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From: Tom Lane <[email protected]>
To: Felipe López Montes <[email protected]>
Cc: [email protected]
Subject: Re: PostgreSQL Choosing Full Index Over Partial Index
Date: Wed, 30 Apr 2025 10:14:04 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CACJPJu8_7yFDmOLcTiVCBLRs31UB4smUtUVEeLq1GoBujqfPqQ@mail.gmail.com>
References: <CACJPJu8oY9hb7LSsqHxbn24Gpa_tWBkcwPei=fottvgBeSc+PQ@mail.gmail.com>
	<[email protected]>
	<[email protected]>
	<CACJPJu8_7yFDmOLcTiVCBLRs31UB4smUtUVEeLq1GoBujqfPqQ@mail.gmail.com>

=?UTF-8?Q?Felipe_L=C3=B3pez_Montes?= <[email protected]> writes:
> Thanks a lot for your response Tom.
> May I ask how do you debug those functions?
> Or is it just that you read the code and more or less guess what should be
> the value for each variable with information coming from querying Postgres
> tables?

The guessing was in building a test case, since you didn't provide
a self-contained reproducer...

Once I had a case where the estimated cost was smaller for the larger
index, I just worked through the code to see where the costs diverged.
It helped that I already know the structure of that code well, but
there wasn't guesswork involved, other than where to set breakpoints
to narrow down the source more quickly.

I forgot to mention on this thread that I posted a possible fix at
[1].

			regards, tom lane

[1] https://www.postgresql.org/message-id/870686.1745860834%40sss.pgh.pa.us

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