[PATCH] Indexing IS (NOT) NULL for btrees

[CC any replies, thanks]

This is basically a refined version of the last one.

- It restricts itself to btree indexes
- Doesn't play tricks with RestrictInfos and focuses on a new type
of ScanKey. Thus, the EXPLAIN output looks correct now also.
- Does IS NOT NULL as well as IS NULL
- Probably better use of statistics.

It's messier because now more places need to know about it, but
functionalitywise it's like the last one.

For the archive:
http://svana.org/kleptog/pgsql/indexnulls3.diff
--
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.