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From: Pantelis Theodosiou <[email protected]>
To: Alvaro Herrera <[email protected]>
Cc: [email protected]
Cc: [email protected]
Subject: Re: Documentation of EXCEPT ALL may have a bug
Date: Sat, 10 Feb 2018 13:02:53 +0000
Message-ID: <CAE3TBxxFn0JcDDkvz-bpGcrkSHfvt1u857miO3+=JYY=SofMCA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

On Sat, Feb 10, 2018 at 11:59 AM, Alvaro Herrera <[email protected]>
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:

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.

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