Re: GROUP BY DISTINCT - Mailing list pgsql-hackers

On 3/18/21 6:25 PM, Tomas Vondra wrote:
> On 3/16/21 3:52 PM, Tomas Vondra wrote:
>>
>>
>> On 3/16/21 9:21 AM, Vik Fearing wrote:
>>> On 3/13/21 12:33 AM, Tomas Vondra wrote:
>>>> Hi Vik,
>>>>
>>>> The patch seems quite ready, I have just two comments.
>>>
>>> Thanks for taking a look.
>>>
>>>> 1) Shouldn't this add another <indexterm> for DISTINCT, somewhere in the
>>>> documentation? Now the index points just to the SELECT DISTINCT part.
>>>
>>> Good idea; I never think about the index.
>>>
>>>> 2) The part in gram.y that wraps/unwraps the boolean flag as an integer,
>>>> in order to stash it in the group lists is rather ugly, IMHO. It forces
>>>> all the places handling the list to be aware of this (there are not
>>>> many, but still ...). And there are no other places doing (bool) intVal
>>>> so it's not like there's a precedent for this.
>>>
>>> There is kind of a precedent for it, I was copying off of TriggerEvents
>>> and func_alias_clause.
>>>
>>
>> I see. I was looking for "(bool) intVal" but you're right TriggerEvents
>> code does something similar.
>>
>>>> I think the clean solution is to make group_clause produce a struct with
>>>> two fields, and just use that. Not sure how invasive that will be
>>>> outside gram.y, though.
>>>
>>> I didn't want to create a whole new parse node for it, but Andrew Gierth
>>> pointed me towards SelectLimit so I did it like that and I agree it is
>>> much cleaner.
>>>
>>
>> I agree, that's much cleaner.
>>
>>>> Also, the all_or_distinct vs. distinct_or_all seems a bit error-prone. I
>>>> wonder if we can come up with some clearer names, describing the context
>>>> of those types.
>>>
>>> I turned this into an enum for ALL/DISTINCT/default and the caller can
>>> choose what it wants to do with default.  I think that's a lot cleaner,
>>> too.  Maybe DISTINCT ON should be changed to fit in that?  I left it
>>> alone for now.
>>>
>>
>> I think DISTINCT ON is a different kind of animal, because that is a
>> list of expressions, not just a simple enum state.
>>
>>> I also snuck in something that all of us overlooked which is outputting
>>> the DISTINCT in ruleutils.c.  I didn't add a test for it but that would
>>> have been an unfortunate bug.
>>>
>>
>> Oh!
>>
>>> New patch attached, rebased on 15639d5e8f.
>>>
>>
>> Thanks. At this point it seems fine to me, no further comments.
>>
> 
> Pushed. Thanks for the patch.
> 

Hmmm, this seems to fail on lapwing with this error:

parse_agg.c: In function 'expand_grouping_sets':
parse_agg.c:1851:23: error: value computed is not used
[-Werror=unused-value]
cc1: all warnings being treated as errors

That line is this:

    foreach_delete_current(result, cell);

and I don't see how any of the values close by could be unused ...

The only possibility I can think of is some sort of issue in the old-ish
gcc release (4.7.2).


regards

-- 
Tomas Vondra
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