On Wed, Aug 26, 2020 at 4:21 PM Andy Fan <zhihui.fan1...@gmail.com> wrote:
> > > On Mon, Aug 17, 2020 at 10:12 PM Andy Fan <zhihui.fan1...@gmail.com> > wrote: > >> >> >> On Mon, Jun 22, 2020 at 9:39 AM Tom Lane <t...@sss.pgh.pa.us> wrote: >> >>> I wrote: >>> > Nope. The entire reason why we have that kluge is that we don't know >>> > until much later how many times we expect to execute the subplan. >>> > AlternativeSubPlan allows the decision which subplan form to use to be >>> > postponed till runtime; but when we're doing things like estimating the >>> > cost and selectivity of a where-clause, we don't know that. >>> >>> > Maybe there's some way to recast things to avoid that problem, >>> > but I have little clue what it'd look like. >>> >>> Actually ... maybe it's not that bad. Clearly there would be a >>> circularity issue for selectivity estimation, but all the alternatives >>> should have the same selectivity. Cost estimation is a different story: >>> by the time we need to do cost estimation for a subexpression, we do in >>> many cases have an idea how often the subexpression will be executed. >>> >>> >> I read your idea of "ripping out all executor support for >> AlternativeSubPlan >> and instead having the planner replace an AlternativeSubPlan with >> the desired specific SubPlan somewhere late in planning, possibly >> setrefs.c." >> in [1]. I was thinking that if we can do such a replacement sooner, >> for example once we know the num_calls for the subplans, Unknown if it >> is possible though. If we can, then we can handle the issue here as well. >> >> The attached is a very PoC version, I'm not sure if it is the right >> direction >> to go. >> > > The idea behind it is if we have a RelOptInfo which have > some AlternativeSubPlan, > and assume these subplans have some correlated vars which can be expressed > as > deps_relids. Then we can convert the AlternativeSubPlan to SubPlan once > bms_is_subset(subplan->deps_relids, rel->relids). > The way of figuring out subplan->deps_relids was wrong in my patch, I will fix it later. But the general idea is the same. > My patch is able to fix the issue reported > here and it only converts the AlternativeSubPlan in rel->reltarget for > demo purpose. > > -- > Best Regards > Andy Fan > -- Best Regards Andy Fan
