From: Tom Lane <tgl@sss.pgh.pa.us>
To: Amit Langote <amitlangote09@gmail.com>
cc: Robert Haas <robertmhaas@gmail.com>,
        Jacob Champion <jchampion@timescale.com>,
        Zhihong Yu <zyu@yugabyte.com>, David Rowley <dgrowleyml@gmail.com>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: generic plans and "initial" pruning
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Comments: In-reply-to Amit Langote <amitlangote09@gmail.com>
	message dated "Fri, 29 Jul 2022 13:20:37 +0900"
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Date: Fri, 29 Jul 2022 00:55:17 -0400
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Amit Langote <amitlangote09@gmail.com> writes:
> On Thu, Jul 28, 2022 at 1:27 AM Robert Haas <robertmhaas@gmail.com> wrote:
>> I wonder whether it's really necessary to added the PartitionPruneInfo
>> objects to a list in PlannerInfo first and then roll them up into
>> PlannerGlobal later. I know we do that for range table entries, but
>> I've never quite understood why we do it that way instead of creating
>> a flat range table in PlannerGlobal from the start. And so by
>> extension I wonder whether this table couldn't be flat from the start
>> also.

> Tom may want to correct me but my understanding of why the planner
> waits till the end of planning to start populating the PlannerGlobal
> range table is that it is not until then that we know which subqueries
> will be scanned by the final plan tree, so also whose range table
> entries will be included in the range table passed to the executor.

It would not be profitable to flatten the range table before we've
done remove_useless_joins.  We'd end up with useless entries from
subqueries that ultimately aren't there.  We could perhaps do it
after we finish that phase, but I don't really see the point: it
wouldn't be better than what we do now, just the same work at a
different time.

			regards, tom lane