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21 Managing Indexes

This chapter contains the following topics:

About Indexes

Indexes are optional structures associated with tables and clusters that allow SQL queries to execute more quickly against a table. Just as the index in this manual helps you locate information faster than if there were no index, an Oracle Database index provides a faster access path to table data. You can use indexes without rewriting any queries. Your results are the same, but you see them more quickly.

Oracle Database provides several indexing schemes that provide complementary performance functionality. These are:

  • B-tree indexes: the default and the most common

  • B-tree cluster indexes: defined specifically for cluster

  • Hash cluster indexes: defined specifically for a hash cluster

  • Global and local indexes: relate to partitioned tables and indexes

  • Reverse key indexes: most useful for Oracle Real Application Clusters applications

  • Bitmap indexes: compact; work best for columns with a small set of values

  • Function-based indexes: contain the precomputed value of a function/expression

  • Domain indexes: specific to an application or cartridge.

Indexes are logically and physically independent of the data in the associated table. Being independent structures, they require storage space. You can create or drop an index without affecting the base tables, database applications, or other indexes. The database automatically maintains indexes when you insert, update, and delete rows of the associated table. If you drop an index, all applications continue to work. However, access to previously indexed data might be slower.

Guidelines for Managing Indexes

This section discusses guidelines for managing indexes and contains the following topics:

Create Indexes After Inserting Table Data

Data is often inserted or loaded into a table using either the SQL*Loader or an import utility. It is more efficient to create an index for a table after inserting or loading the data. If you create one or more indexes before loading data, then the database must update every index as each row is inserted.

Creating an index on a table that already has data requires sort space. Some sort space comes from memory allocated for the index creator. The amount for each user is determined by the initialization parameter SORT_AREA_SIZE. The database also swaps sort information to and from temporary segments that are only allocated during the index creation in the user's temporary tablespace.

Under certain conditions, data can be loaded into a table with SQL*Loader direct-path load, and an index can be created as data is loaded.

See Also:

Oracle Database Utilities for information about using SQL*Loader for direct-path load

Index the Correct Tables and Columns

Use the following guidelines for determining when to create an index:

  • Create an index if you frequently want to retrieve less than 15% of the rows in a large table. The percentage varies greatly according to the relative speed of a table scan and how the row data is distributed in relation to the index key. The faster the table scan, the lower the percentage; the more clustered the row data, the higher the percentage.

  • To improve performance on joins of multiple tables, index columns used for joins.

    Note:

    Primary and unique keys automatically have indexes, but you might want to create an index on a foreign key.
  • Small tables do not require indexes. If a query is taking too long, then the table might have grown from small to large.

Columns That Are Suitable for Indexing

Some columns are strong candidates for indexing. Columns with one or more of the following characteristics are candidates for indexing:

  • Values are relatively unique in the column.

  • There is a wide range of values (good for regular indexes).

  • There is a small range of values (good for bitmap indexes).

  • The column contains many nulls, but queries often select all rows having a value. In this case, use the following phrase:

    WHERE COL_X > -9.99 * power(10,125)
    

    Using the preceding phrase is preferable to:

    WHERE COL_X IS NOT NULL
    

    This is because the first uses an index on COL_X (assuming that COL_X is a numeric column).

Columns That Are Not Suitable for Indexing

Columns with the following characteristics are less suitable for indexing:

  • There are many nulls in the column, and you do not search on the not null values.

LONG and LONG RAW columns cannot be indexed.

Virtual Columns

You can create unique or non-unique indexes on virtual columns. A table index defined on a virtual column is equivalent to a function-based index on the table.

Order Index Columns for Performance

The order of columns in the CREATE INDEX statement can affect query performance. In general, specify the most frequently used columns first.

If you create a single index across columns to speed up queries that access, for example, col1, col2, and col3; then queries that access just col1, or that access just col1 and col2, are also speeded up. But a query that accessed just col2, just col3, or just col2 and col3 does not use the index.

Note:

In some cases, such as when the leading column has very low cardinality, the database may use a skip scan of this type of index. See Oracle Database Concepts for more information about index skip scan.

Limit the Number of Indexes for Each Table

A table can have any number of indexes. However, the more indexes there are, the more overhead is incurred as the table is modified. Specifically, when rows are inserted or deleted, all indexes on the table must be updated as well. Also, when a column is updated, all indexes that contain the column must be updated.

Thus, there is a trade-off between the speed of retrieving data from a table and the speed of updating the table. For example, if a table is primarily read-only, then having more indexes can be useful; but if a table is heavily updated, then having fewer indexes could be preferable.

Drop Indexes That Are No Longer Required

Consider dropping an index if:

  • It does not speed up queries. The table could be very small, or there could be many rows in the table but very few index entries.

  • The queries in your applications do not use the index.

  • The index must be dropped before being rebuilt.

Indexes and Deferred Segment Creation

Index segment creation is deferred when the associated table defers segment creation. This is because index segment creation reflects the behavior of the table with which it is associated.

See Also:

"Understand Deferred Segment Creation" for further information

Estimate Index Size and Set Storage Parameters

Estimating the size of an index before creating one can facilitate better disk space planning and management. You can use the combined estimated size of indexes, along with estimates for tables, the undo tablespace, and redo log files, to determine the amount of disk space that is required to hold an intended database. From these estimates, you can make correct hardware purchases and other decisions.

Use the estimated size of an individual index to better manage the disk space that the index uses. When an index is created, you can set appropriate storage parameters and improve I/O performance of applications that use the index. For example, assume that you estimate the maximum size of an index before creating it. If you then set the storage parameters when you create the index, then fewer extents are allocated for the table data segment, and all of the index data is stored in a relatively contiguous section of disk space. This decreases the time necessary for disk I/O operations involving this index.

The maximum size of a single index entry is approximately one-half the data block size.

Storage parameters of an index segment created for the index used to enforce a primary key or unique key constraint can be set in either of the following ways:

  • In the ENABLE ... USING INDEX clause of the CREATE TABLE or ALTER TABLE statement

  • In the STORAGE clause of the ALTER INDEX statement

Specify the Tablespace for Each Index

Indexes can be created in any tablespace. An index can be created in the same or different tablespace as the table it indexes. If you use the same tablespace for a table and its index, then it can be more convenient to perform database maintenance (such as tablespace or file backup) or to ensure application availability. All the related data is always online together.

Using different tablespaces (on different disks) for a table and its index produces better performance than storing the table and index in the same tablespace. Disk contention is reduced. But, if you use different tablespaces for a table and its index, and one tablespace is offline (containing either data or index), then the statements referencing that table are not guaranteed to work.

Consider Parallelizing Index Creation

You can parallelize index creation, much the same as you can parallelize table creation. Because multiple processes work together to create the index, the database can create the index more quickly than if a single server process created the index sequentially.

When creating an index in parallel, storage parameters are used separately by each query server process. Therefore, an index created with an INITIAL value of 5M and a parallel degree of 12 consumes at least 60M of storage during index creation.

See Also:

Oracle Database VLDB and Partitioning Guide for information about using parallel execution

Consider Creating Indexes with NOLOGGING

You can create an index and generate minimal redo log records by specifying NOLOGGING in the CREATE INDEX statement.

Note:

Because indexes created using NOLOGGING are not archived, perform a backup after you create the index.

Creating an index with NOLOGGING has the following benefits:

  • Space is saved in the redo log files.

  • The time it takes to create the index is decreased.

  • Performance improves for parallel creation of large indexes.

In general, the relative performance improvement is greater for larger indexes created without LOGGING than for smaller ones. Creating small indexes without LOGGING has little effect on the time it takes to create an index. However, for larger indexes the performance improvement can be significant, especially when you are also parallelizing the index creation.

Understand When to Use Unusable or Invisible Indexes

Use unusable or invisible indexes when you want to improve the performance of bulk loads, test the effects of removing an index before dropping it, or otherwise suspend the use of an index by the optimizer.

Unusable indexes

An unusable index is ignored by the optimizer and is not maintained by DML. One reason to make an index unusable is to improve bulk load performance. (Bulk loads go more quickly if the database does not need to maintain indexes when inserting rows.) Instead of dropping the index and later re-creating it, which requires you to recall the exact parameters of the CREATE INDEX statement, you can make the index unusable, and then rebuild it.

You can create an index in the unusable state, or you can mark an existing index or index partition unusable. In some cases the database may mark an index unusable, such as when a failure occurs while building the index. When one partition of a partitioned index is marked 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. Truncating a table makes an unusable index valid.

When you make an existing index unusable, its index segment is dropped.

The functionality of unusable indexes depends on the setting of the SKIP_UNUSABLE_INDEXES initialization parameter. When SKIP_UNUSABLE_INDEXES is TRUE (the default), then:

  • DML statements against the table proceed, but unusable indexes are not maintained.

  • DML statements terminate with an error if there are any unusable indexes that are used to enforce the UNIQUE constraint.

  • For nonpartitioned indexes, the optimizer does not consider any unusable indexes when creating an access plan for SELECT statements. The only exception is when an index is explicitly specified with the INDEX() hint.

  • For a partitioned index where one or more of the partitions is unusable, the optimizer can use table expansion. With table expansion, the optimizer transforms the query into a UNION ALL statement, with some subqueries accessing indexed partitions and other subqueries accessing partitions with unusable indexes. The optimizer can choose the most efficient access method available for a partition. See Oracle Database SQL Tuning Guide for more information about table expansion.

When SKIP_UNUSABLE_INDEXES is FALSE, then:

  • If any unusable indexes or index partitions are present, then any DML statements that would cause those indexes or index partitions to be updated are terminated with an error.

  • For SELECT statements, if an unusable index or unusable index partition is present, but the optimizer does not choose to use it for the access plan, then the statement proceeds. However, if the optimizer does choose to use the unusable index or unusable index partition, then the statement terminates with an error.

Invisible Indexes

You can create invisible indexes or make an existing 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. Unlike unusable indexes, an invisible index is maintained during DML statements. Although you can make a partitioned index invisible, you cannot make an individual index partition invisible while leaving the other partitions visible.

Using invisible indexes, you can do the following:

  • Test the removal of an index before dropping it.

  • Use temporary index structures for certain operations or modules of an application without affecting the overall application.

  • Add an index to a set of columns on which an index already exists.

Understand When to Create Multiple Indexes on the Same Set of Columns

You can create multiple indexes on the same set of columns when the indexes are different in some way. For example, you can create a B-tree index and a bitmap index on the same set of columns. When you have multiple indexes on the same set of columns, only one of these indexes can be visible at a time, and any other indexes must be invisible.

You might create different indexes on the same set of columns because they provide the flexibility to meet your requirements. You can also create multiple indexes on the same set of columns to perform application migrations without dropping an existing index and recreating it with different attributes.

Different types of indexes are useful in different scenarios. For example, B-tree indexes are often used in online transaction processing (OLTP) systems with many concurrent transactions, while bitmap indexes are often used in data warehousing systems that are mostly used for queries. Similarly, locally and globally partitioned indexes are useful in different scenarios. Locally partitioned indexes are easy to manage because partition maintenance operations automatically apply to them. Globally partitioned indexes are useful when you want the partitioning scheme of an index to be different from its table's partitioning scheme.

You can create multiple indexes on the same set of columns when at least one of the following index characteristics is different:

  • The indexes are of different types.

    See "About Indexes" and Oracle Database Concepts for information about the different types of indexes.

    However, the following exceptions apply:

    • You cannot create a B-tree index and a B-tree cluster index on the same set of columns.

    • You cannot create a B-tree index and an index-organized table on the same set of columns.

  • The indexes use different partitioning.

    Partitioning can be different in any of the following ways:

    • Indexes that are not partitioned and indexes that are partitioned

    • Indexes that are locally partitioned and indexes that are globally partitioned

    • Indexes that differ in partitioning type (range or hash)

  • The indexes have different uniqueness properties.

    You can create both a unique and a non-unique index on the same set of columns.

Consider Costs and Benefits of Coalescing or Rebuilding Indexes

Improper sizing or increased growth can produce index fragmentation. To eliminate or reduce fragmentation, you can rebuild or coalesce the index. But before you perform either task weigh the costs and benefits of each option and choose the one that works best for your situation. Table 21-1 is a comparison of the costs and benefits associated with rebuilding and coalescing indexes.

Table 21-1 Costs and Benefits of Coalescing or Rebuilding Indexes

Rebuild Index Coalesce Index

Quickly moves index to another tablespace

Cannot move index to another tablespace

Higher costs: requires more disk space

Lower costs: does not require more disk space

Creates new tree, shrinks height if applicable

Coalesces leaf blocks within same branch of tree

Enables you to quickly change storage and tablespace parameters without having to drop the original index

Quickly frees up index leaf blocks for use


In situations where you have B-tree index leaf blocks that can be freed up for reuse, you can merge those leaf blocks using the following statement:

ALTER INDEX vmoore COALESCE;

Figure 21-1 illustrates the effect of an ALTER INDEX COALESCE on the index vmoore. Before performing the operation, the first two leaf blocks are 50% full. Therefore, you have an opportunity to reduce fragmentation and completely fill the first block, while freeing up the second.

Figure 21-1 Coalescing Indexes

Description of Figure 21-1 follows
Description of "Figure 21-1 Coalescing Indexes"

Consider Cost Before Disabling or Dropping Constraints

Because unique and primary keys have associated indexes, you should factor in the cost of dropping and creating indexes when considering whether to disable or drop a UNIQUE or PRIMARY KEY constraint. If the associated index for a UNIQUE key or PRIMARY KEY constraint is extremely large, then you can save time by leaving the constraint enabled rather than dropping and re-creating the large index. You also have the option of explicitly specifying that you want to keep or drop the index when dropping or disabling a UNIQUE or PRIMARY KEY constraint.

Creating Indexes

This section describes how to create indexes.

Note:

These operations also collect index statistics.

To create an index in your own schema, at least one of the following prerequisites must be met:

  • The table or cluster to be indexed is in your own schema.

  • You have INDEX privilege on the table to be indexed.

  • You have CREATE ANY INDEX system privilege.

To create an index in another schema, all of the following prerequisites must be met:

  • You have CREATE ANY INDEX system privilege.

  • The owner of the other schema has a quota for the tablespaces to contain the index or index partitions, or UNLIMITED TABLESPACE system privilege.

This section contains the following topics:

Creating an Index Explicitly

You can create indexes explicitly (outside of integrity constraints) using the SQL statement CREATE INDEX. The following statement creates an index named emp_ename for the ename column of the emp table:

CREATE INDEX emp_ename ON emp(ename)
      TABLESPACE users
      STORAGE (INITIAL 20K
      NEXT 20k);

Notice that several storage settings and a tablespace are explicitly specified for the index. If you do not specify storage options (such as INITIAL and NEXT) for an index, then the default storage options of the default or specified tablespace are automatically used.

See Also:

Oracle Database SQL Language Reference for syntax and restrictions on the use of the CREATE INDEX statement

Creating a Unique Index Explicitly

Indexes can be unique or non-unique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column (or columns). Non-unique indexes do not impose this restriction on the column values.

Use the CREATE UNIQUE INDEX statement to create a unique index. The following example creates a unique index:

CREATE UNIQUE INDEX dept_unique_index ON dept (dname)
      TABLESPACE indx;

Alternatively, you can define UNIQUE integrity constraints on the desired columns. The database enforces UNIQUE integrity constraints by automatically defining a unique index on the unique key. This is discussed in the following section. However, it is advisable that any index that exists for query performance, including unique indexes, be created explicitly.

See Also:

Oracle Database SQL Tuning Guide for more information about creating an index for performance

Creating an Index Associated with a Constraint

Oracle Database enforces a UNIQUE key or PRIMARY KEY integrity constraint on a table by creating a unique index on the unique key or primary key. This index is automatically created by the database when the constraint is enabled. No action is required by you when you issue the CREATE TABLE or ALTER TABLE statement to create the index, but you can optionally specify a USING INDEX clause to exercise control over its creation. This includes both when a constraint is defined and enabled, and when a defined but disabled constraint is enabled.

To enable a UNIQUE or PRIMARY KEY constraint, thus creating an associated index, the owner of the table must have a quota for the tablespace intended to contain the index, or the UNLIMITED TABLESPACE system privilege. The index associated with a constraint always takes the name of the constraint, unless you optionally specify otherwise.

Note:

An efficient procedure for enabling a constraint that can make use of parallelism is described in "Efficient Use of Integrity Constraints: A Procedure".

Specifying Storage Options for an Index Associated with a Constraint

You can set the storage options for the indexes associated with UNIQUE and PRIMARY KEY constraints using the USING INDEX clause. The following CREATE TABLE statement enables a PRIMARY KEY constraint and specifies the storage options of the associated index:

CREATE TABLE emp (
     empno NUMBER(5) PRIMARY KEY, age INTEGER)
     ENABLE PRIMARY KEY USING INDEX
     TABLESPACE users;

Specifying the Index Associated with a Constraint

If you require more explicit control over the indexes associated with UNIQUE and PRIMARY KEY constraints, the database lets you:

  • Specify an existing index that the database is to use to enforce the constraint

  • Specify a CREATE INDEX statement that the database is to use to create the index and enforce the constraint

These options are specified using the USING INDEX clause. The following statements present some examples.

Example 1:

CREATE TABLE a (
     a1 INT PRIMARY KEY USING INDEX (create index ai on a (a1)));

Example 2:

CREATE TABLE b(
     b1 INT, 
     b2 INT, 
     CONSTRAINT bu1 UNIQUE (b1, b2) 
                    USING INDEX (create unique index bi on b(b1, b2)),
     CONSTRAINT bu2 UNIQUE (b2, b1) USING INDEX bi);

Example 3:

CREATE TABLE c(c1 INT, c2 INT);
CREATE INDEX ci ON c (c1, c2);
ALTER TABLE c ADD CONSTRAINT cpk PRIMARY KEY (c1) USING INDEX ci;

If a single statement creates an index with one constraint and also uses that index for another constraint, the system will attempt to rearrange the clauses to create the index before reusing it.

Creating a Large Index

When creating an extremely large index, consider allocating a larger temporary tablespace for the index creation using the following procedure:

  1. Create a new temporary tablespace using the CREATE TABLESPACE or CREATE TEMPORARY TABLESPACE statement.

  2. Use the TEMPORARY TABLESPACE option of the ALTER USER statement to make this your new temporary tablespace.

  3. Create the index using the CREATE INDEX statement.

  4. Drop this tablespace using the DROP TABLESPACE statement. Then use the ALTER USER statement to reset your temporary tablespace to your original temporary tablespace.

Using this procedure can avoid the problem of expanding your usual, and usually shared, temporary tablespace to an unreasonably large size that might affect future performance.

Creating an Index Online

You can create and rebuild indexes online. Therefore, you can update base tables at the same time you are building or rebuilding indexes on that table. You can perform DML operations while the index build is taking place, but DDL operations are not allowed. Parallel execution is not supported when creating or rebuilding an index online.

The following statements illustrate online index build operations:

CREATE INDEX emp_name ON emp (mgr, emp1, emp2, emp3) ONLINE;

Note:

Keep in mind that the time that it takes on online index build to complete is proportional to the size of the table and the number of concurrently executing DML statements. Therefore, it is best to start online index builds when DML activity is low.

Creating a Function-Based Index

Function-based indexes facilitate queries that qualify a value returned by a function or expression. The value of the function or expression is precomputed and stored in the index.

In addition to the prerequisites for creating a conventional index, if the index is based on user-defined functions, then those functions must be marked DETERMINISTIC. Also, you just have the EXECUTE object privilege on any user-defined function(s) used in the function-based index if those functions are owned by another user.

Note:

CREATE INDEX stores the timestamp of the most recent function used in the function-based index. This timestamp is updated when the index is validated. When performing tablespace point-in-time recovery of a function-based index, if the timestamp on the most recent function used in the index is newer than the timestamp stored in the index, then the index is marked invalid. You must use the ANALYZE INDEX...VALIDATE STRUCTURE statement to validate this index.

To illustrate a function-based index, consider the following statement that defines a function-based index (area_index) defined on the function area(geo):

CREATE INDEX area_index ON rivers (area(geo));

In the following SQL statement, when area(geo) is referenced in the WHERE clause, the optimizer considers using the index area_index.

SELECT id, geo, area(geo), desc
     FROM rivers     
     WHERE Area(geo) >5000;

Table owners should have EXECUTE privileges on the functions used in function-based indexes.

Because a function-based index depends upon any function it is using, it can be invalidated when a function changes. If the function is valid, then you can use an ALTER INDEX...ENABLE statement to enable a function-based index that has been disabled. The ALTER INDEX...DISABLE statement lets you disable the use of a function-based index. Consider doing this if you are working on the body of the function.

Note:

An alternative to creating a function-based index is to add a virtual column to the target table and index the virtual column. See "About Tables" for more information.

See Also:

Creating a Key-Compressed Index

As your database grows in size, consider using index compression to save disk space. Creating an index using key compression eliminates repeated occurrences of key column prefix values. Key compression is most useful for non-unique indexes with a large number of duplicates on the leading columns.

Key compression breaks an index key into a prefix and a suffix entry. Compression is achieved by sharing the prefix entries among all the suffix entries in an index block. This sharing can lead to substantial savings in space, allowing you to store more keys for each index block while improving performance.

Key compression can be useful in the following situations:

  • You have a non-unique index where ROWID is appended to make the key unique. If you use key compression here, then the duplicate key is stored as a prefix entry on the index block without the ROWID. The remaining rows become suffix entries consisting of only the ROWID.

  • You have a unique multicolumn index.

You enable key compression using the COMPRESS clause. The prefix length (as the number of key columns) can also be specified to identify how the key columns are broken into a prefix and suffix entry. For example, the following statement compresses duplicate occurrences of a key in the index leaf block:

CREATE INDEX  hr.emp_ename ON emp(ename)
   TABLESPACE users
   COMPRESS 1;

You can also specify the COMPRESS clause during rebuild. For example, during rebuild you can disable compression as follows:

ALTER INDEX hr.emp_ename REBUILD NOCOMPRESS;

See Also:

Creating an Unusable Index

When you create an index in the UNUSABLE state, it is ignored by the optimizer and is not maintained by DML. An unusable index must be rebuilt, or dropped and re-created, before it can be used.

If the index is partitioned, then all index partitions are marked UNUSABLE.

The database does not create an index segment when creating an unusable index.

The following procedure illustrates how to create unusable indexes and query the database for details about the index.

To create an unusable index: 

  1. If necessary, create the table to be indexed.

    For example, create a hash-partitioned table called hr.employees_part as follows:

    sh@PROD> CONNECT hr
    Enter password: **
    Connected.
    
    hr@PROD> CREATE TABLE employees_part
      2    PARTITION BY HASH (employee_id) PARTITIONS 2
      3    AS SELECT * FROM employees;
     
    Table created.
    
    hr@PROD> SELECT COUNT(*) FROM employees_part;
     
      COUNT(*)
    ----------
           107
    
  2. Create an index with the keyword UNUSABLE.

    The following example creates a locally partitioned index on employees_part, naming the index partitions p1_i_emp_ename and p2_i_emp_ename, and making p1_i_emp_ename unusable:

    hr@PROD> CREATE INDEX i_emp_ename ON employees_part (employee_id)
      2    LOCAL (PARTITION p1_i_emp_ename UNUSABLE, PARTITION p2_i_emp_ename);
     
    Index created.
    
  3. (Optional) Verify that the index is unusable by querying the data dictionary.

    The following example queries the status of index i_emp_ename and its two partitions, showing that only partition p2_i_emp_ename is unusable:

    hr@PROD> SELECT INDEX_NAME AS "INDEX OR PARTITION NAME", STATUS
      2  FROM   USER_INDEXES
      3  WHERE  INDEX_NAME = 'I_EMP_ENAME'
      4  UNION ALL
      5  SELECT PARTITION_NAME AS "INDEX OR PARTITION NAME", STATUS
      6  FROM   USER_IND_PARTITIONS
      7  WHERE  PARTITION_NAME LIKE '%I_EMP_ENAME%';
     
    INDEX OR PARTITION NAME        STATUS
    ------------------------------ --------
    I_EMP_ENAME                    N/A
    P1_I_EMP_ENAME                 UNUSABLE
    P2_I_EMP_ENAME                 USABLE
    
  4. (Optional) Query the data dictionary to determine whether storage exists for the partitions.

    For example, the following query shows that only index partition p2_i_emp_ename occupies a segment. Because you created p1_i_emp_ename as unusable, the database did not allocate a segment for it.

    hr@PROD> COL PARTITION_NAME FORMAT a14
    hr@PROD> COL SEG_CREATED FORMAT a11
    hr@PROD> SELECT p.PARTITION_NAME, p.STATUS AS "PART_STATUS",
      2         p.SEGMENT_CREATED AS "SEG_CREATED",   
      3  FROM   USER_IND_PARTITIONS p, USER_SEGMENTS s
      4  WHERE  s.SEGMENT_NAME = 'I_EMP_ENAME';
     
    PARTITION_NAME PART_STA SEG_CREATED 
    -------------- -------- ----------- 
    P2_I_EMP_ENAME USABLE   YES       
    P1_I_EMP_ENAME UNUSABLE NO
    

See Also:

Creating an Invisible Index

An invisible index is an index that is ignored by the optimizer unless you explicitly set the OPTIMIZER_USE_INVISIBLE_INDEXES initialization parameter to TRUE at the session or system level.

To create an invisible index: 

  • Use the CREATE INDEX statement with the INVISIBLE keyword.

    The following statement creates an invisible index named emp_ename for the ename column of the emp table:

    CREATE INDEX emp_ename ON emp(ename)
          TABLESPACE users
          STORAGE (INITIAL 20K
          NEXT 20k)
          INVISIBLE;
    

Creating Multiple Indexes on the Same Set of Columns

You can create multiple indexes on the same set of columns when the indexes are different in some way. See "Understand When to Create Multiple Indexes on the Same Set of Columns" for information.

To create multiple indexes on the same set of columns, the following prerequisites must be met:

For example, the following steps create a B-tree index and a bitmap index on the same set of columns in the oe.orders table:

  1. Create a B-tree index on the customer_id and sales_rep_id columns in the oe.orders table:

    CREATE INDEX oe.ord_customer_ix1    ON oe.orders (customer_id, sales_rep_id);
    

    The oe.ord_customer_ix1 index is visible by default.

  2. Alter the index created in Step 1 to make it invisible:

    ALTER INDEX oe.ord_customer_ix1 INVISIBLE;
    

    Alternatively, you can add the INVISIBLE clause in Step 1 to avoid this step.

  3. Create a bitmap index on the customer_id and sales_rep_id columns in the oe.orders table:

    CREATE BITMAP INDEX oe.ord_customer_ix2    ON oe.orders (customer_id, sales_rep_id);
    

    The oe.ord_customer_ix2 index is visible by default.

    If the oe.ord_customer_ix1 index created in Step 1 is visible, then the CREATE BITMAP INDEX statement in this step returns an error.

Altering Indexes

To alter an index, your schema must contain the index, or you must have the ALTER ANY INDEX system privilege. With the ALTER INDEX statement, you can:

  • Rebuild or coalesce an existing index

  • Deallocate unused space or allocate a new extent

  • Specify parallel execution (or not) and alter the degree of parallelism

  • Alter storage parameters or physical attributes

  • Specify LOGGING or NOLOGGING

  • Enable or disable key compression

  • Mark the index unusable

  • Make the index invisible

  • Rename the index

  • Start or stop the monitoring of index usage

You cannot alter index column structure.

More detailed discussions of some of these operations are contained in the following sections:

See Also:

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 (NEXT 40);

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;

See Also:

Oracle Database SQL Language Reference for syntax and restrictions on the use of the ALTER INDEX statement

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 21-1.

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 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;

To rebuild an index in a different user's schema online, the following additional system privileges are required:

  • CREATE ANY TABLE

  • CREATE ANY INDEX

Note:

Online index rebuilding has stricter limitations on the maximum key length that can be handled, compared to other methods of rebuilding an index. If an ORA-1450 (maximum key length exceeded) error occurs when rebuilding online, try rebuilding offline, coalescing, or dropping and recreating the index.

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.

You must rebuild or drop and re-create an unusable index or index partition before using it.

The following procedure illustrates how to make an index and index partition unusable, and how to query the object status.

To make an index unusable: 

  1. Query the data dictionary to determine whether an existing index or index partition is usable or unusable.

    For example, issue the following query (output truncated to save space):

    hr@PROD> SELECT INDEX_NAME AS "INDEX OR PART NAME", STATUS, SEGMENT_CREATED
      2  FROM   USER_INDEXES
      3  UNION ALL
      4  SELECT PARTITION_NAME AS "INDEX OR PART NAME", STATUS, SEGMENT_CREATED
      5  FROM   USER_IND_PARTITIONS;
     
    INDEX OR PART NAME             STATUS   SEG
    ------------------------------ -------- ---
    I_EMP_ENAME                    N/A      N/A
    JHIST_EMP_ID_ST_DATE_PK        VALID    YES
    JHIST_JOB_IX                   VALID    YES
    JHIST_EMPLOYEE_IX              VALID    YES
    JHIST_DEPARTMENT_IX            VALID    YES
    EMP_EMAIL_UK                   VALID    NO
    .
    .
    .
    COUNTRY_C_ID_PK                VALID    YES
    REG_ID_PK                      VALID    YES
    P2_I_EMP_ENAME                 USABLE   YES
    P1_I_EMP_ENAME                 UNUSABLE NO
     
    22 rows selected.
    

    The preceding output shows that only index partition p1_i_emp_ename is unusable.

  2. Make an index or index partition unusable by specifying the UNUSABLE keyword.

    The following example makes index emp_email_uk unusable:

    hr@PROD> ALTER INDEX emp_email_uk UNUSABLE;
     
    Index altered.
    

    The following example makes index partition p2_i_emp_ename unusable:

    hr@PROD> ALTER INDEX i_emp_ename MODIFY PARTITION p2_i_emp_ename UNUSABLE;
     
    Index altered.
    
  3. (Optional) Query the data dictionary to verify the status change.

    For example, issue the following query (output truncated to save space):

    hr@PROD> SELECT INDEX_NAME AS "INDEX OR PARTITION NAME", STATUS, 
      2  SEGMENT_CREATED
      3  FROM   USER_INDEXES
      4  UNION ALL
      5  SELECT PARTITION_NAME AS "INDEX OR PARTITION NAME", STATUS, 
      6  SEGMENT_CREATED
      7  FROM   USER_IND_PARTITIONS;
     
    INDEX OR PARTITION NAME        STATUS   SEG
    ------------------------------ -------- ---
    I_EMP_ENAME                    N/A      N/A
    JHIST_EMP_ID_ST_DATE_PK        VALID    YES
    JHIST_JOB_IX                   VALID    YES
    JHIST_EMPLOYEE_IX              VALID    YES
    JHIST_DEPARTMENT_IX            VALID    YES
    EMP_EMAIL_UK                   UNUSABLE NO
    .
    .
    .
    COUNTRY_C_ID_PK                VALID    YES
    REG_ID_PK                      VALID    YES
    P2_I_EMP_ENAME                 UNUSABLE NO
    P1_I_EMP_ENAME                 UNUSABLE NO
     
    22 rows selected.
    

    A query of space consumed by the i_emp_ename and emp_email_uk segments shows that the segments no longer exist:

    hr@PROD> SELECT SEGMENT_NAME, BYTES
      2  FROM   USER_SEGMENTS
      3  WHERE  SEGMENT_NAME IN ('I_EMP_ENAME', 'EMP_EMAIL_UK');
     
    no rows selected
    

See Also:

Making an Index Invisible or Visible

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;
    

Note:

If there are multiple indexes on the same set of columns, then only one of these indexes can be visible at any point in time. If you try to make an index on a set of columns visible, and another index on the same set of columns is visible, then an error is returned.

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 USER_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 USER_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.

Monitoring Space Use of Indexes

If key values in an index are inserted, updated, and deleted frequently, the index can lose its acquired space efficiency over time. Monitor index efficiency of space usage at regular intervals by first analyzing the index structure, using the ANALYZE INDEX...VALIDATE STRUCTURE statement, and then querying the INDEX_STATS view:

SELECT PCT_USED FROM INDEX_STATS WHERE NAME = 'index';

The percentage of index space usage varies according to how often index keys are inserted, updated, or deleted. Develop a history of average efficiency of space usage for an index by performing the following sequence of operations several times:

  • Analyzing statistics

  • Validating the index

  • Checking PCT_USED

  • Dropping and rebuilding (or coalescing) the index

When you find that index space usage drops below its average, you can condense the index space by dropping the index and rebuilding it, or coalescing it.

Dropping Indexes

To drop an index, the index must be contained in your schema, or you must have the DROP ANY INDEX system privilege.

Some reasons for dropping an index include:

  • The index is no longer required.

  • The index is not providing anticipated performance improvements for queries issued against the associated table. For example, the table might be very small, or there might be many rows in the table but very few index entries.

  • Applications do not use the index to query the data.

  • The index has become invalid and must be dropped before being rebuilt.

  • The index has become too fragmented and must be dropped before being rebuilt.

When you drop an index, all extents of the index segment are returned to the containing tablespace and become available for other objects in the tablespace.

How you drop an index depends on whether you created the index explicitly with a CREATE INDEX statement, or implicitly by defining a key constraint on a table. If you created the index explicitly with the CREATE INDEX statement, then you can drop the index with the DROP INDEX statement. The following statement drops the emp_ename index:

DROP INDEX emp_ename;

You cannot drop only the index associated with an enabled UNIQUE key or PRIMARY KEY constraint. To drop a constraints associated index, you must disable or drop the constraint itself.

Note:

If a table is dropped, all associated indexes are dropped automatically.

See Also:

Indexes Data Dictionary Views

The following views display information about indexes:

View Description
DBA_INDEXES

ALL_INDEXES

USER_INDEXES

DBA view describes indexes on all tables in the database. ALL view describes indexes on all tables accessible to the user. USER view is restricted to indexes owned by the user. Some columns in these views contain statistics that are generated by the DBMS_STATS package or ANALYZE statement.
DBA_IND_COLUMNS

ALL_IND_COLUMNS

USER_IND_COLUMNS

These views describe the columns of indexes on tables. Some columns in these views contain statistics that are generated by the DBMS_STATS package or ANALYZE statement.
DBA_IND_PARTITIONS

ALL_IND_PARTITIONS

USER_IND_PARTITIONS

These views display, for each index partition, the partition-level partitioning information, the storage parameters for the partition, and various partition statistics that are generated by the DBMS_STATS package.
DBA_IND_EXPRESSIONS

ALL_IND_EXPRESSIONS

USER_IND_EXPRESSIONS

These views describe the expressions of function-based indexes on tables.
DBA_IND_STATISTICS

ALL_IND_STATISTICS

USER_IND_STATISTICS

These views contain optimizer statistics for indexes.
INDEX_STATS Stores information from the last ANALYZE INDEX...VALIDATE STRUCTURE statement.
INDEX_HISTOGRAM Stores information from the last ANALYZE INDEX...VALIDATE STRUCTURE statement.
USER_OBJECT_USAGE Contains index usage information produced by the ALTER INDEX...MONITORING USAGE functionality.

See Also:

Oracle Database Reference for a complete description of these views