Re: index prefetching

On 1/17/24 09:45, Konstantin Knizhnik wrote:
> I have integrated your prefetch patch in Neon and it actually works!
> Moreover, I combined it with prefetch of leaf pages for IOS and it also
> seems to work.
>

Cool! And do you think this is the right design/way to do this?

> Just small notice: you are reporting `blks_prefetch_rounds` in explain,
> but it is not incremented anywhere.
> Moreover, I do not precisely understand what it mean and wonder if such
> information is useful for analyzing query executing plan.
> Also your patch always report number of prefetched blocks (and rounds)
> if them are not zero.
>

Right, this needs fixing.

> I think that adding new information to explain it may cause some
> problems because there are a lot of different tools which parse explain
> report to visualize it,
> make some recommendations top improve performance, ... Certainly good
> practice for such tools is to ignore all unknown tags. But I am not sure
> that everybody follow this practice.
> It seems to be more safe and at the same time convenient for users to
> add extra tag to explain to enable/disable prefetch info (as it was done
> in Neon).
>

I think we want to add this info to explain, but maybe it should be
behind a new flag and disabled by default.

> Here we come back to my custom explain patch;) Actually using it is not
> necessary. You can manually add "prefetch" option to Postgres core (as
> it is currently done in Neon).
>

Yeah, I think that's the right solution.

regards

--
Tomas Vondra
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