Re: Documentation of EXCEPT ALL may have a bug

On Sat, Feb 10, 2018 at 1:02 PM, Pantelis Theodosiou <ypercube(at)gmail(dot)com>
wrote:

>
>
> On Sat, Feb 10, 2018 at 11:59 AM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
> wrote:
>
>> PG Doc comments form wrote:
>>
>> > create table t(x int);
>> > create table u(x int);
>> >
>> > insert into t values (1), (2), (2), (3), (3);
>> > insert into u values (1), (2);
>> >
>> > select * from t except all select * from u;
>>
>> > x
>> > ---
>> > 3
>> > 3
>> > 2
>> > (3 rows)
>>
>> I find this pretty odd behavior. Is this not an outright bug? I
>> haven't read the SQL standard on this matter[*], but if they define
>> EXCEPT ALL to work like this, then it seems pretty useless. (Maybe they
>> just didn't intend EXCEPT ALL to be useful?) If this is indeed the
>> case, maybe we should amend the docs not only to explain this behavior
>> but also to warn against the construct.
>>
>
> I think that's the way it is defined (but I agree, I don't remember ever
> seeing a use for it)
>
>
>>
>> [*] I didn't try terribly hard, but couldn't actually find where the
>> behavior is defined. What I have on hand is a draft of SQL:2011 where
>> this appears to be defined in 7.13 <query expression> but I was unable
>> to find the rules for set operations. It refers to 9.12 Grouping
>> operations but that defines conformance rules only.
>>
>> --
>>
>
> I may have a different version but I see:
>

That is in:
7.13 <query expression>
General Rules 3, Case b:

>
> iii)
> T contains the following rows:
> 1) Let R be a row that is a duplicate of some row in ET1 or of some row in
> ET2 or both. Let
> m be the number of duplicates of R in ET1 and let n be the number of
> duplicates of R in
> ET2, where m ≥ 0 and n ≥ 0.
> 2) If DISTINCT is specified or implicit, then ....
> ...
>
> 3) If ALL is specified, then
> Case:
> A) If UNION is specified, then the number of duplicates of R that T
> contains is (m + n).
> B) If EXCEPT is specified, then the number of duplicates of R that T
> contains is the
> maximum of (m – n) and 0 (zero).
> C) If INTERSECT is specified, then the number of duplicates of R that T
> contains is the
> minimum of m and n.
>
>