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

I'm out of it more than usual..

At Tue, 08 Dec 2020 09:45:53 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> At Mon, 7 Dec 2020 17:18:31 +0530, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote in
> > On Mon, Dec 7, 2020 at 12:32 PM k(dot)jamison(at)fujitsu(dot)com
> > <k(dot)jamison(at)fujitsu(dot)com> wrote:
> > >
> > > On Friday, December 4, 2020 8:27 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > Hi,
> > > I have reported before that it is not always the case that the "cached" flag of
> > > srnblocks() return true. So when I checked the truncate test case used in my
> > > patch, it does not enter the optimization path despite doing INSERT before
> > > truncation of table.
> > > The reason for that is because in TRUNCATE, a new RelFileNode is assigned
> > > to the relation when creating a new file. In recovery, XLogReadBufferExtended()
> > > always opens the RelFileNode and calls smgrnblocks() for that RelFileNode for the
> > > first time. And for recovery processing, different RelFileNodes are used for the
> > > INSERTs to the table and TRUNCATE to the same table.
> > >
> >
> > Hmm, how is it possible if Insert is done before Truncate? The insert
> > should happen in old RelFileNode only. I have verified by adding a
> > break-in (while (1), so that it stops there) heap_xlog_insert and
> > DropRelFileNodesAllBuffers(), and both get the same (old) RelFileNode.
> > How have you verified what you are saying?

It's irrelvant that the insert happens on the old relfilenode. We drop
buffers for the old relfilenode on truncation anyway.

What I did is:

a: Create a physical replication pair.
b: On the master, create a table. (without explicitly starting a tx)
c: On the master, insert a tuple into the table.
d: On the master truncate the table.

On the standby, smgrnblocks is called for the old relfilenode of the
table at c, then the same function is called for the same relfilenode
at d and the function takes the cached path.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center