What is skip scanning?
In prior releases, a composite index would be used to retrieve the records, if the index prefix (leading) column was included in the predicate of the statement. With Oracle9i, the optimizer can use a composite index even if the prefix column value is not known. The optimizer uses an algorithm called skip scanning to retrieve ROWIDs for values that do not use the prefix column. 
How does a skip scan work? During a skip scan, the B*-tree is probed for each distinct value in the prefix column. Under each prefix column value, the normal search algorithm takes over. The result is a series of searches through subsets of the index, each of which appears to result from a query using a specific value of the prefix column. However, with the skip scan, the value of the prefix column in each subset is obtained from the initial index probe rather than from the command predicate. The optimizer uses the statistics to determine whether a skip scan retrieval would be more efficient than a full table scan, or other possible retrieval paths, when parsing
本文转自maclean_007 51CTO博客，原文链接:http://blog.51cto.com/maclean/1276731
