Invisible Indexes in Oracle Database 11g Release 1

Oracle 11g allows indexes to be marked as invisible. Invisible indexes are maintained like any other index, but they are ignored by the optimizer unless the OPTIMIZER_USE_INVISIBLE_INDEXES parameter is set to TRUE at the instance or session level. Indexes can be created as invisible by using the INVISIBLEkeyword, and their visibility can be toggled using the ALTER INDEX command.

CREATE INDEX index_name ON table_name(column_name) INVISIBLE;

ALTER INDEX index_name INVISIBLE;
ALTER INDEX index_name VISIBLE;

The following script creates and populates a table, then creates an invisible index on it.

CREATE TABLE ii_tab (
  id  NUMBER
);

BEGIN
  FOR i IN 1 .. 10000 LOOP
    INSERT INTO ii_tab VALUES (i);
  END LOOP;
  COMMIT;
END;
/

CREATE INDEX ii_tab_id ON ii_tab(id) INVISIBLE;

EXEC DBMS_STATS.gather_table_stats(USER, 'ii_tab', cascade=> TRUE);

A query using the indexed column in the WHERE clause ignores the index and does a full table scan.

SET AUTOTRACE ON
SELECT * FROM ii_tab WHERE id = 9999;

----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |     1 |     3 |     7   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| II_TAB |     1 |     3 |     7   (0)| 00:00:01 |
----------------------------------------------------------------------------

Setting the OPTIMIZER_USE_INVISIBLE_INDEXES parameter makes the index available to the optimizer.

ALTER SESSION SET OPTIMIZER_USE_INVISIBLE_INDEXES=TRUE;
SELECT * FROM ii_tab WHERE id = 9999;

------------------------------------------------------------------------------
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |           |     1 |     3 |     1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| II_TAB_ID |     1 |     3 |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------

Making the index visible means it is still available to the optimizer when the OPTIMIZER_USE_INVISIBLE_INDEXES parameter is reset.

ALTER SESSION SET OPTIMIZER_USE_INVISIBLE_INDEXES=FALSE;
ALTER INDEX ii_tab_id VISIBLE;

------------------------------------------------------------------------------
| Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |           |     1 |     3 |     1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| II_TAB_ID |     1 |     3 |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------

Invisible indexes can be useful for processes with specific indexing needs, where the presence of the indexes may adversely affect other functional areas. They are also useful for testing the impact of dropping an index.

The current visibility status of an index is indicated by the VISIBILITY column of the [DBA|ALL|USER]_INDEXES views.

Source: http://www.oracle-base.com/articles/11g/invisible-indexes-11gr1.php