Re: Improve UNION's output rowcount estimate

From: Richard Guo <guofenglinux(at)gmail(dot)com>
To: Chengpeng Yan <chengpeng_yan(at)outlook(dot)com>
Cc: David Rowley <dgrowleyml(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Improve UNION's output rowcount estimate
Date: 2026-07-01 06:34:18
Message-ID: CAMbWs4_DE1Fh9MzJ3xE6PEkW9tcA2h_Ydu+-BJoX1ZskwOZT1w@mail.gmail.com
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On Thu, Jun 25, 2026 at 12:12 PM Chengpeng Yan
<chengpeng_yan(at)outlook(dot)com> wrote:
> The added regression test looks reasonable to me, since it checks the
> plan change that motivated the patch. One small question: would it be
> worth adding a direct row-estimate check, perhaps with a helper like
> planner_est.sql’s explain_mask_costs() so that rows= stays visible while
> cost/width are masked? The existing plan-shape test may already be
> sufficient, though.

I think the plan-shape test is sufficient here. I've committed this
patch to master. Thanks for all the reviews.

- Richard

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