Spotlight on Oracle 10.8 - Getting Started Guide

Avoid Accidental Table Scans

One of the most fundamental SQL tuning problems is the accidental table scan. Accidental table scans usually occur when the SQL programmer tries to perform a search on an indexed column that can’t be supported by an index. This can occur when:

• Using != (not equals to). Even if the not equals condition satisfies only a small number of rows, Oracle does not use an index to satisfy such a condition. Often, you can re-code these queries using > or IN conditions, which can be supported by index lookups.
• Searching for NULLS. Oracle won’t use an index to find null values, since null values are not usually stored in an index (the exception is a concatenated index entry where only some of the values are NULL). If you’re planning to search for values that are logically missing, consider changing the column to NOT NULL with a DEFAULT clause. For example, you could set a default value of UNKNOWN and use the index to find these values. Interestingly, recent versions of Oracle can index to find values that are NOT NULL — if the cost-based optimizer determines that such an approach is cost-effective.
• Using functions on indexed columns. Any function or operation on an indexed column prevents Oracle from using an index on that column. For instance, Oracle can’t use an index to find SUBSTR(SURNAME,1,4)=’SMIT’. Instead of manipulating the column, try to manipulate the search condition. In the previous example, a better formulation would be SURNAME LIKE ‘SMIT%’.

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