On Mon, Oct 12, 2020 at 3:46 PM Hamid Akhtar <hamid.akh...@gmail.com> wrote:
> > > On Tue, Sep 29, 2020 at 2:57 PM Heikki Linnakangas <hlinn...@iki.fi> > wrote: > >> On 29/09/2020 11:49, Hamid Akhtar wrote: >> > So, not actually random replacement here, rather a change with >> > baserel->allvisfrac taken into consideration (as given below): >> > ---- >> > index_random_page_cost = Min(spc_seq_page_cost + spc_random_page_cost * >> > (1.0 - baserel->allvisfrac), spc_random_page_cost); >> > ---- >> > >> > Does this make sense? >> >> No. genericcostestimate() is only concerned with accesses to the index, >> not the the heap accesses that are needed with Index Scans. 'allvisfrac' >> should not affect the number of *index* pages fetched in any way. >> >> - Heikki >> > > Currently, the costing for indexonlyscan only differs based on > 'allvisfrac'. IIUC, the current implementation changes the number of pages > being fetched based on 'allvisfrac'. > > This patch actually makes indexonlyscan specific changes > to genericcostestimate function. Currently, regardless of the value of > 'allvisfrac', it is being assumed that the cost of fetching index pages is > random page cost. That is not aligned with the current cost calculation for > indexonlyscan. Therefore, I'm suggesting to reduce the random page in a > similar fashion in case of indexonlyscan. > > I'm adding this to the commitfest. > Retrospectively looking at the patch, I see your point. Your criticism is valid. I'll revalidate this issue and rework the patch if necessary. > > -- > Highgo Software (Canada/China/Pakistan) > URL : www.highgo.ca > ADDR: 10318 WHALLEY BLVD, Surrey, BC > CELL：+923335449950 EMAIL: mailto:hamid.akh...@highgo.ca > SKYPE: engineeredvirus > -- Highgo Software (Canada/China/Pakistan) URL : www.highgo.ca ADDR: 10318 WHALLEY BLVD, Surrey, BC CELL：+923335449950 EMAIL: mailto:hamid.akh...@highgo.ca SKYPE: engineeredvirus
