Thread: Re: [SQL] ARRAY() returning NULL instead of ARRAY[] resp. {}
В Втр, 24/05/2005 в 00:06 -0400, Tom Lane пишет:
> Joe Conway <mail@joeconway.com> writes:
> > Markus Bertheau wrote:
> >> why does SELECT ARRAY(SELECT 1 WHERE FALSE) return NULL instead of
> >> ARRAY[] resp. '{}'?
>
> > Why would you expect an empty array instead of a NULL?
>
> I think he's got a good point, actually. We document the ARRAY-with-
> parens-around-a-SELECT syntax as
>
> The resulting one-dimensional array will have an element for
> each row in the subquery result, with an element type matching
> that of the subquery's output column.
>
> To me, that implies that a subquery result of no rows generates a
> one-dimensional array of no elements, not a null array.
By analogy, array_upper('{}'::TEXT[], 1) should return 0 instead of
NULL.
Markus
--
Markus Bertheau ☭ <twanger@bluetwanger.de>
Markus Bertheau ☭ <twanger@bluetwanger.de> writes:
> By analogy, array_upper('{}'::TEXT[], 1) should return 0 instead of
> NULL.
No, that doesn't follow ... we've traditionally considered '{}' to
denote a zero-dimensional array. A 1-D array of no elements is
'[1:0]={}', just as Joe shows ... or at least it would be except
for an overenthusiastic error check:
regression=# select '[1:0]={}' :: int[];
ERROR: upper bound cannot be less than lower bound
I think this should be a legal boundary case. In general, it should be
possible to form zero-size arrays of any number of dimensions.
regards, tom lane
В Пнд, 06/06/2005 в 10:44 -0400, Tom Lane пишет:
> Markus Bertheau ☭ <twanger@bluetwanger.de> writes:
> > By analogy, array_upper('{}'::TEXT[], 1) should return 0 instead of
> > NULL.
>
> No, that doesn't follow ... we've traditionally considered '{}' to
> denote a zero-dimensional array.
But array_upper(ARRAY(SELECT 1 WHERE FALSE), 1) should return 0,
and array_upper(ARRAY[], 1), when the ARRAY[] syntax is supported,
should return 0.
Do I get that right?
Markus
--
Markus Bertheau ☭ <twanger@bluetwanger.de>
Tom Lane wrote:
> Markus Bertheau ☭ <twanger@bluetwanger.de> writes:
>
>>By analogy, array_upper('{}'::TEXT[], 1) should return 0 instead of
>>NULL.
>
> No, that doesn't follow ... we've traditionally considered '{}' to
> denote a zero-dimensional array. A 1-D array of no elements is
> '[1:0]={}', just as Joe shows ... or at least it would be except
> for an overenthusiastic error check:
>
> regression=# select '[1:0]={}' :: int[];
> ERROR: upper bound cannot be less than lower bound
>
> I think this should be a legal boundary case. In general, it should be
> possible to form zero-size arrays of any number of dimensions.
>
I've been playing with exactly this over the weekend. Of course, as
usual, the devil is in the details. For instance, using the above
notation, how would I specify a zero-element 1D array starting at a
lower bound index of 0? The answer following the above pattern would be:
select '[0:-1]={}'::int[];
You could not use '[0:0]={}'::int[], because that would be a one-element
array. I propose the following instead:
regression=# select '[1:]={}' :: int[]; int4
------ {}
(1 row)
regression=# select array_dims('[1:]={}' :: int[]); array_dims
------------ [1:]
(1 row)
In other words, an missing upper bound indicates zero elements.
Now the next question; what does a 2D zero-element array look like? I
think this makes sense:
regression=# select '[1:2][1:]={{},{}}'::int[]; int4
------ {}
(1 row)
Except (I think) array_out() should probably output something closer to
the input literal. Any thoughts on this?
Joe
Markus Bertheau ☭ wrote:
> В Пнд, 06/06/2005 в 10:44 -0400, Tom Lane пишет:
>>Markus Bertheau ☭ <twanger@bluetwanger.de> writes:
>>
>>>By analogy, array_upper('{}'::TEXT[], 1) should return 0 instead of
>>>NULL.
>>
>>No, that doesn't follow ... we've traditionally considered '{}' to
>>denote a zero-dimensional array.
>
> But array_upper(ARRAY(SELECT 1 WHERE FALSE), 1) should return 0,
> and array_upper(ARRAY[], 1), when the ARRAY[] syntax is supported,
> should return 0.
>
Actually, consistent with my last post, I think array_upper() on a
zero-element array should return NULL. A zero-element array has a
defined lower bound, but its upper bound is not zero -- it is really
undefined.
Joe
Joe Conway wrote:
> Actually, consistent with my last post, I think array_upper() on a
> zero-element array should return NULL. A zero-element array has a
> defined lower bound, but its upper bound is not zero -- it is really
> undefined.
Just to clarify my response, this is what I propose:
regression=# select array_upper('[2][1:]={{},{}}'::int[],1); array_upper
------------- 2
(1 row)
regression=# select array_upper('[2][1:]={{},{}}'::int[],2) IS NULL; ?column?
---------- t
(1 row)
Joe
В Пнд, 06/06/2005 в 08:58 -0700, Joe Conway пишет:
> Joe Conway wrote:
> > Actually, consistent with my last post, I think array_upper() on a
> > zero-element array should return NULL. A zero-element array has a
> > defined lower bound, but its upper bound is not zero -- it is really
> > undefined.
>
> Just to clarify my response, this is what I propose:
>
> regression=# select array_upper('[2][1:]={{},{}}'::int[],1);
> array_upper
> -------------
> 2
> (1 row)
>
> regression=# select array_upper('[2][1:]={{},{}}'::int[],2) IS NULL;
> ?column?
> ----------
> t
> (1 row)
Hmm, this gets really complicated and inconsistent. Complicated means
unusable. What about modifying the dimension syntax such that the second
number means number of elements instead of upper bound? That particular
problem would go away then, and array_upper('[0:0]={}'::int[]) can
return the correct 0 then.
What I'm actually worrying about is that array_upper(array(select 1
where false)) returns 0.
An option would be to drop the possibility to let the array start at
another index than 0. I don't know why it was decided to do that in the
first place. It seems a rather odd feature to me.
Markus
--
Markus Bertheau ☭ <twanger@bluetwanger.de>
Markus Bertheau ☭ wrote:
> Hmm, this gets really complicated and inconsistent. Complicated means
> unusable. What about modifying the dimension syntax such that the second
> number means number of elements instead of upper bound? That particular
> problem would go away then, and array_upper('[0:0]={}'::int[]) can
> return the correct 0 then.
>
> What I'm actually worrying about is that array_upper(array(select 1
> where false)) returns 0.
>
> An option would be to drop the possibility to let the array start at
> another index than 0. I don't know why it was decided to do that in the
> first place. It seems a rather odd feature to me.
>
Actually I like both of these ideas, and have advocated the second one
myself before. But it isn't backward compatible -- anyone else have an
opinion? SQL2003 actually specifies that an array *should* start at 1:
4.10.2 Arrays
An array is a collection A in which each element is associated with
exactly one ordinal position in A. If n is the cardinality of A, then
the ordinal position p of an element is an integer in the range 1 (one)
≤ p ≤ n.
Joe
Is this a TODO item?
---------------------------------------------------------------------------
Markus Bertheau ? wrote:
> ? ???, 06/06/2005 ? 08:58 -0700, Joe Conway ?????:
> > Joe Conway wrote:
> > > Actually, consistent with my last post, I think array_upper() on a
> > > zero-element array should return NULL. A zero-element array has a
> > > defined lower bound, but its upper bound is not zero -- it is really
> > > undefined.
> >
> > Just to clarify my response, this is what I propose:
> >
> > regression=# select array_upper('[2][1:]={{},{}}'::int[],1);
> > array_upper
> > -------------
> > 2
> > (1 row)
> >
> > regression=# select array_upper('[2][1:]={{},{}}'::int[],2) IS NULL;
> > ?column?
> > ----------
> > t
> > (1 row)
>
> Hmm, this gets really complicated and inconsistent. Complicated means
> unusable. What about modifying the dimension syntax such that the second
> number means number of elements instead of upper bound? That particular
> problem would go away then, and array_upper('[0:0]={}'::int[]) can
> return the correct 0 then.
>
> What I'm actually worrying about is that array_upper(array(select 1
> where false)) returns 0.
>
> An option would be to drop the possibility to let the array start at
> another index than 0. I don't know why it was decided to do that in the
> first place. It seems a rather odd feature to me.
>
> Markus
> --
> Markus Bertheau ? <twanger@bluetwanger.de>
>
>
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-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
359-1001+ If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square,
Pennsylvania19073
Bruce Momjian wrote: > Is this a TODO item? > Probably. I posted some questions regarding whether or not to break backward compatiblity, and received no replies. In the meanwhile, I've been doing a major system integration in Korea for the last 2 weeks, and won't get back to home, or to anything like a reasonably normal schedule until after July 2. I doubt I'll have time to do much between now and feature freeze. Joe
OK, what is the TODO item text? --------------------------------------------------------------------------- Joe Conway wrote: > Bruce Momjian wrote: > > Is this a TODO item? > > > > Probably. I posted some questions regarding whether or not to break > backward compatiblity, and received no replies. In the meanwhile, I've > been doing a major system integration in Korea for the last 2 weeks, and > won't get back to home, or to anything like a reasonably normal schedule > until after July 2. I doubt I'll have time to do much between now and > feature freeze. > > Joe > > ---------------------------(end of broadcast)--------------------------- > TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org > -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001+ If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania19073