On Wed, Jan 11, 2017 at 4:05 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> While investigating why Rushabh Lathia's Gather Merge patch sometimes
>> fails to pick a Gather Merge plan even when it really ought to do so,
>> I ran smack into this problem. I discovered that this is more than a
>> cosmetic issue. The costing itself is actually badly broken.
>>
>> The reason why this is happening is that final_cost_nestloop(),
>> final_cost_hashjoin(), and final_cost_mergejoin() don't care a whit
>> about whether the path they are generating is partial. They apply the
>> row estimate for the joinrel itself to every such path generated for
>> the join, except for parameterized paths which are a special case. I
>> think this generally has the effect of discouraging parallel joins,
>> because the inflated row count also inflates the join cost. I think
>> the right thing to do is probably to scale the row count estimate for
>> the joinrel's partial paths by the leader_contribution value computed
>> in cost_seqscan.
>>
>> Despite my general hatred of back-patching things that cause plan
>> changes, I'm inclined to think the fix for this should be back-patched
>> to 9.6, because this is really a brown-paper-bag bug. If the
>> consensus is otherwise I will of course defer to that consensus.
>
> And here is a patch which seems to fix the problem.
Since nobody seems to have any comment here, I've committed and
back-patched this to 9.6.
--
Robert Haas
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