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 <ZkK3fm599U2rnRdd@momjian.us>
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From: Andy Fan <zhihuifan1213@163.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>,
 pgsql-hackers@lists.postgresql.org
Subject: Re: First draft of PG 17 release notes
Date: Tue, 14 May 2024 10:32:14 +0800
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Bruce Momjian <bruce@momjian.us> writes:

> On Sat, May 11, 2024 at 01:27:25PM +0800, Andy Fan wrote:
>> 
>> Hello Bruce,
>> 
>> > I have committed the first draft of the PG 17 release notes;  you can
>> > see the results here:
>> >
>> > 	https://momjian.us/pgsql_docs/release-17.html
>> 
>> Thank you for working on this!
>> 
>> > I welcome feedback.  For some reason it was an easier job than usual.
>> 
>> Do you think we need to add the following 2 items?
>> 
>> - 9f133763961e280d8ba692bcad0b061b861e9138 this is an optimizer
>>   transform improvement.
>
> It was unclear from the commit message exactly what user-visible
> optimization this allowed.  Do you have details?

Yes, It allows the query like "SELECT * FROM t1 WHERE t1.a in (SELECT a
FROM t2 WHERE t2.b = t1.b)" be pulled up a semi join, hence more join
methods / join orders are possible.

>
>> - a8a968a8212ee3ef7f22795c834b33d871fac262 this is an optimizer costing
>>   improvement.
>
> Does this allow faster UNION ALL with LIMIT, perhaps?

Yes, for example:  (subquery-1) UNION ALL (subquery-2) LIMIT n;

When planning the subquery-1 or subquery-2, limit N should be
considered. As a consequence, maybe hash join should be replaced with
Nested Loop. Before this commits, it is ignored if it is flatten into 
appendrel, and the "flatten" happens very often.

David provided a summary for the both commits in [1].

[1]
https://www.postgresql.org/message-id/CAApHDvqAQgq27LgYmJ85VVGTR0%3DhRW6HHq2oZgK0ZiYC_a%2BEww%40mail.gmail.com 

-- 
Best Regards
Andy Fan