On Thu, Jul 23, 2020 at 9:22 PM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> 2MB 4MB 8MB 64MB 256MB
> -----------------------------------------------------------
> hash 6.71 6.70 6.73 6.44 5.81
> hash CP_SMALL_TLIST 5.28 5.26 5.24 5.04 4.54
> sort 3.41 3.41 3.41 3.57 3.45
>
> So sort writes ~3.4GB of data, give or take. But hashagg/master writes
> almost 6-7GB of data, i.e. almost twice as much. Meanwhile, with the
> original CP_SMALL_TLIST we'd write "only" ~5GB of data. That's still
> much more than the 3.4GB of data written by sort (which has to spill
> everything, while hashagg only spills rows not covered by the groups
> that fit into work_mem).
>
> I initially assumed this is due to writing the hash value to the tapes,
> and the rows are fairly narrow (only about 40B per row), so a 4B hash
> could make a difference - but certainly not this much. Moreover, that
> does not explain the difference between master and the now-reverted
> CP_SMALL_TLIST, I think.
This is all really good analysis, I think, but this seems like the key
finding. It seems like we don't really understand what's actually
getting written. Whether we use hash or sort doesn't seem like it
should have this kind of impact on how much data gets written, and
whether we use CP_SMALL_TLIST or project when needed doesn't seem like
it should matter like this either.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company