Message-ID: <8026fe90-e661-4448-b9cc-bcbb65629b7b@gmail.com>
Date: Thu, 10 Oct 2024 17:23:40 +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: 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>
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 <CAMbWs49Pcnm_cX=7azYY9d8+qRrOOKDjJDCa0KGRk9kWWMOz3A@mail.gmail.com>
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 <CAHewXNmFmXFxK7bsgAhqv_ToGxK4y7niNVL3GAzgWEOKuiAg5w@mail.gmail.com>
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From: Andrei Lepikhov <lepihov@gmail.com>
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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.

-- 
regards, Andrei Lepikhov