On Fri, 2025-04-25 at 15:42 +0200, Marc Millas wrote: > got something strange to me: > Same db ie. same data, around 1.2TB,one on pg13, one on pg16 > same 16 GB of shared_buffers, > I am the single user. > both have track_io_timing on > > on pg13, if I run a big request with explain (analyze,buffers), > I see around 6 GB read > if I do rerun the very same request, no more read(s), all data in the shared buffers cache. fine > If I check with pg_buffercache what's in it, I see the biggest tables of my request within > the biggest users (in number of blocks used). All this is fine. > > next, if I do the very same on the pg16 machine, whatever the number of times I rerun the > explain (analyze, buffers) of the same request, each time, the explain shows the same volume > of reads. again and again. > If I check with pg_buffercache, the set of objects stay the same, WITHOUT the objects of my > request, just like if those objects where sticky.
I can't see the plans, so I can only guess.
Perhaps the v16 plan uses a sequential scan on a table that is more than a quarter of shared_buffers in size, so that PostgreSQL uses a ring buffer to read it instead of blowing out more than a quarter of its buffer cache.
