Re: Add memory/disk usage for WindowAgg nodes in EXPLAIN

On Wed, 10 Jul 2024 at 21:36, Tatsuo Ishii <ishii(at)postgresql(dot)org> wrote:
> v2-0005-Add-memory-disk-usage-for-Window-Aggregate-nodes-.patch: This
> adds memory/disk usage for Window Aggregate nodes in EXPLAIN (ANALYZE)
> command. Note that if David's proposal
> https://www.postgresql.org/message-id/CAHoyFK9n-QCXKTUWT_xxtXninSMEv%2BgbJN66-y6prM3f4WkEHw%40mail.gmail.com
> is committed, this will need to be adjusted.

Hi,

I pushed the changes to WindowAgg so as not to call tuplestore_end()
on every partition. Can you rebase this patch over that change?

It would be good to do this in a way that does not add any new state
to WindowAggState, you can see that I had to shuffle fields around in
that struct because the next_parition field would have caused the
struct to become larger. I've not looked closely, but I expect this
can be done by adding more code to tuplestore_updatemax() to also
track the disk space used if the current storage has gone to disk. I
expect the maxSpace field can be used for both, but we'd need another
bool field to track if the max used was by disk or memory.

I think the performance of this would also need to be tested as it
means doing an lseek() on every tuplestore_clear() when we've gone to
disk. Probably that will be dominated by all the other overheads of a
tuplestore going to disk (i.e. dumptuples() etc), but it would be good
to check this. I suggest setting work_mem = 64 and making a test case
that only just spills to disk. Maybe do a few thousand partitions
worth of that and see if you can measure any slowdown.

David