Re: First draft of PG 17 release notes

Bruce Momjian <bruce(at)momjian(dot)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