Re: Confine vacuum skip logic to lazy_scan_skip

Hi,

On Fri, 5 Jan 2024 at 02:25, Jim Nasby <jim(dot)nasby(at)gmail(dot)com> wrote:
>
> On 1/4/24 2:23 PM, Andres Freund wrote:
>
> On 2024-01-02 12:36:18 -0500, Melanie Plageman wrote:
>
> Subject: [PATCH v2 1/6] lazy_scan_skip remove unnecessary local var rel_pages
> Subject: [PATCH v2 2/6] lazy_scan_skip remove unneeded local var
> nskippable_blocks
>
> I think these may lead to worse code - the compiler has to reload
> vacrel->rel_pages/next_unskippable_block for every loop iteration, because it
> can't guarantee that they're not changed within one of the external functions
> called in the loop body.
>
> Admittedly I'm not up to speed on recent vacuum changes, but I have to wonder if the concept of skipping should go away in the context of vector IO? Instead of thinking about "we can skip this range of blocks", why not maintain a list of "here's the next X number of blocks that we need to vacuum"?

Sorry if I misunderstood. AFAIU, with the help of the vectored IO;
"the next X number of blocks that need to be vacuumed" will be
prefetched by calculating the unskippable blocks ( using the
lazy_scan_skip() function ) and the X will be determined by Postgres
itself. Do you have something different in your mind?

--
Regards,
Nazir Bilal Yavuz
Microsoft