FULL JOIN and mergjoinable conditions...

Today I got the error:

ERROR: FULL JOIN is only supported with mergejoinable join conditions

Which is really annoying since a full join is exactly what I wanted. I
guess the alternative is to do a left join and a right join and merge
them? Is it just that no-one has come up with a way to code this
efficiently?

Maybe someone has a better way to express this. The problem is I have
two tables with ranges and I wanted to generate a result with the
overlaps and blanks where there are things missed. For example:

Table A Table B
Tag Start End Tag Start End
A 1 2 A 2 7
B 6 9 B 9 9
C 10 12 C 13 15

So the query looks like:

SELECT * from A full outer join B on (a.end >= b.start and b.end >= a.start)

The result would be something like:

A 1 2 A 2 7
B 6 9 A 2 7
B 6 9 B 9 9
C 10 12 \N \N \N
\N \N \N C 13 15

Any ideas?
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.