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From: David Rowley <dgrowleyml@gmail.com>
Date: Wed, 16 Apr 2025 08:49:46 +1200
Message-ID: <CAApHDvoqHSX8OOLQy4Eu2_mz2MWMyHsoArnCy70zHuAV47ssnA@mail.gmail.com>
Subject: Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment
To: Robert Haas <robertmhaas@gmail.com>
Cc: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>, Tom Lane <tgl@sss.pgh.pa.us>, 
	Lukas Fittl <lukas@fittl.com>, Daniel Gustafsson <daniel@yesql.se>, 
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, 
	Nikolay Samokhvalov <samokhvalov@gmail.com>, Andrei Lepikhov <lepihov@gmail.com>
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On Wed, 16 Apr 2025 at 04:25, Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Mon, Apr 14, 2025 at 8:23=E2=80=AFPM David Rowley <dgrowleyml@gmail.co=
m> wrote:
> > "Estimates: capacity=3DN distinct keys=3DN lookups=3DN hit ratio=3DN.N%=
"
>
> Is lookups=3DN here the estimated number of lookups i.e. what we think
> nloops will end up being?

Yes. The estimate is the "calls" variable in cost_memoize_rescan(),
which is fairly critical in the hit ratio estimate calculation.

Technically this is just the Nested Loop's outer_path->rows. There was
an argument earlier in the thread for putting this in along with the
other fields to make things easier to read. I did argue that it was
redundant due to the fact that the reader can look at the row estimate
for the outer side of the Nest Loop, but maybe it's small enough to go
in using the above format.

David