Message-ID: <8a09d059-8bcf-48de-8c8e-771d8221091c@gmail.com>
Date: Fri, 11 Oct 2024 09:43:52 +0700
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Subject: Re: BUG #18652: Planner can not find pathkey item to sort for query
 with expression and expression index
From: Andrei Lepikhov <lepihov@gmail.com>
To: Tender Wang <tndrwang@gmail.com>
Cc: Richard Guo <guofenglinux@gmail.com>, 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>
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 <8026fe90-e661-4448-b9cc-bcbb65629b7b@gmail.com>
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On 10/10/24 17:23, Andrei Lepikhov wrote:
> On 10/10/24 16:02, Tender Wang wrote:
>>
>>
>> Andrei Lepikhov <lepihov@gmail.com <mailto:lepihov@gmail.com>> 于2024 
>> 年      > 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?
>>
>>
>> Yeah, I'm also curious why only cost Sort but not adding Sort node in 
>> create_merge_append_path().
>> The comments say that " We'll need to insert a Sort node, so include 
>> cost for that".  Does another place
>> insert the Sort node?
> Before inserting the Sort node, we must identify the column 
> corresponding to each path key. And here is the problem: IndexScan has 
> two elements in the target list, but SeqScan has only one. It already 
> looks strange to me. How do we UNION two sources with different numbers 
> of resulting columns? It seems to me we have a bug under the Append node.Sorry for the mess: my sentence is totally wrong: it is the MergeAppend 
node with two elements in the target list (the second one is resjunk). I 
think I should research the logic of how append arranges its pathkeys 
beforehand.

-- 
regards, Andrei Lepikhov