Re: Cache relation sizes?

On Tue, Nov 17, 2020 at 4:13 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Mon, Nov 16, 2020 at 11:01 PM Konstantin Knizhnik
> <k(dot)knizhnik(at)postgrespro(dot)ru> wrote:
> > I will look at your implementation more precisely latter.
>
> Thanks! Warning: I thought about making a thing like this for a
> while, but the patch itself is only a one-day prototype, so I am sure
> you can find many bugs... Hmm, I guess the smgrnblocks_fast() code
> really needs a nice big comment to explain the theory about why this
> value is fresh enough, based on earlier ideas about implicit memory
> barriers. (Something like: if you're extending, you must have
> acquired the extension lock; if you're scanning you must have acquired
> a snapshot that only needs to see blocks that existed at that time and
> concurrent truncations are impossible; I'm wondering about special
> snapshots like VACUUM, and whether this is too relaxed for that, or if
> it's covered by existing horizon-based interlocking...).
>

Yeah, it is good to verify VACUUM stuff but I have another question
here. What about queries having functions that access the same
relation (SELECT c1 FROM t1 WHERE c1 <= func(); assuming here function
access the relation t1)? Now, here I think because the relation 't1'
is already opened, it might use the same value of blocks from the
shared cache even though the snapshot for relation 't1' when accessed
from func() might be different. Am, I missing something, or is it
dealt in some way?

--
With Regards,
Amit Kapila.