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Why is Oracle not using my index?

Oracle Database Tips by Donald BurlesonApril 3, 2015

Question:  I just created a function-based index on a table and I was distressed to see that the cost-based optimizer is not using the highly selective index.  Why is Oracle not using my index?

Answer:  When your optimizer_mode is set to minimize response time (e.g. first_rows_1, first_rows_10, &c), you should always see Oracle using the most "selective" index, the indexes that will return the smallest number of rows.  First, we need to know a few details about your optimizer settings:

  • Query re-write: You need to set these parameters to get Oracle to use a function-based index:

                ALTER SESSION SET QUERY_REWRITE_INTEGRITY = TRUSTED;
               ALTER SESSION SET QUERY_REWRITE_ENABLED = TRUE;

  • optimizer_mode:  If you are using the default value of all_rows, the query my think that a full-table can is "better" than your function-based index.  Remember, the all_rows optimizer mode optimizes to minimize computing resources and favor full-scans over index scans.  Conversely, the first_rows_n optimizer_mode optimizes to minimize response time and favors index access.
  • table dbms_stats:  If your function-based index has not been fully analyzed by dbms_stats, the optimizer may be ignoring the index.  You need to re-analyze the target table with dbms_stats.  This will also re-analyze the indexes on the target table.
  • index dbms_stats:  A function based index can be ignored when you fail to create histograms on the function-based index.  See here using statistics for indexes
  • system dbms_stats  When Oracle is not using an index, it can also be helpful to run dbms_stats.gather_system_stats.
  • Bugs:  It is rare, but you may need to check Oracle technical support for bugs relating to index usage.  If you suspect a bug and you are in a test environment you can also issue the "alter system flush shared pool" command to re-initialize all SQL.
  • Other indexes:  You may have other indexes that Oracle perceives as being "better" for the query. 

In sum, the Oracle optimizer should immediately see a new index and begin using it immediately, invalidating all SQL that might benefit from the index.

Forcing Oracle to use an index

When Oracle does not use an index, you can force him to use the index with diagnostic tools.  Testing to force Oracle to use an index is easy.  We use the SQL*Plus "set autotrace on" and "set timing on" commands and time the queries, once with the default and again using an index hint. 

The "best" query for response time is usually the one that fetches the rows with the fewest consistent gets.  These are ONLY testing tools, but you will still need to find the reason my Oracle is ignoring your index and not using the index in your query.

  • index hint:  You can force Oracle to use your index with an index hint.
  • optimizer_index_cost_adj:  If you are already using cost-based optimization and you have already run system stats, try setting optimizer_index_cost_adj=10.  This will temporarily force the use of the index:

alter session set optimizer_index_cost_adj=10;.

 
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