Module12-indexes.htm; updated June 25, 2013; Some figures shown in these notes are from Oracle document D11321GC11. Ref: Oracle® Database Administrator's Guide 11g Release 2 (11.2) E25494-03; Oracle® Database Concepts 11g Release 2 (11.2) E25789-01

Module 12 – Managing Indexes

Objectives

·        Learn basic concepts about indexes including types of indexes.

·        Create B-Tree, Bitmap, Hash Cluster, Global/Local, Reverse Key, and Function-based indexes.

·        Reorganize indexes.

·        Drop indexes.

·        Obtain data dictionary information about indexes.

Introduction

 

Indexes are totally optional structures that are intended to speed up the execution of SQL statements against table data and cluster data. 

 

From your earlier studies, you should realize that Indexes are used for direct access to a particular row or set of rows in a table.  From your study of database management systems, you learned that indexes are most typically organized as some type of tree structure.  You may not have studied Bitmap Indexes – you will also study this type of index in this course.

 

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.

 

 

Index Concepts and Facts

 

An index can be composed of a single column for a table, or it may be comprised of more than one column for a table.  An index based on more than one column is termed a concatenated (or composite) index.

 

Examples:

·        The SSN column serves as an index key that tracks individual students at a university.

·        The concatenated primary key index for an ENROLL table (SSN + SectionID + Term + Year) is used to track the enrollment of a student in a particular course section.

·        The maximum number of columns for a concatenated index is 32; but the combined size of the columns cannot exceed about one-half of a data block size.

 

·        A unique index allows no two rows to have the same index entry.  An example would be an index on student SSN. 

·        A non-unique index allows more than one row to have the same index entry (this is also called a secondary key index).  An example would be an index on U.S. Mail zip codes.

·        A function-based index is created when using functions or expressions that involve one or more columns in the table that is being indexed.  A function-based index pre-computes the value of the function or expression and stores it in the index.  Function-based indexes can be created as either a B-tree or a bitmap index.

·        A partitioned index allows an index to be spread across several tablespaces - the index would have more than one segment and typically access a table that is also partitioned to improve scalability of a system.  This type of index decreases contention for index lookup and increases manageability. 

 

To create an index in your own schema:

·        The table to be indexed must be in your schema, OR

·        You have the INDEX privilege on the table to be indexed, OR

·        You have the CREATE ANY INDEX system privilege.

 

To create an index in another schema:

·        You have the CREATE ANY INDEX system privilege, AND

·        The owner of the other schema has a quota for the tablespace that will store the index segment (or UNLIMITED TABLESPACE privilege).

 

 

Loading Data

 

Data initially loaded into a table will load more efficiently if the index is created after the table is created.  This is because the index must be updated after each row insertion if the index is created before loading data.

 

Creating an index on an existing table requires sort space – typically memory values that are paged in and out of segments in the TEMP tablespace allocated to a user.  Users are also allocated memory for index creation based on the SORT_AREA_SIZE parameter – if memory is insufficient, then swapping takes place.

 

Guidelines for Creating an Index

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.

o   The percentage varies greatly according to the relative speed of a table scan and how the distribution of the row data in relation to the index key.

o   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.

 

Index columns with these characteristics:

o   The order of the columns in the CREATE INDEX statement affect query performance – specify the most frequently used columns first.

o   An index that specifies column1+column2+column3 can improve performance for queries with WHERE clauses on column1 or on column1+column2, but will not be used for queries with a WHERE clause that just uses column2 or column3.

·        Values are relatively unique in the column (but not number columns that store currency values).

·        There is a wide range of values (good for regular B-tree indexes).

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

·        The column is a virtual column (can create both unique and non-unique 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 > <some numeric value goes her>

 

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).

 

Small tables don't need indexes – but it is a good idea to still index the Primary Key.

 

Do not index columns where:

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

·        LONG and LONG RAW columns – these cannot be indexed.

 

 

 

B-Tree Index

 

B-Tree Structure

 

This figure illustrates in a very simple way the structure of a B-Tree index.  A B-tree index usually stores a list of primary key values and a list of associated ROWID values that point to the row location of the record with a given primary key value.  In this figure the ROWID values are represented by the pointers at the leaf level.

 

 

 

The top level of the index is called the Root.  The Root points to entries at lower levels of the index - these lower levels are termed Branch Blocks.

 

A node in the index may store multiple (more than one) key values - in fact, almost all B-Trees have nodes with multiple values - the tree structure grows upward and nodes split when they reach a specified size.

 

At the Leaf level the index entries point to rows in the table.  At the Leaf level the index entries are linked in a bi-directional chain that allows scanning rows in both ascending and descending order of key values - this supports sequential processing as well as direct access processing.

 

In a non-partitioned table, Key values are repeated if multiple rows have the same key value – this would be a non-unique index (unless the index is compressed).  Index entries are not made for rows that have all of the key columns NULL.

 

Leaf Index Format:  The index entry at the Leaf level is made up of three components.

·        Entry Header - stores number of columns in the index and locking information about the row to which the index points.

·        Key Column Length-Value Pairs - defines the size of the column in the key followed by the actual column value.   These pairs are repeated for each column in a composite index.

·        ROWID - this is the ROWID of a row in a table that contains the key value associated with this index entry.

 

Data Manipulation Language Effects:  Any DML on a table also causes the Oracle Server to maintain the associated indexes.

·        When a row is inserted into a table, the index must also be updated.  This requires the physical insertion of an index entry into the index tree structure.

·        When a row is deleted from a table, the index only has the entry "logically" deleted (turn a delete bit from off to on).  The space for the deleted row is not available for new entries until all rows in the block are deleted.

·        When a row key column is updated for a table, the index has both a logical deletion and a physical insertion into the index.

1.   If an indexed table has lots of rows to be inserted, set PCTFREE high to accommodate new index values.

2.   If the table is static, set PCTFREE low.

3.   PCTUSED cannot be specified for indexes.

 

Creating B-Tree Indexes

 

An example CREATE INDEX command for a normal B-tree index is given here.

 

CREATE [UNIQUE] INDEX USER350.Orders_Index
    ON USER350.Orders(OrderId)
    PCTFREE 20
    INITTRANS 6 MAXTRANS 10
    LOGGING
    TABLESPACE Index01;

 

·        The UNIQUE clause specifies unique entries - the default is NONUNIQUE.  Note that the owner's schema (USER350) is specified – this is optional.

·         The PCTFREE parameter is only effective when the index is created - after that, new index block entries are made and PCTFREE is ignored.  PCTFREE is ignored because entries are not updated - instead a logical delete and physical insert of a new index entry is made. 

·        PCTUSED cannot be specified for an index because updates are not made to index entries. 

 

Use a low PCTFREE when the indexed column is system generated as would be the case with a sequence (sequence indexes tend to increase in an ascending fashion) because new entries tend to be made to new data blocks - there are no or few insertions into data blocks that already contain index entries.

 

Use a high PCTFREE when the indexed column or set of columns can take on random values that are not predictable.  Such is the case when a new Orderline row is inserted - the ProductID column may be a non-unique foreign key index and the product to be sold on an Orderline is not predictable for any given order.

 

The Default and Minimum for INITTRANS is 2.  The limit on MAXTRANS is 255 - this number would be inordinately large.

 

By default, LOGGING is on so that the index creation is logged into the redo log file.  Specifying NOLOGGING would increase the speed of index creation initially, but would not enable recovery at the time the index is created.

 

Interestingly, Oracle will use existing indexes to create new indexes whenever the key for the new index corresponds to the leading part of the key of an existing index.

 

Indexes and Constraints

 

The UNIQUE and PRIMARY KEY constraints on tables are enforced by creating indexes on the unique key or primary key – creation of the index is automatic when such a constraint is enabled.

 

You can specify a USING INDEX clause to control the creation process.  The index created takes the name of the constraint unless otherwise specified.

 

Example creating a PRIMARY KEY constraint while creating a table.

 

CREATE TABLE District (

    District_Number     NUMBER(5)

        CONSTRAINT District_PK PRIMARY KEY,

    District_Name       VARCHAR2(50)

    )

  ENABLE PRIMARY KEY USING INDEX

  TABLESPACE Index01

  PCTFREE 0;

 

 

Key-Compressed Index

 

This is a B-tree with compression – compression eliminates duplicate occurrences of a key in an index leaf block.

 

CREATE INDEX Emp_Name ON Emp (Last_Name, First_Name)

    TABLESPACE Index01

    COMPRESS 1;

 

This approach breaks an index key into a prefix and suffix entry in the index block.  Compression causes sharing of the prefix entries among all suffix entries and save lots of space allowing the storage of more keys in a block.

 

Use key compression when:

·        The index is non-unique where the ROWID column is appended to the key to make the index key unique.

·        The index is a non-unique multicolumn index – example:  Zip_Code + Last_Name.

 

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.

 

This procedure avoids the problem of expanding the usually shared, temporary tablespace to an unreasonably large size that might affect future performance.

 

 

 

Bitmap Index

 

Bitmap Index Structure

 

Bitmap indexes are alternatives to B-tree indexes and are only used to create secondary key indexes.  They are used in certain situations:

·        If a table has millions of rows and there are very few distinct values for index entries, for example, indexes on zip codes could have many, many rows for a single zip code value, then a bitmap index may perform well. 

·        A bitmap index may best support WHERE conditions involving the OR operator.

·        A bitmap index may work best for low update activity tables or read-only key columns.

 

A bitmap index is also organized as a B-tree structure; however, the leaf nodes in a bitmap index stores a bitmap for each key value - it does NOT store the ROWIDs.  Instead, each bit corresponds to a possible ROWID.  If the bit is set on, the row with the corresponding ROWID contains the key value.  This figure illustrates this concept.  In the figure, the key consists of only one column, and the first entry has a logical key value of Blue, the second is Green, the third Red, and the fourth Yellow.

 

 

 

Comparisons and Limitations:  Updates to key columns for bitmap indexes require locking a significant portion of the index in order to perform the update.  This table compares B-tree and bitmap indexes.

 

B-Tree

Bitmap

Suitable for high-cardinality columns

Suitable for low-cardinality columns

Updates on keys relatively inexpensive

Updates to key columns very expensive

Inefficient for queries using OR logical operator

Efficient for queries using OR logical operator

Used mostly for On-Line Transaction Processing

Very useful for Data Warehousing

 

 

Creating Bitmap Indexes

 

Example CREATE BITMAP INDEX command:

 

CREATE BITMAP INDEX USER350.Products_Region_Idx
    ON USER350.Products_Region (RegionId ASC)
    PCTFREE 40
    INITTRANS 6 MAXTRANS 10
    LOGGING
    TABLESPACE Index01;

 

A BITMAP index cannot be specified as unique since they are only used for secondary key indexes.

 

You can specify an init.ora parameter CREATE_BITMAP_AREA_SIZE to specify the amount of memory allocated for storing bitmap segments. 

·        The default memory allocated is 8MB. 

·        If the memory allocated for bitmap index segment creation is larger, then indexes are created faster. 

·        If there are only a few unique values for the index key field, then you can allocate much less memory to the creation of bitmap index segments, perhaps only a few kilobytes.

 

 

Other General Topics

 

This section covers altering, rebuilding, coalescing, and dropping indexes.  It also covers validating indexes and identifying unused indexes.  Guidelines are provided for when to index columns.

 

Altering Index Storage Parameters

 

The ALTER INDEX command is used primarily to alter storage parameters.  Other uses include:

 

·         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.

Use of the ALTER INDEX command is straight-forward as illustrated in this example.

 

ALTER INDEX USER350.Products_Region_Idx
    STORAGE (NEXT 200K MAXEXTENTS 200);

 

One of the most common changes is to increase MAXEXTENTS for an index as a system grows in size.

 

This example enforces the primary key constraint that might have previous been relaxed.

 

ALTER TABLE Student
     ENABLE PRIMARY KEY USING INDEX;

 

 

Allocating and De-allocating Index Space:  If a DBA is going to insert a lot of rows into a table, performance can be improved by adding extents to the indexes associated with the table prior to doing the inserts.  This improves performance by avoiding the dynamic addition of index extents.  This can be accomplished by using the ALTER INDEX command with the ALLOCATE EXTENT option.

 

ALTER INDEX USER350.Products_Region_Idx
    ALLOCATE EXTENT (SIZE 400K
    DATAFILE
      '/u01/student/dbockstd/oradata/USER350index01.dbf');

 

You can DEALLOCATE unused index space after the insertions are complete.  This will free up space that is not in use within the tablespace.

 

ALTER INDEX USER350.Products_Region_Idx
    DEALLOCATE UNUSED;

 

Creating an Index Online

 

Creating an Online Index allows Data Manipulation Language (DML) operations against the table while the index build is being completed.  DDL operations on the table are not allowed during the index creation process.

 

CREATE INDEX Manager_Employee_Idx ON Employee

    (Manager_ID, Emp_SSN) ONLINE;

 

There are some restrictions:

·        Temporary tables cannot be created/rebuilt online.

·        Partitioned indexes must be created/rebuilt one partition at a time.

·        You cannot de-allocate unused space during an online rebuild.

·        You cannot change the PCTFREE parameter for the whole index.

 

 

Rebuilding Indexes

 

You may improve system performance by rebuilding an index that is highly fragmented due to lots of physical insertions and logical deletions.  Such may be the case for a SALES_ORDER table where filled orders are deleted over time.

 

ALTER INDEX USER350.Products_Region_Idx
    REBUILD
    TABLESPACE Index01;

 

·        If you rebuild an index from an existing index, the rebuild operation is more efficient because the index information does not need to be sorted - it is already sorted. 

·        You can specify a new tablespace if necessary for the rebuilt index. 

·        The older index is deleted after the new index is rebuilt.  You must have sufficient space in the tablespace for the old/new index during the rebuild operation. 

·        Queries use the old index while the rebuild operation is under way.

·        You can also specify the ONLINE option when rebuilding an index – this allows DML updates to process even while the index is being rebuilt.

 

ALTER INDEX USER350.Products_Region_Idx
    REBUILD ONLINE;

 

Function-Based Indexes

 

These indexes support queries that use a value returned by a function or expression in a WHERE clause.

·        The index computes and stores the value from the function or expression in the index.

·        If user-based functions are used, the function must be marked DETERMINISTIC (stochastic functions won't work).

·        If the function is owned by another user, you must have the EXECUTE privilege on the function object.

·        The table must be analyzed after the index is created.

·        The query must be guaranteed to not need NULL values from the indexed expression because NULL values are not stored  in indexes.

 

Example --- this defines a function-based index (Last_Name_Caps_Idx) based on the UPPER function – this facilitates use of the UPPER function when specifying a WHERE clause condition and ensures use of the index to return values efficiently.

 

CREATE TABLE Faculty (

    Faculty_ID NUMBER(5)

       CONSTRAINT Employee_PK PRIMARY KEY,

    Last_Name  VARCHAR2(20),

    First_Name VARCHAR2(20)

    )

  TABLESPACE Data01;

INSERT INTO Faculty VALUES (1, 'Bock', 'Douglas');

INSERT INTO Faculty VALUES (2, 'Bordoloi', 'Bijoy');

INSERT INTO Faculty VALUES (3, 'Sumner', 'Mary');

INSERT INTO Faculty Values (4, 'BOCK', 'Ronald');

 

Now create the (non-unique) index, then select from the table.

 

CREATE INDEX Last_Name_Caps_Idx ON Faculty (UPPER(Last_Name))

TABLESPACE Index01;

 

Note that the Oracle optimizer considers using the index because of the use of the UPPER function in the WHERE clause.

 

SELECT Faculty_ID, Last_Name, First_Name

FROM Faculty

WHERE UPPER(Last_Name) LIKE 'BOCK';

 

    EMP_ID LAST_NAME            FIRST_NAME

---------- -------------------- --------------------

         1 Bock                 Douglas

         4 BOCK                 Ronald

 

 

 

Coalescing Indexes

 

This is an alternative to the ALTER INDEX…REBUILD command – it eliminates index fragmentation.  Coalesce on an index is a block rebuild that is done online. 

 

In situations where a B-tree index has leaf blocks that can be freed up for reuse, you can merge leaf blocks using the ALTER INDEX…COALESCE statement.

 

ALTER INDEX USER350.Products_Region_Idx COALESCE;

In this figure, the first two leaf node blocks are about 50% full – fragmentation is evident.  After coalescing, blocks are merged freeing up a block for reuse.

 

 

When to Rebuild or Coalesce

 

Rebuild Index

Coalesce Index

To quickly move an index to another tablespace.

Cannot move the index to another tablespace.

Requires more disk space.

Operation does not require more disk space.

Creates new index tree – height of tree may shrink.

Only coalesces leaf blocks.

Enables changing storage and tablespace parameters.

Frees up index leaf blocks for use.

 

 

 

Checking Index Validity

 

The ANALYZE INDEX command with the VALIDATE STRUCTURE option can be used to check for block corruption in an index.

 

ANALYZE INDEX dbock.pk_course VALIDATE STRUCTURE;

 

You can query the INDEX_STATS view and obtain information about the index.

 

SQL> ANALYZE INDEX Course_PK VALIDATE STRUCTURE;

 

Index analyzed.

 

SELECT blocks, pct_used, distinct_keys, lf_rows, del_lf_rows

FROM index_stats;

 

    BLOCKS   PCT_USED DISTINCT_KEYS    LF_ROWS DEL_LF_ROWS

---------- ---------- ------------- ---------- -----------

        15          1             2          2           0

 

An index needs to be reorganized when the proportion of deleted rows (DEL_LF_ROWS) to existing rows (LF_ROWS – short for Leaf) is greater than 30%. 

 

 

Dropping an Index

 

DBAs drop indexes if they are no longer needed by an application.  Here are reasons for dropping an index:

·         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.

·        A large data load is to be processed – drop indexes prior to large data loads - this improves performance and tends to use index space more efficiently.

·        You also drop indexes that are marked by the system as INVALID because of some type of instance failure, or if the index is corrupt.

 

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.

 

Example: 

DROP INDEX index_name;

 

You cannot drop an index used to implement an integrity constraint

·         You would first need to remove the integrity constraint by altering the table, then drop the index.

·        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.

 

 

Identifying Unused Indexes

 

Statistics about the usage of an index can be gathered and displayed in V$OBJECT_USAGE.

 

·        Start usage monitoring of an index:

ALTER INDEX USER350.dept_id_idx

MONITORING USAGE;

 

·        Stop usage monitoring of an index:

ALTER INDEX USER350.dept_id_idx

NOMONITORING USAGE;

 

·        If the information gathered indicates that an index is never used, the index can be dropped.

·        In addition, eliminating unused indexes cuts down on overhead that the Oracle server has to do for DML, thus performance is improved. 

 

Each time the MONITORING USAGE clause is specified, V$OBJECT_USAGE will be reset for the specified index.  The previous information is cleared or reset, and a new start time is recorded.

 

V$OBJECT_USAGE Columns

·        INDEX_NAME: The index name

·        TABLE_NAME: The corresponding table

·        MONITORING: Indicates whether monitoring is ON or OFF

·        USED: Indicates YES or NO whether index has been used during the monitoring time

·        START_MONITORING: Time monitoring began on index

·        END_MONITORING: Time monitoring stopped on index

 

 

Guidelines for Creating Indexes

 

A table can have any number of indexes, but the more indexes, the more overhead that is associated with DML operations:

·        A single row insertion or deletion requires updating all indexes on the table. 

·        An update operation may or may not affect indexes depending on the column updated.

 

The use of indexes involves a tradeoff - query performance tends to speed up, but data manipulation language operations tend to slow down.

·        Query performance improves because the index speeds up row retrieval. 

·        DML operations slow down because along with row insertions, deletions, and updates, the indexes associated with a table must also have insertions and deletions completed.

·        For very volatile tables, minimize the number of indexes used.

 

When possible, store indexes to a tablespace that does not have rollback segments, temporary segments, and user/data tables – DBAs usually create a separate tablespace just used for index segments.

 

You can minimize fragmentation by using extent sizes that are at least multiples of 5 times the DB_BLOCK_SIZE for the database.

 

Create an index when row retrieval involves less than 15% of a large table's rows – retrieval of more rows is generally more efficient with a full table scan.

 

You may want to use NOLOGGING when creating large indexes - you can improve performance by avoiding redo generation.

·        Indexes created with NOLOGGING requires a backup as their creation is not archived.

·        Example:

 

CREATE BITMAP INDEX USER350.Products_Region_Idx
    ON USER350.Products_Region (RegionId ASC)
    NOLOGGING;

Usually index entries are smaller than the rows they index.  Data blocks that store index entries tend to store more entries in each block, so the INITRANS parameter should be higher on indexes than on their related tables.

 

 

Checking Index Information in the Data Dictionary

 

You can check the DBA_INDEXES view (or ALL_INDEXES and USER_INDEXES) to get information on the INDEX_NAME, TABLESPACE_NAME, INDEX_TYPE, UNIQUENESS, and STATUS.

 

COLUMN tablespace_name FORMAT A15;

COLUMN index_type FORMAT A10;

SELECT Index_Name, Tablespace_Name, Index_Type,
    Uniqueness, Status
    FROM Dba_Indexes

    WHERE OWNER='DBOCK';

 

INDEX_NAME                     TABLESPACE_NAME INDEX_TYPE UNIQUENES STATUS

------------------------------ --------------- ---------- --------- --------

PK_ROOM                        USERS           NORMAL     UNIQUE    VALID

PK_BEDCLASSIFICATION           USERS           NORMAL     UNIQUE    VALID

PK_BED                         USERS           NORMAL     UNIQUE    VALID

PK_PATIENT                     USERS           NORMAL     UNIQUE    VALID

<more rows display>
 

The STATUS column indicates if the index is valid.  The INDEX_TYPE column indicates if the index is a bitmap or normal index.

 

You can query the DBA_IND_COLUMNS view (or ALL_IND_COLUMNS and USER_IND_COLUMNS) to determine which columns of a table are used for a particular index.

 

The DBA_IND_EXPRESSIONS (or ALL_IND_EXPRESSIONS and USER_IND_EXPRESSIONS) views describe the expressions of function-based indexes.

 

The V$OBJECT_USAGE view has information on index usage. produced by the ALTER INDEX … MONITORING USAGE command.

 

 



END OF NOTES