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

Andy Fan <zhihui(dot)fan1213(at)gmail(dot)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.

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.

Some other random comments:

* Don't especially like the way you broke query_is_distinct_for()
into two functions, especially when you then introduced a whole
lot of other code in between. That's just making reviewer's job
harder to see what changed. It makes the comments a bit disjointed
too, that is where you even had any. (Zero introductory comment
for query_is_distinct_agg is *not* up to project coding standards.
There are a lot of other undercommented places in this patch, too.)

* Definitely don't like having query_distinct_through_join re-open
all the relations. The data needed for this should get absorbed
while plancat.c has the relations open at the beginning. (Non-nullness
of columns, in particular, seems like it'll be useful for other
purposes; I'm a bit surprised the planner isn't using that already.)

* In general, query_distinct_through_join seems hugely messy, expensive,
and not clearly correct. If it is correct, the existing comments sure
aren't enough to explain what it is doing or why.

* Not entirely convinced that a new GUC is needed for this, but if
it is, you have to document it.

* I wouldn't bother with bms_array_free(), nor with any of the other
cleanup you've got at the bottom of query_distinct_through_join.
The planner leaks *lots* of memory, and this function isn't going to
be called so many times that it'll move the needle.

* 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.

* I see you decided to add a new regression test file select_distinct_2.
That's a poor choice of name because it conflicts with our rules for the
naming of alternative output files. Besides which, you forgot to plug
it into the test schedule files, so it isn't actually getting run.
Is there a reason not to just add the new test cases to select_distinct?

* There are some changes in existing regression cases that aren't
visibly related to the stated purpose of the patch, eg it now
notices that "select distinct max(unique2) from tenk1" doesn't
require an explicit DISTINCT step. That's not wrong, but I wonder
if maybe you should subdivide this patch into more than one patch,
because that must be coming from some separate change. I'm also
wondering what caused the plan change in expected/join.out.

regards, tom lane