Extensive "Index Range Scan" Access Wait

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Extensive I/O wait was experienced, as a result of range scans on the index. Although this may be normal, it can often indicate a matching level problem, indicating that the structure of the index can be improved.

Extensive "Index Range Scan" Access findings

	Description
What to do next	Examine statements using the index in the Statements tab. Examine column usage, selectivity and matching level (see example) for the top statements, in the Columns table to assess the efficiency of the index.
Advice	If the index structure does not fit the Where predicates of the top statements consider doing one of the following: n   Add columns to the index in the right sequence (columns with high selectivity and equal predicates should be first) to improve the matching level. This leads   to a better filtering of leaf pages. n   Change the sequence of columns in the index to the optimal sequence (columns with high selectivity and equal predicates should be first) to improve the matching level. This leads to a better filtering of leaf pages.
Example	Table: TAB1 (C1 number, C2 number, C3 number, C10 Date) Index: IX1 (C1,C2,C5) Statement: select * from TAB1 where C1=:h and C5=10; Execution plan uses IX1 in Index Range Scan In this statement the matching level of the index is 1. This means that Oracle uses only C1 to filter index leaf pages, it cant match C5=10 against the index tree because of the absence of a C2= predicate. Because C1 is not selective, many irrelevant index leaf pages can be read. Oracle will apply the C5=10 predicate on the index keys to screen irrelevant table ROWIDs. An index on C1 followed by C5 would be more efficient for the query.

  

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