Re: foreign join error "variable not found in subplan target list"

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От Richard Guo
Тема Re: foreign join error "variable not found in subplan target list"
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Msg-id CAMbWs49z470Eex4gYXaB=RB6LNJc-oE_mOQWaCTeUjBFm8dnWw@mail.gmail.com
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Ответ на Re: foreign join error "variable not found in subplan target list"  (Etsuro Fujita <etsuro.fujita@gmail.com>)
Ответы Re: foreign join error "variable not found in subplan target list"  (Etsuro Fujita <etsuro.fujita@gmail.com>)
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On Tue, Aug 30, 2022 at 3:50 PM Etsuro Fujita <etsuro.fujita@gmail.com> wrote:
On Mon, Aug 29, 2022 at 6:46 PM Richard Guo <guofenglinux@gmail.com> wrote:
> I'm looking again on how we adjust the PathTarget, and I think we may
> need to update the cost and width fields if there are any new columns
> added. Maybe we can leverage set_pathtarget_cost_width for that.

I'm not really sure we really need to make the estimates accurate,
because 1) the resulting EPQ plan is used only for rechecks, so the
estimates would not be that important IMO, and 2) in fact, we don't
adjust the estimates of a given EPQ plan in postgresGetForeignPlan()
even when replacing the plan's tlist and/or removing local conditions
from the plan.  Is it worth expending the cycles here?
 
Hmm, yes, the EPQ path does not need to compete with others in add_path,
so its cost does not matter too much. And the comment just above
GetExistingLocalJoinPath says:

 * Since the plan created using this path will presumably only be used to
 * execute EPQ checks, efficiency of the path is not a concern.

But still I feel it's not a good practice to not update the cost and
width fields after calling add_new_columns_to_pathtarget(). How about we
add some comments here explaining why we do not need to adjust the
estimates for the EPQ path?

Thanks
Richard

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