Re: generic plans and "initial" pruning

Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> On Thu, Jul 28, 2022 at 1:27 AM Robert Haas <robertmhaas(at)gmail(dot)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