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  1. Re: BUG #19357: PostgreSQL generates a custom plan that performs worse than the generic plan for a certain query.

    ZhangChi <798604270@qq.com> — 2025-12-17T15:52:23Z

    Hi Tom Lane,


I strongly agree with your point. My objective is not to make PostgreSQL capable of optimizing all queries.&nbsp;My intention is simply to note that, for this test case, PostgreSQL is fully capable of optimizing the query, as demonstrated by the generic plan. However, I am unsure why the performance of the custom plan actually degrades. I believe that understanding the reasons behind this performance degradation (or suboptimality) could be important. This facilitates PostgreSQL’s selection of the optimal query plan.


Best,
Chi


         Original
         
       
From: Tom Lane <tgl@sss.pgh.pa.us&gt;
Date: 2025-12-17 23:39
To: Greg Sabino Mullane <htamfids@gmail.com&gt;
Cc: 798604270 <798604270@qq.com&gt;, pgsql-bugs <pgsql-bugs@lists.postgresql.org&gt;
Subject: Re: BUG #19357: PostgreSQL generates a custom plan that performs worse than the generic plan for a certain query.



       Greg&nbsp;Sabino&nbsp;Mullane&nbsp;<htamfids@gmail.com&gt;&nbsp;writes:
&gt;&nbsp;Please&nbsp;simplify&nbsp;your&nbsp;test&nbsp;query&nbsp;as&nbsp;much&nbsp;as&nbsp;possible&nbsp;and&nbsp;use&nbsp;"text"&nbsp;format
&gt;&nbsp;in&nbsp;your&nbsp;explain&nbsp;results;&nbsp;those&nbsp;will&nbsp;improve&nbsp;your&nbsp;chances&nbsp;of&nbsp;getting&nbsp;a
&gt;&nbsp;useful&nbsp;reply.&nbsp;:)

More&nbsp;to&nbsp;the&nbsp;point:&nbsp;it's&nbsp;hard&nbsp;to&nbsp;get&nbsp;excited&nbsp;about&nbsp;optimizing&nbsp;a&nbsp;query
that&nbsp;was&nbsp;so&nbsp;obviously&nbsp;built&nbsp;by&nbsp;a&nbsp;fuzzer&nbsp;rather&nbsp;than&nbsp;corresponding&nbsp;to
any&nbsp;real-world&nbsp;use-case.&nbsp;&nbsp;We&nbsp;have&nbsp;finite&nbsp;manpower&nbsp;and&nbsp;we'd&nbsp;much&nbsp;rather
put&nbsp;it&nbsp;into&nbsp;solving&nbsp;users'&nbsp;real&nbsp;problems.

			regards,&nbsp;tom&nbsp;lane