Re: Partial join

"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Uh ... why? The pushed-down restrictions should result in pruning
> away any prunable partitions at the scan level, leaving nothing for
> the partitionwise join code to do.

It seems reasonable to me that the join condition can no longer be verified, since 'sc.sl = sg.sl' is now replaced by 'sg.sl = 5' so the join condition can no longer be validated.

It's true that the pruning would prune everything but one partition, in case we'd just have a single column partition key. But we don't. I don't see how pruning partitions should help in this case, since we are left with multiple partitions for both relations.

Regards
Arne

________________________________
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Sent: Thursday, August 1, 2019 4:14:54 PM
To: Richard Guo
Cc: Arne Roland; pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Partial join

Richard Guo <riguo(at)pivotal(dot)io> writes:
> For the third query, a rough investigation shows that, the qual 'sl =
> 5' and 'sc.sl = sg.sl' will form an equivalence class and generate two
> implied equalities: 'sc.sl = 5' and 'sg.sl = 5', which can be pushed
> down to the base rels. One consequence of the deduction is when
> constructing restrict lists for the joinrel, we lose the original
> restrict 'sc.sl = sg.sl', and this would fail the check
> have_partkey_equi_join(), which checks if there exists an equi-join
> condition for each pair of partition keys. As a result, this joinrel
> would not be considered as an input to further partitionwise joins.

> We need to fix this.

Uh ... why? The pushed-down restrictions should result in pruning
away any prunable partitions at the scan level, leaving nothing for
the partitionwise join code to do.

regards, tom lane