On 21 December 2017 at 23:51, Beena Emerson <memissemer...@gmail.com> wrote: > On Thu, Dec 21, 2017 at 2:31 PM, David Rowley > <david.row...@2ndquadrant.com> wrote: >> On 19 December 2017 at 21:54, Beena Emerson <memissemer...@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. > Thank you for working on this. I will look into this and merge with > my current version of patch and Amit's v16 patches and post a new > patch soon. okay. I've just rebased mine on top of Amit's v16 now and will send shortly. I already took some code from your v7 patch to make my v2 work. Which parts do you think still need to be merged? -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
