Re: [PATCH] Erase the distinctClause if the result is unique by definition

Thank you Tom for the review! 

On Mon, Mar 2, 2020 at 4:46 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:

Andy Fan <zhihui.fan1213@gmail.com> writes:
> Please see if you have any comments.   Thanks

The cfbot isn't at all happy with this.  Its linux build is complaining
about a possibly-uninitialized variable, and then giving up:

https://travis-ci.org/postgresql-cfbot/postgresql/builds/656722993

The Windows build isn't using -Werror, but it is crashing in at least
two different spots in the regression tests:

https://ci.appveyor.com/project/postgresql-cfbot/postgresql/build/1.0.81778

I've not attempted to identify the cause of that.

 

Before I submit the patch, I can make sure "make check-world" is successful, but

since the compile option is not same,  so I didn't catch the possibly-uninitialized 

variable.   As for the crash on the windows,  I didn't get the enough information 

now,  I will find a windows server and reproduce the cases.

I just found the link http://commitfest.cputube.org/ this morning, I will make sure

the next patch can go pass this test.  

At a high level, I'm a bit disturbed that this focuses only on DISTINCT
and doesn't (appear to) have any equivalent intelligence for GROUP BY,
though surely that offers much the same opportunities for optimization.
It seems like it'd be worthwhile to take a couple steps back and see
if we couldn't recast the logic to work for both.

OK,  Looks group by is a bit harder  than distinct since the aggregation function. 

I will go through the code to see why to add this logic.  

 

* There seem to be some pointless #include additions, eg in planner.c
the added code doesn't look to justify any of them.  Please also
avoid unnecessary whitespace changes, those also slow down reviewing.