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From: Daniel Gustafsson <[email protected]>
To: Lukas Fittl <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: David Rowley <[email protected]>
Subject: Re: Add estimated hit ratio to Memoize in EXPLAIN to explain cost adjustment
Date: Thu, 6 Jul 2023 09:56:18 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAApHDvrBctjUFjY77BFd_MqPD+yf4nOZSKzbdzP70XntkknY8A@mail.gmail.com>
References: <CAP53Pky29GWAVVk3oBgKBDqhND0BRBN6yTPeguV_qSivFL5N_g@mail.gmail.com>
	<CAApHDvrBctjUFjY77BFd_MqPD+yf4nOZSKzbdzP70XntkknY8A@mail.gmail.com>

> On 7 Mar 2023, at 10:51, David Rowley <[email protected]> wrote:
> 
> On Sun, 5 Mar 2023 at 13:21, Lukas Fittl <[email protected]> wrote:
>> Alternatively (or in addition) we could consider showing the "ndistinct" value that is calculated in cost_memoize_rescan - since that's the most significant contributor to the cache hit ratio (and you can influence that directly by improving the ndistinct statistics).
> 
> I think the ndistinct estimate plus the est_entries together would be
> useful. I think showing just the hit ratio number might often just
> raise too many questions about how that's calculated. To calculate the
> hit ratio we need to estimate the number of entries that can be kept
> in the cache at once and also the number of input rows and the number
> of distinct values.  We can see the input rows by looking at the outer
> side of the join in EXPLAIN, but we've no idea about the ndistinct or
> how many items the planner thought could be kept in the cache at once.
> 
> The plan node already has est_entries, so it should just be a matter
> of storing the ndistinct estimate in the Path and putting it into the
> Plan node so the executor has access to it during EXPLAIN.

Lukas: do you have an updated patch for this commitfest to address David's
comments?

--
Daniel Gustafsson

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