In older versions of Oracle (circa v8), the optimizer would use a concatenated index only if the leading edge column(s) appeared in the
WHERE clause. In modern versions, the optimizer uses a concatenated index even if the leading edge column(s) aren't present in the
WHERE clause. This ability to use an index without reference to leading edge columns is known as the skip-scan feature.
Share this highlighthttp://www.safaribooksonline.com/a/pro-oracle-database/3896/