Originally posted by tamilselvan
In a concatenated Index, the order of columns are important. The leading (first) column should be of a high cardinality value, followed by other columns in that order.
With the appearance of index compression in Oracle8i, the order of columns should be just the opposite if the cardinality is the only deciding factor. Columns with least cardinality should be the leading columns in the composite index and not the other way around.

So in tamiselvan example the order of columns in index should be C-B-E and not E-B-C. And of course index should be compressed.