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

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От Ashutosh Bapat
Тема Re: [HACKERS] advanced partition matching algorithm forpartition-wise join
Дата
Msg-id CAExHW5twZqP1NU1G52EWMB1nxkNGFo5G=3yd1FEJ3d7ZbhdkzA@mail.gmail.com
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Ответ на Re: [HACKERS] advanced partition matching algorithm forpartition-wise join  (Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>)
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On Tue, May 14, 2019 at 10:00 AM Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> wrote:
On 2019/05/14 13:23, Amit Langote wrote:
> Tom
> strongly objected to that idea saying that such join paths are kind of
> silly [1], even outside the context of partitionwise join.  He suggested
> that we abandon partitionwise join in such cases, because having to build
> a dummy base relation for pruned partitions only to generate silly-looking
> paths would be an ugly kludge.

I forgot to mention that he even committed a patch to disable
partitionwise joins in such cases, which was also applied to v11 branch.

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=d70c147fa217c4bae32ac1afb86ab42d98b36fdf

Note that there were also other reasons for committing, beside what I
described in my previous email.

 
I haven't seen the actual commit, but we could use these patches to enable partition-wise join when partitions are pruned. For that the partition descriptor of the pruned partition table should be arranged as if those partitions are missing in the table itself. However, we will still need code to handle the cases when the partitions are missing on the nullable side. Tom mentioned the idea of using just projection to produce join tuples with rows on the outer side appended with null columns from the nullable side. If we can implement that, we can remove the restrictions in this patch.

--
Best Wishes,
Ashutosh Bapat

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