Module 06_ Planning Indexes

8/9/2019 Module 06_ Planning Indexes

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Module 6:Planning, Creating &Maintaining Indexes

Vidya Vrat Agarwal. | MCT, MCSD


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Overview

Introduction to Indexes

B Tree

Clustered Index

NonClustered IndexCreating and Dropping Index

Creating Unique Index

Creating Composite Index

Creating Index Options FillFactor & Pad_Index

Maintaining Indexes


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What is an Index

Indexes in databases are similar to indexes in books.In a book, an index allows you to find informationquickly without reading the entire book.

In a database, an index allows the database programto find data in a table without scanning the entiretable.

An Index is an Internal Table Structure that Databaseuses to provide quick access to rows of a table.


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Introduction to Indexes

How Data Is Stored

Rows are stored in data pages

Heaps are a collection of data pages for a table

How Data Is Accessed

Scanning all data pages in a table (Table Scan)

Using anIndex that points to data on a page (Indexes)Data Pages

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Use of Table ScanWhen a Table Scan is performed SQL Server starts at the

beginning of tablelooking through every row in the table. Asit finds rows that match the Criteria of Query, it includesthem in theResult Set.

Use of IndexesOnce the index is chosen, SQL Server navigates the tree

structure to the point of data that matches criteria andextracts only the records it needs.

During the query optimization process, the optimizer takes alook at all the available indexes and choose the best one.


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Whether to Create Indexes

Why to Create an Index ( Advantages)

Speeds up data access

Enforces uniqueness of rows

Speeds joins between tables

Why Not to Create an Index ( Disadvantages)

Consumes disk space

Incurs overhead

Data Modification takes longer as Indexes have to beupdated.


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Indexing Guidelines

Columns to Index

Primary and foreign keys

Those frequently searched in ranges

Those frequently accessed in sorted order

Columns Not to Index

Those seldom referenced in queries

Those that contain few unique valuesThose defined withbit, text, or image data types


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How SQL Server Uses Indexes

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SQL Server:

Determines Whether an IndexExists and Is Useful

Navigates Through the Index

Evaluates the Search Value Against EachKey Value andRepeats This Evaluation Until One of Following Occurs:

The search value is not greater than or equal to thekey valueThe search value is greater than or equal to the last keyvalue on the index page

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Maintaining Heaps

SQL Server:

Uses Index Allocation Map (IAM) Pages

Contain information on where the extents of a heapare stored

Navigate through the heap and find available space for new rows being inserted

Connect data pages

Reclaims Space for New Rows in the Heap When a RowIs Deleted

Heaps are a collection of data pages for a table


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B - Tree


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B - Tree

Balanced Tree or B Tree attempts to provide a consistent andrelatively low cost method of finding way to a particular piece of information.

A Tree starts at the Root node. ThisRoot node can point

directly to the actual location of the data (if there is smallamount of data).

Non- Leaf nodes are in somewhere between the root and thenode that tells, where data is physically stored.

Leaf Level nodes are the nodes where obtain the leaf reference to the actual physical data.


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Clustered Index

A Clustered index is Unique for any table i.e. can have onlyone per table.

The Characteristic of Clustered Index is thatLeaf Levelcontains the actual Data Pages.

Any new record is inserted according to its actual physicalorder in the clustered index.

In case of a new record that needs to be inserted into themiddle of the index structure, a normalPage Splitoccurs.


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Clustered Indexes

Clustered indexes are useful for columns that are searchedfrequently for ranges of key values, or are accessed in sorted order.When you create a clustered index, consider the following facts andguidelines:

Each table can have only one clustered index.

The physical row order of the table and the order of rows inthe index are the same. You should create clusteredindexes before you create any nonclustered indexesbecause a clustered index changes the physical row order of the table. Rows are sorted into a sequenced order andmaintained in that order.

Key value uniqueness is maintained explicitly, with theUNIQUE keyword, or implicitly, with an internal uniqueidentifier. These unique identifiers are internal to SQLServer and are not accessible to the user.


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Clustered Indexes

The average size of a clustered index is about five percentof the table size.

However, clustered index size varies depending on the sizeof the indexed column.When a row is deleted, space is reclaimed and is availablefor a new row.

During index creation, SQL Server temporarily uses disk

space from the current database. A clustered index requiresabout 1.2 times the table size for working space when theindex is created. The disk space that is used during indexcreation is reclaimed automatically after the index is created.


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Using Nonclustered Indexes

The Leaf Level is not the Data Page instead it is the level toobtain a pointer to the data.

Nonclustered Indexes Are theSQL Server Default

Existing Nonclustered Indexes Are Automatically Rebuilt

When:An existing clustered index is dropped

A clustered index is created

You can have up to 249 nonclustered indexes per table.


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USE NorthwindCREATE CLUSTERED INDEX CL_lastnameON employees(lastname)

USE NorthwindCREATE CLUSTERED INDEX CL_lastnameON employees(lastname)

Creating and Dropping Indexes

Using the CREATE INDEX Statement

Indexes are created automatically on tables withPRIMARY KEY or UNIQUE constraints

Using the DROP INDEX Statement

USE NorthwindDROP INDEX employees.CL_lastname

USE NorthwindDROP INDEX employees.CL_lastname


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Create Index Statement

SQL Server automatically creates indexes when a PRIMARY KEY or UNIQUE constraint is created on a table. Defining a PRIMARY KEYor UNIQUE constraint is preferred over creating standard indexes.

Indexes can be created on views.

SQL Server stores index information in the sysindexes systemtable.

Before you create an index on a column, determine whether indexes already exist on that column.

Keep your indexes small by defining them on columns that aresmall in size. Typically, smaller indexes are more efficient thanindexes with larger key values.

Select columns on the basis of uniqueness so that each key value

identifies a small number of rows.When you create a clustered index, all existing nonclusteredindexes are rebuilt.


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Drop Index Statement

Use the DROP INDEX statement to remove an index on a table. When youdrop an index, consider the following facts:

SQL Server reclaims disk space that is occupied by the index when you

execute the DROP INDEX statement.

You cannot use the DROP INDEX statement on indexes that are created by

PRIMARY KEY or UNIQUE constraints. You must drop the constraint inorder to drop these indexes.

When you drop a table, all indexes for that table are also dropped.

When you drop a clustered index, all nonclustered indexes on the tableare

rebuilt automatically.

You must be in the database in which an index resides in order to dropthat

index.

The DROP INDEX statement cannot be used on system tables .


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Creating Unique Indexes

USE NorthwindCREATE UNIQUE NONCLUSTERED INDEX U_CustID

ON customers(CustomerID)

USE NorthwindCREATE UNIQUE NONCLUSTERED INDEX U_CustIDON customers(CustomerID)

RANCHRANCH Sant GourmetSant Gourmet Jonas BergulfsenJonas Bergulfsen

Duplicate key values are not allowedwhen a new row is added to the table

Duplicate key values are not allowedwhen a new row is added to the table

Customers Customers Customers

CustomerID CustomerID CompanyName CompanyName ContactName ContactName

QUICKBONAP12

QUICKBONAP12

QUICK-StopBon app'Walking

QUICK-StopBon app'Walking

Horst KlossLaurence LebihanHenry David Thoreau

Horst KlossLaurence LebihanHenry David ThoreauRANCH Rancho grande Sergio Gutirrez


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Creating Composite Indexes

USE NorthwindCREATE UNIQUE NONCLUSTERED INDEX U_OrdID_ProdIDON [Order Details] (OrderID, ProductID)

USE NorthwindCREATE UNIQUE NONCLUSTERED INDEX U_OrdID_ProdIDON [Order Details] (OrderID, ProductID)

Composite KeyComposite Key

Column 1 Column 2

Order Details Order Details Order Details

OrderID OrderID

ProductID ProductID

UnitPrice UnitPrice

Quantity Quantity

102481024810248

102481024810248

114272

114272

14.0009.80034.800

14.0009.80034.800

12105

12105

Discount Discount

0.00.00.0

0.00.00.0


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Obtaining Information on Existing Indexes

Using the sp_helpindex System Stored Procedure

Using the sp_help tablename System Stored Procedure

USE NorthwindEXEC sp_helpindex Customers

USE NorthwindEXEC sp_helpindex Customers


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Creating Index Options

Using the FILLFACTOR Option

Using the PAD_INDEX Option


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Using the FILLFACTOR Option

Specifies How Much to Fill the PageImpacts Leaf-Level Pages

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Fillfactor 50 = Leaf Pages 50% Full

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Using the PAD_INDEX Option

The PAD_INDEX OptionApplies to Non-Leaf-LevelIndexPages

If PAD_INDEX Is Not Specified, the Default Leaves Spacefor One Row Entry in Non-Leaf-Level Pages

Number of Rows on Non-Leaf-Level Pages Is Never LessThan Two

PAD_INDEX Uses the Fillfactor Value

USE NorthwindCREATE INDEX OrderID_ind

ON Orders(OrderID)WITH PAD_INDEX, FILLFACTOR=70

USE NorthwindCREATE INDEX OrderID_ind

ON Orders(OrderID)WITH PAD_INDEX, FILLFACTOR=70


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Maintaining Indexes

Data Fragmentation

DBCC SHOWCONTIG Statement

DROP_EXISTING Option


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Data Fragmentation

How Fragmentation Occurs

SQL Server reorganizes index pages when data is modified

Reorganization causes index pages to split

Methods of Managing FragmentationDrop and recreate an index and specify a fillfactor value

Rebuild an index and specify a fillfactor value

Business Environment

Data fragmentation can be good for OLTP environmentData fragmentation can be bad for Analysis Servicesenvironment


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DBCC SHOWCONTIG Statement

What DBCC SHOWCONTIG Determines

Whether a table or index is heavily fragmented

Whether data and index pages are fullWhen to Execute

If tables have been heavily modified

If tables contain imported dataIf tables seem to cause poor query performance


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CREATE UNIQUE NONCLUSTERED INDEX U_OrdID_ProdIDON [Order Details] (OrderID, ProductID)WITH DROP_EXISTING, FILLFACTOR=65

CREATE UNIQUE NONCLUSTERED INDEX U_OrdID_ProdIDON [Order Details] (OrderID, ProductID)WITH DROP_EXISTING, FILLFACTOR=65

DROP_EXISTING Option

Rebuilding an IndexReorganizes leaf pagesRemoves fragmentationRecalculates index statistics

Use the DROP_EXISTING option to change thecharacteristics of an index or to rebuild indexes withouthaving to drop the index and recreate it.


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Performance Considerations

Create Indexes on Foreign Keys

Create the Clustered Index Before Nonclustered Indexes

Consider Creating Composite IndexesCreate Multiple Indexes for a Table That Is ReadFrequently

Use the Index Tuning Wizard


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Check Your Understanding.


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Q.1. What is an Index.? What are the data access

methods.?


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Q.2. What are the Advantages of Creating an Index.?


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Q.3. What are the Disadvantages of creating an Index.?


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Q.4. What are the Columns to Index.?


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Q.5. What are the Columns Not to Index.?


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Q.6. How SQL Server uses Index.?


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Q.7. What is Heap .?


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Q.8. What is IAM .?


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Q.9. What is B Tree.?


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Q.10. What are the types of Indexes.?


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Q.11. What is Clustered Index.?


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Q.14. How many Columns a Unique and Composite Indexcan have.?


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Q.15. What is the Mistake done by a databaseProgrammer in the given SQL Statement .?

CREATE UNIQUE NONCLUSTEREDINDEX U_OrdID_ProdIDON Order Details (OrderID, ProductID)


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Q.16. What is FillFactor.?


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Q.17.What is Pad_index.?


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Q.19. What is the feature of DROP_EXISTING.?


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Want to Be an MCP


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Q.2. Which SQL Server service handles alerts?

a) MSSQL Server b) Server Agent

c) Microsoft Distributed Transaction Coordinator

d)Microsoft Search


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Q.3. You have recently created several jobs in order toautomate basic tasks. You discover, however, that the jobs are not always running properly. In which system

database should you look for job history?a) Master

b) Msdb

c) Model

d) Distribution


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Q.4. You are designing a database. Data integrity is aprime concern. Which type of database object will youmost likely implement?

a) User-defined data type

b) View

c) Constraintd) Non-clustered index


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Q.5. Where is the system catalog found?

a) In the master databaseb) In every database

c) In the database catalog

d)In system tables


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Q.6. You receive a phone call from a junior databaseadministrator. She is having trouble writing a query toupdate values in an identity field. What should you tell

her to do?a) Use the IDENTITYCOL keyword.

b) Identity fields cannot be updated.

c) Change the values to null and then repopulate thecolumn.

d) Make a second identity field and populate thecolumn.


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Review

Introduction to IndexesB Tree

Clustered Index

NonClustered Index

Creating and Dropping Index

Creating Unique Index

Creating Composite Index

Creating Index Options FillFactor & Pad_Index

Maintaining Indexes


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NO pain, No Gain

Thank You.

Documents

Module 06_ Planning Indexes