Understanding behavior of SELECT with multiple unnested columns

I've been working on some queries involving multiple unnested columns. At
first, I expected the number of rows returned would be the product of the
array lengths, so that this query would return 4 rows:

SELECT unnest2(array['a','b']),unnest2(array['1','2']);

when in fact it returns 2:

unnest2 | unnest2
---------+---------
a | 1
b | 2

Which is all well and good. (Better, in fact, for my purposes.) But then
this query returns 6 rows:

SELECT unnest2(array['a','b','c']),unnest2(array['1','2']);
unnest2 | unnest2
---------+---------
a | 1
b | 2
c | 1
a | 2
b | 1
c | 2

Throw an unnested null column in and you get zero rows, which I also didn't
expect:

SELECT
unnest2(array['a','b','c']),unnest2(array['1','2']),unnest(NULL::varchar[]);
unnest2 | unnest2 | unnest
---------+---------+--------
(0 rows)

After some head scratching, I think I understand what to expect from these
unnests, but I'm unclear of the logic behind what is going on. I'm hoping
someone can explain it a bit. Also, on a practical level, would anyone
know how to query so that SELECT
unnest2(array['a','b','c']),unnest2(array['1','2']) would return three rows
instead of six, like so:

a 1
b 2
c (NULL)

As that would be perfect for my purposes. Thanks in advance!

Ken

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