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From: Peter Geoghegan <pg@bowt.ie>
Date: Wed, 29 Apr 2026 10:43:47 -0400
Message-ID: <CAH2-Wzk6-mzP4utr9=sKT9XQi+NgonD_X30z1sf_hgnKQvPpGg@mail.gmail.com>
Subject: Re: Fix size estimation for parallel B-Tree scans with skip arrays
To: Siddharth Kothari <sidkot@google.com>
Cc: pgsql-bugs@lists.postgresql.org, Vaibhav Jain <jainva@google.com>, 
	Madhukar <madhukarprasad@google.com>, Xun Cheng <xuncheng@google.com>
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On Wed, Apr 29, 2026 at 2:54=E2=80=AFAM Siddharth Kothari <sidkot@google.co=
m> wrote:
> Root cause:
>
> In src/backend/access/nbtree/nbtree.c, the loop in btestimateparallelscan=
 assumes that every index column might require a skip array and adds sizeof=
(int) to the estimated size:
>
> However, every skip array actually needs space for its slot in the btps_a=
rrElems array AND space to store its scan key's sk_flags.

Your diagnosis looks correct to me. As you said, the problem is that
we only add btps_arrElems space overhead for input scan keys -- we
neglect to do the same for any skip array scan key that might be
output by nbtree preprocessing later on.

I'll commit this patch later today.

Thanks!
--=20
Peter Geoghegan