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Agenda

Index types

Create various indexes

Drop index

Get index information

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Index

An index is a tree structure that allowsdirect access to a row in a table

Indexes can be classified on their

logical design or on their physicalimplementation Logical: groups indexes from an application

perspective Physical: derived from the way the indexes

are stored

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Single Column and ConcatenatedIndexes

Single column index: has only one column inthe index keyAn index on the employee IDcolumn of an

employees table

Concatenated index: also known ascomposite index. Created on multiple columnsin a tableAn index on the departmentandjobcolumns of

an employees table Maximum number of columns in a composite key

index is 32

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Unique and Non-unique Indexes

Unique index: guarantees that no tworows of a table have duplicate values inthe column that defines the index

An index key in a unique index can point toonly one row in the table

Non-unique index: a single key canhave multiple rows associated with it

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Function-based Index

Created when using functions orexpressions that involve one or morecolumns in the table being indexed

Function-based indexes can be treatedas either a B-tree or a bitmap index

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Partitioned and Non-partitionedIndexes

Partitioned indexes are used for largetables to store index entriescorresponding to an index in several

segments

Partitioning allows an index to be

spread across many tablespaces Decrease contention for index lookup, and

increase manageability

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Index Leaf Entries

Entry header: storesnumber of columns andlocking info

Key column length-valuepairs: size of the a columnin the key, followed by the

value for the columnROWID

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B-tree Index and Null

There is no index entry correspondingto a row that has all key columns thatare NULL

Therefore a WHERE clause specifyingNULL will result in a full table scan

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Bitmap Index

A bitmap index is also organized as a B-tree, but the leaf node stores a bitmapfor each key value instead of a list of

ROWIDsEach bit in the bitmap corresponds to a

possible ROWID

If the bit is set, it means that the row withthe corresponding ROWID contains the keyvalue

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Bitmap Index Structure

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Bitmap Index Advantages

Bitmap indexes are more advantageous thanB-tree indexes in certain situations:

When a table has millions of rows and the key

columns have low cardinalityWhen queries often use a combination of

multiple WHERE conditions involving OR

operatorWhen there is read-only or low update

activity on the key columns

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Using Bitmap Index

The B-tree is used to locate the leaf nodesthat contain bitmap segments for a givenvalue of the key Start ROWID and the bitmap segments are used

to locate the rows that contain the key value

When changes are made to the key column inthe table, bitmaps must be modified.

Resulted in locking the bitmap segmentA row covered by the bitmap cannot be updated

by other transactions until the 1st transaction ends

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B-tree vs Bitmap Indexes

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Create Normal B-tree Indexes

Create an index on the EMPLOYEEStable using the LAST_NAME column

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Create Index: Guidelines

Balance query and DML needs Indexes speed up query performance and

slow down DML operations. Always

minimize the number of indexes needed onvolatile tables

Place in separate tablespace

Consider NOLOGGING for large indexes The creation and certain types of data

loads are not logged in the redo log file

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Create Bitmap Indexes

Use the CREATE_BITMAP_AREA_SIZE to

specify the amount of memory allocated forbitmap creation

Default value 8MB

Larger value, faster index creation

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Change Storage Parameters forIndexes

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Allocate Index Space

Add extents to an index before a periodof high insert activity on a table

Adding extents prevents dynamic

extensions of indexes and the resultingdegradation in performance

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De-allocate Index Space

Use the DEALLOCATE clause of ALTERINDEX command to release unusedspace above the high-water mark in an

index

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Rebuild Indexes

Use theALTER INDEX command toMove an index to a different tablespace

Improve space utilization by removing

deleted entries Change a reverse key index to a normal B-

tree index and vice versa

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Online Rebuild of Indexes

Building or rebuilding an index can betime-consuming, especially if the tableis very large

Rebuild indexes can be done withminimal table locking

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Coalescing Indexes

When you encounterindex fragmentation, youcan rebuild or coalesce

the indexCoalesce on an index is

a block rebuild that is

done online

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Check Index Validity

Analyze the index to perform thefollowing

Check all the index blocks for block

corruption. Populate the INDEX_STATS view with

information about the index

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Example: Query INDEX_STATS

Reorganize the index if it has a high

proportion of deleted rowsFor example: when the ration of

DEL_LF_ROWS to LF_ROWS exceeds 30%

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Drop Indexes

An index that is no longer needed byapplications can be dropped

An index may be dropped prior to

performing bulk loads. Improve performance of the load

Re-create the index after the load

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Identify Unused Index

To start monitoring the usage of anindex

To stop monitoring the usage of an

index

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Obtain Index Information

DBA_INDEXES: provide information onthe indexes

DBA_IND_COLUMNS: provide

information on the columns indexesDBA_IND_EXPRESSIONS: provide

information on function based indexes

V$OBJECT_USAGE: provide informationon the usage of an index

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Summary

Create different types of indexesReorganize index

Drop index

Get index information from the datadictionary