Message-ID: <ea7e53bf-5aad-4a0a-8f73-a21dd8f6969b@gmail.com>
Date: Thu, 10 Oct 2024 13:50:01 +0700
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Subject: Re: BUG #18652: Planner can not find pathkey item to sort for query
 with expression and expression index
To: Richard Guo <guofenglinux@gmail.com>
Cc: exclusion@gmail.com, pgsql-bugs@lists.postgresql.org,
 Tom Lane <tgl@sss.pgh.pa.us>
References: <18652-deaa782ebcca85d1@postgresql.org>
 <2527690.1728510183@sss.pgh.pa.us>
 <CAMbWs49Pcnm_cX=7azYY9d8+qRrOOKDjJDCa0KGRk9kWWMOz3A@mail.gmail.com>
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From: Andrei Lepikhov <lepihov@gmail.com>
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On 10/10/24 10:52, Richard Guo wrote:
> On Thu, Oct 10, 2024 at 5:43 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> It looks like we are generating a Path tree in which one of the
>> inputs to a MergeAppend is a plain unsorted seqscan, which'd
>> be all right except it doesn't expose the required sort value
>> in its targetlist.
> 
> Correct.  In addition, find_computable_ec_member() fails to find a
> computable expression from its targetlist.
> For the indexscan path, find_computable_ec_member() is able to find
> (t.i + 0) which can be computed from its tlist item 't.i'.
> 
> For the seqscan path, though, find_computable_ec_member() is not able
> to find ((t_1.i + 1) + 0) from its tlist item '(t_1.i + 1)'.
> 
> I think this is because find_computable_ec_member() only tries to
> match Vars.  Maybe we should teach it to also match OpExprs?
Looking into that case, I don't understand only one thing:
generate_orderedappend_paths decided to try MergeAppend; the 
create_append_path routine added the Sort cost,  but the Sort node 
itself wasn't added. Maybe the origin problem is the lack of feasibility 
examinations?

-- 
regards, Andrei Lepikhov