Re: pg_buffercache: Add per-relation summary stats

On Tue, Apr 7, 2026 at 9:25 PM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 07/04/2026 16:47, Andres Freund wrote:
> > On 2026-04-07 16:07:45 +0300, Heikki Linnakangas wrote:
> >> On 28/03/2026 06:18, Ashutosh Bapat wrote:
> >>> Parallely myself and Palak Chaturvedi developed a quick patch to
> >>> modernise pg_buffercache_pages() and use tuplestore so that it doesn't
> >>> have to rely on NBuffers being the same between start of the scan,
> >>> when memory allocated, when the scan ends - a condition possible with
> >>> resizing buffer cache. It seems to improve the timings by about 10-30%
> >>> on my laptop for 128MB buffercache size. Without this patch the time
> >>> taken to execute Lukas's query varies between 10-15ms on my laptop.
> >>> With this patch it varies between 8-9ms. So the timing is more stable
> >>> as a side effect. It's not a 10x improvement that we are looking for
> >>> but it looks like a step in the right direction. That improvement
> >>> seems to come purely because we avoid creating a heap tuple. I wonder
> >>> if there are some places up in the execution tree where full
> >>> heaptuples get formed again instead of continuing to use minimal
> >>> tuples or places where we perform some extra actions that are not
> >>> required.
> >
> > I don't think that's the reason for the improvement - tuplestore_putvalues()
> > forms a minimal tuple, and the cost to form a minimal tuple and a heap tuple
> > aren't meaningfully different.
>
> Yeah, I wasn't fully convinced of that part either, which is why I left
> it out of the commit message. I mostly wanted to get rid of the
> double-buffering where we first accumulated all the data in an array.
>

Just because of the name of the function, I thought
tuplestore_puttuple stores virtual tuple. But looking at the function
it's clearly using minimal tuples. Sorry for my misunderstanding.

> > I think the problem is that we materialize rowmode SRFs as a tuplestore if
> > they are in the from list.  You can easily see this even with just
> > generate_series():
> >
> > postgres[1520825][1]=# SELECT count(*) FROM generate_series(1, 1000000);
> > ┌─────────┐
> > │  count  │
> > ├─────────┤
> > │ 1000000 │
> > └─────────┘
> > (1 row)
> >
> > Time: 117.939 ms
> > postgres[1520825][1]=# SELECT count(*) FROM (SELECT generate_series(1, 1000000));
> > ┌─────────┐
> > │  count  │
> > ├─────────┤
> > │ 1000000 │
> > └─────────┘
> > (1 row)
> >
> > Time: 58.914 ms
>
> Oh, to be honest I didn't remember that we *don't* materialize when it's
> in the target list.
>

IIUC, query using SRF in targetlist runs faster because it does "not"
use tuplestore. I consistently saw 10%-30% performance improvement
after using tuplestore in pg_buffercache_pages(). Is that purely
because of avoiding an in-memory array?

--
Best Wishes,
Ashutosh Bapat