Re: Improved Cost Calculation for IndexOnlyScan

On Mon, Oct 12, 2020 at 3:46 PM Hamid Akhtar <hamid(dot)akhtar(at)gmail(dot)com> wrote:

>
>
> On Tue, Sep 29, 2020 at 2:57 PM Heikki Linnakangas <hlinnaka(at)iki(dot)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.

>
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--
Highgo Software (Canada/China/Pakistan)
URL : www.highgo.ca
ADDR: 10318 WHALLEY BLVD, Surrey, BC
CELL：+923335449950 EMAIL: mailto:hamid(dot)akhtar(at)highgo(dot)ca
SKYPE: engineeredvirus