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So what's an "empty" array anyway?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: So what's an "empty" array anyway?
Date: 2008-10-21 17:50:37
Message-ID: 28026.1224611437@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Currently, the constructs
	'{}'::arraytype
	ARRAY[]::arraytype
return zero-dimensional arrays, as does the underlying function
construct_empty_array().  I can't immediately find any way at SQL
level to produce an empty array with one or more dimensions.
However, construct_array and construct_md_array will happily build
zero-length arrays of dimension 1 or higher, leading to weirdnesses
such as illustrated here:
http://archives.postgresql.org/pgsql-general/2008-10/msg00915.php

Seems like we ought to clean this up.  I'm not sure which way to jump
though: should we decree that arrays of no elements must always have
zero dimensions, or should we get rid of that and standardize on, say,
1-D array with lower bound 1 and upper bound 0?

A somewhat related issue that I noticed while poking at this is that
array_dims() returns NULL for a zero-dimension array.  That seems a bit
bogus too; wouldn't an empty string be saner?  Of course the issue
goes away if we get rid of zero-dimension arrays.

Thoughts?

			regards, tom lane

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