From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200506042252.j54MqVu14936@candle.pha.pa.us>
Subject: Re: ARRAY() returning NULL instead of ARRAY[] resp. {}
In-Reply-To: <42A2169E.9020700@joeconway.com>
To: Joe Conway <mail@joeconway.com>
Date: Sat, 4 Jun 2005 18:52:31 -0400 (EDT)
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
	Markus Bertheau <twanger@bluetwanger.de>, pgsql-sql@postgresql.org
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Joe Conway wrote:
> Bruce Momjian wrote:
> > Joe Conway wrote:
> >>
> >>Any thoughts on how this should be handled for an empty 1D array?
> > 
> > No one responed to this email, so I will try.  Is this the one
> > dimmentional array you were talking about?
> > 
> > 	test=> select array_dims('{}'::integer[]);
> > 	 array_dims
> > 	------------
> > 	
> > 	(1 row)
> 
> In this case, what you get is actually a dimensionless array. Literally, 
> you get this:
> 
> 	if (nitems == 0)
> 	{
> 		/* Return empty array */
> 		retval = (ArrayType *) palloc0(sizeof(ArrayType));
> 		retval->size = sizeof(ArrayType);
> 		retval->elemtype = element_type;
> 		PG_RETURN_ARRAYTYPE_P(retval);
> 	}
> 
> I.e. the array structure is allocated, the size is set (which is 
> required since arrays are varlena), and the element type is initialized. 
> There is no initialization of ndim, ARR_DIMS(), or ARR_LBOUND().
> 
> In this case, since there are no dimensions, array_dims() probably does 
> the right thing by returning NULL.
> 
> > Why is [1:0] wrong to return?
> > 
> 
> I'm not sure it is wrong -- it just seems a bit strange. The difference 
> is that in order to return an empty *one-dimensional* array, ndim, 
> ARR_DIMS(), and ARR_LBOUND() are all appropriately set (by the patched 
> code). Basically, ndim == 1, ARR_DIMS() is a single element int array (C 
> array that is) indicating 0 elements for dimension 1, and ARR_LBOUND() 
> is a single element int array indicating a lower bound of 1. This leads 
> to the array_dims() return value of [1:0]. The value 1 is unquestionably 
> correct for the lower bound index, but what should be reported for the 
> upper bound? We can't return [1:1], because that would indicate that we 
> have one element.

OK, so '[1:0]' seems correct.  How would to specify such an array
manually in a string?

-- 
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