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help / color / mirror / Atom feedFrom: Andrei Lepikhov <[email protected]>
To: Robert Haas <[email protected]>
To: David Rowley <[email protected]>
Cc: Ilia Evdokimov <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Lukas Fittl <[email protected]>
Cc: Daniel Gustafsson <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Nikolay Samokhvalov <[email protected]>
Subject: Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment
Date: Mon, 14 Apr 2025 21:49:23 +0200
Message-ID: <[email protected]> (raw)
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On 4/14/25 18:31, Robert Haas wrote:
> On Tue, Apr 1, 2025 at 5:07 PM David Rowley <[email protected]> wrote:
>> I tried to move things along to address Tom's concern about not
>> wanting to show this in EXPLAIN's standard output. I suggested in [1]
>> that we could use EXPLAIN ANALYZE, but nobody commented on that.
>> EXPLAIN ANALYZE is much more verbose than EXPLAIN already, and if we
>> put these in EXPLAIN ANALYZE then the viewer can more easily compare
>> planned vs actual. I had mentioned that Hash Join does something like
>> this for buckets.
> Having looked at v6, I think it would help, at least if the reader is
> sufficiently knowledgeable. From the values displayed, it looks like
> someone could reconstruct the evict_ratio value in
> cost_memoize_rescan(), and if they know the loop count, also the
> hit_ratio. But that seems hard: if you weren't reading the code, how
> would you know how to do it? Even if you are reading the code, are you
> sure you'd reconstruct it correctly? I wonder why we think it's better
> to display this than a more "cooked" number like the estimated hit
> ratio that was proposed in v1?
+1.
My reasons: a) it more "physical"; b) We don't need to look for the
corresponding NestLoop JOIN node - my explans usually contain 150+ lines
and it is hard to get into the analysis something out of current screen.
--
regards, Andrei Lepikhov
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Subject: Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment
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