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Why is Oracle not using the damn index? (for DBA

ORACLE Interview Questions and Answers (Part 3)

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386. Why is Oracle not using the damn index? (for DBA

This problem normally only arises when the query plan is being generated by the Cost Based Optimizer. The usual cause is because the CBO calculates that executing a Full Table Scan would be faster than accessing the table via the index.
Fundamental things that can be checked are:
. USER_TAB_COLUMNS.NUM_DISTINCT - This column defines the number of distinct values the column holds.
. USER_TABLES.NUM_ROWS - If NUM_DISTINCT = NUM_ROWS then using an index would be preferable to doing a FULL TABLE SCAN. As the NUM_DISTINCT decreases, the cost of using an index increase thereby is making the index less desirable.
. USER_INDEXES.CLUSTERING_FACTOR - This defines how ordered the rows are in the index. If CLUSTERING_FACTOR approaches the number of blocks in the table, the rows are ordered. If it approaches the number of rows in the table, the rows are randomly ordered. In such a case, it is unlikely that index entries in the same leaf block will point to rows in the same data blocks.
. Decrease the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT - A higher value will make the cost of a FULL TABLE SCAN cheaper.
. Remember that you MUST supply the leading column of an index, for the index to be used (unless you use a FAST FULL SCAN or SKIP SCANNING).
. There are many other factors that affect the cost, but sometimes the above can help to show why an index is not being used by the CBO. If from checking the above you still feel that the query should be using an index, try specifying an index hint. Obtain an explain plan of the query either using TKPROF with TIMED_STATISTICS, so that one can see the CPU utilization, or with AUTOTRACE to see the statistics. Compare this to the explain plan when not using an index.

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