From: Bruce Momjian <bruce@momjian.us>
Message-Id: <200701310407.l0V47B317659@momjian.us>
Subject: Re: [BUGS] BUG #2913: Subscript on multidimensional array
	yields no value
In-Reply-To: <14949.1170215057@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 30 Jan 2007 23:07:11 -0500 (EST)
CC: Roman Nowak <roman_nowak@poczta.onet.pl>,
	PostgreSQL-documentation <pgsql-docs@postgresql.org>
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Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > Tom Lane wrote:
> >> This is not a bug, this is a definitional disagreement, and your TODO
> >> entry presupposes an answer that I don't particularly agree with.
> 
> > Well, our documentation suggests thaat [1] is the same as [1:1]:
> > 	http://www.postgresql.org/docs/8.2/static/arrays.html#AEN5791
> 
> It says absolutely no such thing.  A subscript expression involving m:n
> produces a "slice", hence an array of different dimensionality from the
> original, whereas a subscript expression not involving any colon
> produces a single element --- that is, not an array at all.
> 
> You could make a fair case that the (ARRAY[[1,2],[3,4]])[1] example
> should throw an error instead of returning null.  But to claim it is
> the same as a slice expression is a typing violation.

[ moved to docs list ]

Well, I don't understand our array documentation, so odds are others
don't either.  What are we saying here:

	An array slice is denoted by writing lower-bound:upper-bound for one or
	more array dimensions. For example, this query retrieves the first item
	on Bill's schedule for the first two days of the week:
	
	SELECT schedule[1:2][1:1] FROM sal_emp WHERE name = 'Bill';
	
	        schedule
	------------------------
	 {{meeting},{training}}
	(1 row)
	
	We could also have written
	
	SELECT schedule[1:2][1] FROM sal_emp WHERE name = 'Bill';
	
	with the same result. An array subscripting operation is always taken to
	represent an array slice if any of the subscripts are written in the
	form lower:upper. A lower bound of 1 is assumed for any subscript where
	only one value is specified, as in this example:

What is the difference between 'lower-bound:upper-bound' and
'lower:upper'?  Here are the items that confuse me:

	test=> SELECT (ARRAY[[1,2],[3,4]])[1][1];
	 array
	-------
	     1
	(1 row)
	
	test=> SELECT (ARRAY[[1,2],[3,4]])[1:1];
	  array
	---------
	 {{1,2}}
	(1 row)

	test=> SELECT (ARRAY[[1,2],[3,4]])[1];
	 array
	-------
	
	(1 row)
	
The first two make sense to me, but the last one does not.  If someone
explains it, I can update our documentation.

-- 
  Bruce Momjian   bruce@momjian.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +