Feedback-ID: id4a34324:Fastmail
Date: Tue, 10 Mar 2026 20:16:50 -0400
From: Andres Freund <andres@anarazel.de>
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>,
	pgsql-hackers <pgsql-hackers@lists.postgresql.org>,
 Nazir Bilal Yavuz <byavuz81@gmail.com>
Subject: Re: Streamify more code paths
Message-ID: <r2lwb3342lwrvyoz4fuwctfgfx776rm22ttaytvlinbkarakmp@gujbzwverhra>
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Hi,

On 2026-03-10 19:27:59 -0400, Andres Freund wrote:
> > > pgstattuple_large          base= 12429.3ms  patch= 11916.8ms   1.04x
> > > (  4.1%)  (reads=206945->12983, io_time=6501.91->32.24ms)
> > 
> > > pgstattuple_large          base= 12642.9ms  patch= 11873.5ms   1.06x
> > > (  6.1%)  (reads=206945->12983, io_time=6516.70->143.46ms)
> > 
> > Yeah, this looks somewhat strange. The io_time has been reduced
> > significantly, which should also lead to a substantial reduction in
> > runtime.
> 
> It's possible that the bottleneck just moved, e.g to the checksum computation,
> if you have data checksums enabled.
> 
> It's also worth noting that likely each of the test reps measures
> something different, as likely
>   psql_run "$ROOT" "$PORT" -c "UPDATE heap_test SET data = data || '!' WHERE id % 5 = 0;"
> 
> leads to some out-of-page updates.
> 
> You're probably better off deleting some of the data in a transaction that is
> then rolled back. That will also unset all-visible, but won't otherwise change
> the layout, no matter how many test iterations you run.
> 
> 
> I'd also guess that you're seeing a relatively small win because you're
> updating every page. When reading every page from disk, the OS can do
> efficient readahead.  If there are only occasional misses, that does not work.

I think that last one is a big part - if I use
  BEGIN; DELETE FROM heap_test WHERE id % 500 = 0; ROLLBACK;
(which leaves a lot of 

I see much bigger wins due to the pgstattuple changes.

                       time buffered          time DIO
w/o read stream        2222.078 ms            2090.239 ms
w   read stream         299.455 ms             155.124 ms

That's with local storage. io_uring, but numbers with worker are similar.


Greetings,

Andres Freund