Re: proposal: FOREACH-IN-ARRAY (probably for 9.2?)

2010/12/17 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
>> On Fri, Dec 17, 2010 at 10:47 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> This seems like a really bad, confusing idea.  I think it should throw
>>> a type-mismatch error in this case.  If there is any use-case for such a
>>> thing, which I'm quite unconvinced of, it ought to use a separate syntax
>>> rather than overloading the element-by-element syntax.
>
>> I don't agree at all -- iterating arrays by slice is a frequently
>> requested feature (you can kinda sorta do it by slice notation, but
>> arr[n] giving null is a -general FAQ.  This is how people think arrays
>> should work.  I suppose that having this functionality reserved in a
>> tiny corner of plpgsql is not so good, but I think foreach... would
>> become the preferred way to iterate arrays always.
>
> Well, okay, if it's useful we can have it, but I still say it needs to
> be a separate syntax.  The example Pavel gives looks like nothing so
> much as a beginner's error, ie putting [] on the target variable when
> he shouldn't have.

Now the message is unclean - but it can be enhanced. We can a diagnose
situation when result is multidimensional array and target isn't
array, and the we can to throw user friendly message.

>
> Furthermore, it's underspecified: who's to say how many dimensions of
> the array are supposed to get sliced off?  There's no reasonable place
> to extend this syntax to specify that.  It will also be inconsistent
> for "foreach scalar in array" to iterate element-by-element no matter
> how many dimensions array has, while "foreach array in array" does
> something different from that.
>

it reduce just one dimension. Now I expect, and I think so it is
correct, so user knows a used dimension. Just doesn't know a data. So
user can to decide and fill correct type. The design strictly remove
any U.I. from design. So using a incorect type is bug.

Because a FOREACH syntax is new, we can to enhance it to possible direction:

FOREACH VALUE var IN ARRAY expr
LOOP
END LOOP

and then it will iterate per one field without a dimension reduction.
So this possibility is available and I think so could be implemented
too.

Pavel

>                        regards, tom lane
>