Message-ID: <42A46E44.3060100@joeconway.com>
Date: Mon, 06 Jun 2005 08:39:48 -0700
From: Joe Conway <mail@joeconway.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: =?UTF-8?B?TWFya3VzIEJlcnRoZWF1IOKYrQ==?= <twanger@bluetwanger.de>,
	pgsql-sql@postgresql.org, pgsql-hackers@postgresql.org
Subject: Re: ARRAY() returning NULL instead of ARRAY[] resp. {}
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Tom Lane wrote:
> Markus Bertheau =?UTF-8?Q?=E2=98=AD?= <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