Re: [Patch] Optimize dropping of relation buffers using dlist

On Fri, Dec 11, 2020 at 5:54 AM k(dot)jamison(at)fujitsu(dot)com
<k(dot)jamison(at)fujitsu(dot)com> wrote:
>
> On Thursday, December 10, 2020 8:12 PM, Amit Kapila wrote:
> > On Thu, Dec 10, 2020 at 1:40 PM k(dot)jamison(at)fujitsu(dot)com
> > <k(dot)jamison(at)fujitsu(dot)com> wrote:
> > >
> > > Yes, I have tested that optimization works for index relations.
> > >
> > > I have attached the V34, following the conditions that we use "cached"
> > > flag for both DropRelFileNodesBuffers() and DropRelFileNodesBuffers()
> > > for consistency.
> > > I added comment in 0004 the limitation of optimization when there are
> > > TOAST relations that use NON-PLAIN strategy. i.e. The optimization
> > > works if the data types used are integers, OID, bytea, etc. But for
> > > TOAST-able data types like text, the optimization will be skipped and force a
> > full scan during recovery.
> > >
> >
> > AFAIU, it won't take optimization path only when we have TOAST relation but
> > there is no insertion corresponding to it. If so, then we don't need to mention
> > it specifically because there are other similar cases where the optimization
> > won't work like when during recovery we have to just perform TRUNCATE.
> >
>
> Right, I forgot to add that there should be an update like insert to the TOAST
> relation for truncate optimization to work. However, that is only limited to
> TOAST relations with PLAIN strategy. I have tested with text data type, with
> Inserts before truncate, and it did not enter the optimization path.
>

I think you are seeing because text datatype allows creating toast
storage and your data is small enough to be toasted.

> OTOH,
> It worked for data type like integer.
>

It is not related to any datatype, it can happen whenever we don't
have any operation on any of the forks after recovery.

> So should I still not include that information?
>

I think we can extend your existing comment like: "Otherwise if the
size of a relation fork is not cached, we proceed to a full scan of
the whole buffer pool. This can happen if there is no update to a
particular fork during recovery."

--
With Regards,
Amit Kapila.