Well, I'm no Oracle expert either, but Oracle does not ever index NULL
values. So if you do something like:
select foo from my_table where bar is null
you
will force a full table scan. I don't know why Oracle
does it that way - you'd think indexing nulls just like any other
value would be useful. Also, that's for normal b-tree indexes - I'm
not sure whether the same is true of bitmap indexes.
On the other hand,
select foo from my_table where bar is NOT null
will be able to use the index just fine. (Whether Oracle
actually uses the index or not is at the optimizer's discretion, of
course).
On the inserting thing, yes, Don's correct - Oracle coerces empty
string to null on inserts, and I believe updates as well. Funny, I
can't remember ever needing to insert a real empty string
into Oracle. Offhand, I can't think of a good way to do that.
