| From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Andrei Lepikhov <lepihov(at)gmail(dot)com>, Ilia Evdokimov <ilya(dot)evdokimov(at)tantorlabs(dot)com>, Lukas Fittl <lukas(at)fittl(dot)com>, Daniel Gustafsson <daniel(at)yesql(dot)se>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Nikolay Samokhvalov <samokhvalov(at)gmail(dot)com> |
| Subject: | Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment |
| Date: | 2025-03-25 03:32:56 |
| Message-ID: | CAApHDvqPkWmz1Lq23c9CD+mAW=hgPrD289tC30f3H1f6Ng59+g@mail.gmail.com |
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On Tue, 25 Mar 2025 at 11:15, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> FWIW, I share these doubts about whether these values are useful
> enough to include in the default EXPLAIN output. My main beef
> with them though is that they are basically numbers derived along
> the way to producing a cost estimate, and I don't think we break
> out such intermediate results for other node types.
The only likeness I can think of is the "(originally N)" for the
buckets and batches in EXPLAIN ANALYZE for Hash Joins. I see that that
only shows when one of those isn't the same as the planner's
expectations
Maybe there's room to show additional details for Memoize in some
similar way only during EXPLAIN ANALYZE. EXPLAIN ANALYZE for Memoize
currently shows things like the number of hits, misses and evictions.
We could calculate what the planner expected those values to be and
show those. For this case, they're almost certain not to match what
the planner expected, but maybe that's ok.
David
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