From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
cc: Manikandan Swaminathan <maniswami23@gmail.com>,
        pgsql-general@lists.postgresql.org
Subject: Re: Postgres Query Plan using wrong index
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References: <1203098.1743640224@sss.pgh.pa.us> <2BC8AB39-16D7-4423-BE0A-F0F4EA432E2E@gmail.com> <CAApHDvou3ZhHmQ6Qx9O9HbXigTu9oHdTFKibv96TyAbJ8WGYNw@mail.gmail.com>
Comments: In-reply-to David Rowley <dgrowleyml@gmail.com>
	message dated "Thu, 03 Apr 2025 17:41:43 +1300"
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Date: Thu, 03 Apr 2025 01:07:22 -0400
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David Rowley <dgrowleyml@gmail.com> writes:
> On Thu, 3 Apr 2025 at 16:24, Manikandan Swaminathan
> <maniswami23@gmail.com> wrote:
>> why doesn’t making a multivariate statistic make a difference?

> Extended statistics won't help you here. "dependencies" just estimates
> functional dependencies between the columns mentioned in the ON
> clause.  What we'd need to store to do better in your example query is
> positional information of where certain values are within indexes
> according to an ordered scan of the index. I don't quite know how we'd
> represent that exactly, but if we knew that a row matching col_a >
> 4996 wasn't until somewhere near the end of idx_col_a_btree index,
> then we'd likely not want to use that index for this query.

A simple-minded approach could be to just be pessimistic, and
increase our estimate of how many rows would need to be scanned as a
consequence of noticing that the columns have significant correlation.
The shape of that penalty function would be mostly guesswork though,
I fear.  (Even with a clear idea of what to do, making this happen
seems a little complex --- just a SMOP, but I'm not very sure how to
wire it up.)

			regards, tom lane