Re: Partial join

On Thu, Aug 1, 2019 at 5:38 PM Arne Roland <A(dot)Roland(at)index(dot)de> wrote:

> Hello,
>
> I attached one example of a partitioned table with multi column partition
> key. I also attached the output.
> Disabling the hash_join is not really necessary, it just shows the more
> drastic result in the case of low work_mem.
>
> Comparing the first and the second query I was surprised to see that SET
> enable_partitionwise_join could cause the costs to go up. Shouldn't the
> paths of the first query be generated as well?
>
> The third query seems to have a different issue. That one is close to my
> original performance problem. It looks to me like the push down of the sl
> condition stops the optimizer considering a partial join.
> If so would it be sane to keep a copy of the original quals to make the
> partial join possible? Do you have better ideas?
>

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.

Thanks
Richard