pb with big volumes

Hi,

I have a 15 TB db on postgres 14 (soon 15).
shared buffers is 32 GB.

It's a db with max 15 users and often less, and currently 1 or 2.
the biggest table have 133 partitions of 150M to 200M+ rows each.
lots of request access explicitly one of those.

When I, alone, run a query "reading" 15M buffers, it takes 20 minutes
(+-5minutes). inside the query there are 2 group by on a 200M rows
partition, with all the rows in each group by.
When a colleague run the same kind of request (not the same request, but
something reading roughly the same volume ) , on a different set of data,
his request is completed in less than half an hour.
If we run our requests simultaneously... my request take hours. around 3
hours.

I am making a supposition that its some kind of "pumping" effect in the
cache.

I cannot have access to the underlying OS. I can, for sure, do some copy xx
from program 'some command', but its a container with very limited
possibilities, not even 'ps'.
So I would like to monitor from inside the db (so without iostat and the
same) the volumes of read that postgres do to the OS.
I did activate track_io_timing, but the volumes I get in the explain
analyze buffer are roughly the same alone or not alone. (the 15M buffers
told )
to my understanding, the volumes that are shown in pg_stat_database are the
useful ones ie. even if the db as to read it from disk more than once. true
? or false ?

So.. either my supposition is not correct, and I will read with a lot of
interest other ideas
either its correct and I would like to know how to monitor this (in the
current context, installing a dedicated extension is not impossible, but is
a very boring process)

Thanks for your help :-)

regards,

PS: I know that providing the complete data model and the exact requests
can be considered mandatory, but when I change the request I get the very
same behaviour...

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com