Altering Indexes

Altering Storage Characteristics of an Index

Alter the storage parameters of any index, including those created by the database to enforce primary and unique key integrity constraints, using the ALTER INDEX statement. For example, the following statement alters the emp_ename index:

ALTER INDEX emp_ename
     STORAGE (PCTINCREASE 50);

The parameters INITIAL and MINEXTENTS cannot be altered. All new settings for the other storage parameters affect only extents subsequently allocated for the index.

For indexes that implement integrity constraints, you can adjust storage parameters by issuing an ALTER TABLE statement that includes the USING INDEX subclause of the ENABLE clause. For example, the following statement changes the storage options of the index created on table emp to enforce the primary key constraint:

ALTER TABLE emp
     ENABLE PRIMARY KEY USING INDEX;

Rebuilding an Existing Index

Before rebuilding an existing index, compare the costs and benefits associated with rebuilding to those associated with coalescing indexes as described in Table 20-1.

When you rebuild an index, you use an existing index as the data source. Creating an index in this manner enables you to change storage characteristics or move to a new tablespace. Rebuilding an index based on an existing data source removes intra-block fragmentation. Compared to dropping the index and using the CREATE INDEX statement, re-creating an existing index offers better performance.

The following statement rebuilds the existing index emp_name:

ALTER INDEX emp_name REBUILD;

The REBUILD clause must immediately follow the index name, and precede any other options. It cannot be used in conjunction with the DEALLOCATE UNUSED clause.

You have the option of rebuilding the index online. Rebuilding online enables you to update base tables at the same time that you are rebuilding. The following statement rebuilds the emp_name index online:

ALTER INDEX emp_name REBUILD ONLINE;

If you do not have the space required to rebuild an index, you can choose instead to coalesce the index. Coalescing an index is an online operation.

Making an Index Unusable

When you make an index unusable, it is ignored by the optimizer and is not maintained by DML. When you make one partition of a partitioned index unusable, the other partitions of the index remain valid.

An unusable index or index partition must be rebuilt, or dropped and re-created, before it can be used.

To make an index unusable: 

• Submit the following statement:

  ALTER INDEX index_name UNUSABLE;
  

To make an index partition unusable: 

• Submit the following statement:

  ALTER INDEX index_name MODIFY PARTITION partition_name UNUSABLE;
  

To determine if an index is valid or unusable: 

• Submit the following query:

  SELECT INDEX_NAME, STATUS FROM USER_INDEXES
      WHERE INDEX_NAME = 'index_name';
  

  For example, to determine if the index ind1 is unusable, submit this query:

  SELECT INDEX_NAME, STATUS FROM USER_INDEXES
      WHERE INDEX_NAME = 'IND1';
   
  INDEX_NAME                     STATUS
  ------------------------------ --------
  IND1                           UNUSABLE
  

To determine if an index partition is unusable: 

• Submit the following query

  SELECT PARTITION_NAME, STATUS FROM USER_IND_PARTITIONS
     WHERE INDEX_NAME = 'index_name';
  

  For example, to determine the unusable status of the partitions in the index custzip, submit this query:

  SELECT PARTITION_NAME, STATUS FROM USER_IND_PARTITIONS
     WHERE INDEX_NAME = 'CUSTZIP';
  
  PARTITION_NAME                 STATUS
  ------------------------------ --------
  CUSTZIP_P1                     USABLE
  CUSTZIP_P2                     UNUSABLE
  CUSTZIP_P3                     USABLE
  

Making an Index Invisible

An invisible index is ignored by the optimizer unless you explicitly set the OPTIMIZER_USE_INVISIBLE_INDEXES initialization parameter to TRUE at the session or system level. Making an index invisible is an alternative to making it unusable or dropping it. You cannot make an individual index partition invisible. Attempting to do so produces an error.

To make an index invisible: 

• Submit the following SQL statement:

  ALTER INDEX index INVISIBLE;
  

To make an invisible index visible again: 

• Submit the following SQL statement:

  ALTER INDEX index VISIBLE;
  

To determine whether an index is visible or invisible: 

• Query the dictionary views USER_INDEXES, ALL_INDEXES, or DBA_INDEXES.

  For example, to determine if the index ind1 is invisible, issue the following query:

  SELECT INDEX_NAME, VISIBILITY FROM USER_INDEXES
     WHERE INDEX_NAME = 'IND1';
  
  INDEX_NAME   VISIBILITY
  ----------   ----------
  IND1         VISIBLE
  

Renaming an Index

To rename an index, issue this statement:

ALTER INDEX index_name RENAME TO new_name;

Monitoring Index Usage

Oracle Database provides a means of monitoring indexes to determine whether they are being used. If an index is not being used, then it can be dropped, eliminating unnecessary statement overhead.

To start monitoring the usage of an index, issue this statement:

ALTER INDEX index MONITORING USAGE;

Later, issue the following statement to stop the monitoring:

ALTER INDEX index NOMONITORING USAGE;

The view V$OBJECT_USAGE can be queried for the index being monitored to see if the index has been used. The view contains a USED column whose value is YES or NO, depending upon if the index has been used within the time period being monitored. The view also contains the start and stop times of the monitoring period, and a MONITORING column (YES/NO) to indicate if usage monitoring is currently active.

Each time that you specify MONITORING USAGE, the V$OBJECT_USAGE view is reset for the specified index. The previous usage information is cleared or reset, and a new start time is recorded. When you specify NOMONITORING USAGE, no further monitoring is performed, and the end time is recorded for the monitoring period. Until the next ALTER INDEX...MONITORING USAGE statement is issued, the view information is left unchanged.