the point of this patch is not to
>> require exact match of the partitions, right?
>>
>> 2) Do we really need the 'merged' flag in try_partitionwise_join? Can't
>> we simply use the joinrel->merged flag directly? ISTM the we always
>> start with joinrel->merged=false, and then only ever set it to true in
>> some cases. I've tried doing that, per the attached 0002 patch. The
>> regression tests seem to work fine.
>
>
> Thanks. I just added a small prologue to compute_partition_bounds().
>
>>
>>
>> I noticed this because I've tried moving part of the function into a
>> separate function, and removing the variable makes that simpler.
>>
>> The patch also does a couple additional minor tweaks.
>>
>> 3) I think the for nparts comment is somewhat misleading:
>>
>>    int nparts;  /* number of partitions; 0 = not partitioned;
>>                  * -1 = not yet set */
>>
>> which says that nparts=0 means not partitioned. But then there are
>> conditions like this:
>>
>>      /* Nothing to do, if the join relation is not partitioned. */
>>      if (joinrel->part_scheme == NULL || joinrel->nparts == 0)
>>          return;
>>
>> which (ignoring the obsolete comment) seems to say we can have nparts==0
>> even for partitioned tables, no?
>
>
> See my previous mail.
>
>>
>>
>> Anyway, attached is the original patch (0001) and two patches with
>> proposed changes. 0002 removes the "merged" flag as explained in (2),
>> 0003 splits the try_partitionwise_join() function into two parts.
>>
>> I'm saying these changes have to happen and it's a bit crude (and it
>> might be a bit of a bikeshedding).
>
>
> I have added 0005 with the changes I described in this as well as the previous mail. 0004 is just some white space fixes.

Thanks for the comments, Tomas!  Thanks for the patch, Ashutosh!  I
will look at the patch.

Best regards,
Etsuro Fujita