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From: Tom Lane <[email protected]>
To: Malay Keshav <[email protected]>
Cc: [email protected]
Cc: [email protected] <[email protected]>
Subject: Re: [Bug][Ver 11]: Generic query plan selected is worse than custom query plan
Date: Thu, 29 Sep 2022 15:58:02 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAJzqzvo+zATPZTtmFK0gjRQUftOuyMr6mB_bT_Pk+T2QNiSdrw@mail.gmail.com>
References: <CAJzqzvo+zATPZTtmFK0gjRQUftOuyMr6mB_bT_Pk+T2QNiSdrw@mail.gmail.com>

Malay Keshav <[email protected]> writes:
> In our particular case, the Postgres engine decided to pick the generic
> query plan and cache it for all further calls with that query. My
> understanding was that the generic query plan would only be selected if it
> had a better execution time than the custom query plan. Which in our case
> is not true.

> We were able to reproduce this deterministically using the same query
> parameters to trigger the engine to pick the bad generic query plan on the
> 6th run (first 5 runs shows the engine used the efficient query plan). Why
> does the engine pick the generic query plan when its execution time is
> worse than the custom query plan? Is this a bug?

Since you've provided precisely zero detail, it's impossible to answer
in any detail, but this is unlikely to be anything we'd call a bug.
What's presumably happening is that the planner's estimate of the cost
of the generic plan comes out less than its estimate of the cost of
a custom plan, but that estimate is faulty.  Sure, we'd love to improve
all such situations, but it's not an easy task --- and any changes in that
area would not be something we'd consider back-patching into long-stable
branches.

			regards, tom lane

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