SQL/Oracle: when indexes on multiple columns can be used

Question

If I create an index on columns (A, B, C), in that order, my understanding is that the database will be able to use it even if I search only on (A), or (A and B), or (A and B an

Answer 1

There are actually three index-based access methods that Oracle can use when a predicate is placed on a non-leading column of an index.

i) Index skip-scan: http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#PFGRF10105

ii) Fast full index scan: http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#i52044

iii) Index full scan: http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm#i82107

I've most often seen the fast full index scan "in the wild", but all are possible.