Hello,
On Thu, Dec 21, 2017 at 6:01 PM, David Rowley
<david.rowley@2ndquadrant.com> wrote:
> On 21 December 2017 at 23:51, Beena Emerson <memissemerson@gmail.com> wrote:
>> On Thu, Dec 21, 2017 at 2:31 PM, David Rowley
>> <david.rowley@2ndquadrant.com> wrote:
>>> On 19 December 2017 at 21:54, Beena Emerson <memissemerson@gmail.com> wrote:
>>
>>> The problem is down to the logic in choose_custom_plan() only choosing
>>> a generic plan if the average cost of the generic plan is less than
>>> the average custom plan cost. The problem is that the generic plan can
>>> have many extra Append subnodes in comparison to the custom plan, all
>>> of which are taken into account in the total plan cost, but these may
>>> be pruned during execution. The logic in choose_custom_plan() has no
>>> idea about this..
>>
>> I had mentioned this in the first mail on this thread that the generic
>> plan is always preferred and Robert said that it is not in scope of
>> this patch. Maybe we can start a new thread for this.
>
> Sorry, I missed that. I knew you had modified choose_custom_plan()
> too, but I didn't know the diagnosis of the problem had made its way
> here yet.
>
> I wonder if Robert understands the extent of the problem. The patch
> will be useful to prune away partitions when the partitioned table is
> on the inside of a parameterised nested loop join, but I think we'll
> likely get some complaints about PREPARE statements always using a
> custom plan. It's going to make this pretty hard to write regression
> tests for unless we went and invent some GUC "prefer_generic_plan", or
> "generic_plan_cost_multiplier" to provide a way to coax
> choose_custom_plan() into not choosing a custom plan. I admit to not
> having the answer to this but I don't think we should rule out
> discussing possible fixes.
I have attached the regression tests I am currently using to check my
patch. It has few prepared statements using runtime pruning.
--
Beena Emerson
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