Extensive Index Skip Scan Access Wait

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Index extensively accessed via index skip scans which often means that the index structure does not fit the query in the best possible way, and leads Oracle to perform heavy activity against the index

Extensive "Index Skip Scan" Access findings

	Description
What to do next	Perform the following: n   Examine associated statements in the Statements tab. n   Focus on the Index Skip Scan access type in the Access Types table. n   Examine column usage for each statement in the Columns table.
Advice	Perform one of the following options: n   Change the columns sequence within the index that use the index skip scan. Check the effect of this change in the What-if workspace. n   Identify common high selectivity columns for top statements. Create an index matching the statements predicates. n   If you cannot create a new index, use hints, such as, "full" and "index_ffs" to determine if by using these access types you can achieve better performance results.
Example	Table: TAB1 (C1 number, C2 number, C3 number, C10 Date) Index: IX1 (C1,C2) (C1 has two distinct values Yes and No) Statement: select * from TAB1 where C2=10; Execution plan uses Index Skip Scan on IX1. In this case Oracle has to perform two range scans on the index—one with a key of (Yes,10) and another with a key of (No,10)and   then unite the results. The more distinct values defined for C1, the more index scanning required. Defining a new index on (C2), or changing the column sequence in IX1 to be (C2,C1), enables a more efficient access path for the Index Range Scan.

    

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