Re: Using index for skip scan

[Apkleads apkleads]

Rather than restricting the search path using a predicate from the statement, Skip Scans are initiated by probing the index for distinct values of the prefix column. Each of these distinct values is then used as a starting point for a regular index search. The result is several separate searches of a single index that, when combined, eliminate the affect of the prefix column. Essentially, the index has been searched from the second level down.

The optimizer uses statistics to decide if a skip scan would be more efficient than a full table scan.


This approach is advantageous because:

It reduces the number of indexes needed to support a range of queries. This increases performance by reducing index maintenance and decreases wasted space associated with multiple indexes.
The prefix column should be the most discriminating and the most widely used in queries. These two conditions do not always go hand in hand which makes the decision difficult. In these situations skip scanning reduces the impact of makeing the "wrong" decision.