Re: Partial join

Hello Richard,

thanks for your quick reply.

> We need to fix this.

Do you have a better idea than just keeping the old quals - possibly just the ones that get eliminated - in a separate data structure? Is the push down of quals the only case of elimination of quals, only counting the ones which happen before the restrict lists are generated?

Regards

Arne

________________________________
From: Richard Guo <riguo(at)pivotal(dot)io>
Sent: Thursday, August 1, 2019 1:14:44 PM
To: Arne Roland
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Partial join

On Thu, Aug 1, 2019 at 5:38 PM Arne Roland <A(dot)Roland(at)index(dot)de<mailto: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<http://sc.sl> = sg.sl<http://sg.sl>' will form an equivalence class and generate two
implied equalities: 'sc.sl<http://sc.sl> = 5' and 'sg.sl<http://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<http://sc.sl> = sg.sl<http://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