uncovering duplicate, redundant, & missing indexes · leaf level...

Uncovering Duplicate, Redundant, & Missing Indexes

Andy YunSenior Solutions Engineer

Andy YunSenior Solutions Engineer

• SQL Server DBA & DB Developer• Working with SQL Server since 2001• Chicago Suburban User Group Chapter Leader• Speaking since Early 2014

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@SQLBek

[email protected] | [email protected]

https://blogs.sentryone.com/andyyun/

Have You Ever Asked?

1. Are there any indexes that I do not need anymore?

2. Is SQL Server even using of my indexes?

3. What additional indexes could help my current workload?

Why Reassess My Indexes?

1. Optimize & aid as many of my queries as possible

2. Reduce DML query overhead

3. Reduce data storage footprint

Presentation Agenda

1. Index Internals

2. T-SQL Tools

3. Index Analysis Demo

Index Internals:B-Trees & Index Operations

Customer TableCREATE TABLE dbo.Customer (

CustomerID INT IDENTITY(1, 1) PRIMARY KEY CLUSTERED,FirstName VARCHAR(50),LastName VARCHAR(50),Address VARCHAR(50),City VARCHAR(50),State CHAR(2),ZipCode CHAR(5),Email VARCHAR(50),PhoneNumber CHAR(10),FirstVisit DATE,RepeatCustomer BIT

);CREATE NONCLUSTERED INDEX IX_Customer_LastName

ON dbo.Customer (LastName, FirstName) INCLUDE (FirstVisit, Email);

Index B-TreeStructure

Adams,James,29285Martinez,Luis,29214Simon,Paul,25946

Adams,James,29285Chavez,Esme,23555Howard,Kate,29334

Martinez,Luis,29214Raji,Bo,26523Samant,Mike,12873

Simon,Paul,25946Truempy,Nick,14558

Adams,James,2002-02-10,29285Agbonile,Lisa,2004-01-01,18083Alan,Pete,2003-10-06,15384

Chavez,Esme,2003-09-26,23555Chen,Li,2004-05-13,29031Chow,Matt,2001-11-12,28644

Howard,Kate,2002-02-14,29334Hu,Li,2002-01-28,29203Huang,Ilo,2002-07-30,26276

Martinez,Luis,2003-08-19,29214McDon,Jynell,2004-03-09,28084Mehta,Rit,2004-05-19,27436

Raji,Bo,2004-04-20,26523Raman,Arthur,2001-12-20,28990Ramirez,Lori,2002-01-17,29198

Samant,Mike,2003-05-10,12873Sanchez,Ray,2002-01-17,29254Sandberg,Ryne,2003-11-06,23075

Simon,Paul,2002-06-28,25946Simpson,2003-01-08,26717Sims,2003-12-28,26869

Truempy,Nick,2001-07-28,4558Turner,Tina,2002-02-17,29279Umeda,Umesh,2003-10-13,16717

CREATE NONCLUSTERED INDEX IX_Customer_LastName ON

dbo.Customer (LastName, FirstName) INCLUDE (FirstVisit, Email);

Root Level

➢ Adams,James,29285

➢ Martinez,Luis,29214

➢ Simon,Paul,25946

CREATE NONCLUSTERED INDEX IX_Customer_LastName

ON dbo.Customer(LastName, FirstName)

INCLUDE (FirstVisit, Email);

Intermediate Level


➢ Chavez,Esme,23555

➢ Howard,Kate,29334


➢ Raji,Bo,26523

➢ Samant,Mike,12873



➢ Simon,Paul,25946




Leaf Level

➢ Adams,James,2002-02-10,[email protected],29285

➢ Agbonile,Lisa,2004-01-01,[email protected],18083

➢ Alan,Pete,2003-10-06,[email protected]




➢ Chavez,Esme,2003-09-26,[email protected],23555

➢ Chen,Li,2004-05-13,[email protected]

➢ Chow,Matt,2001-11-12,[email protected]


➢ Chavez,Esme,23555

➢ Howard,Kate,29334

Index Scan Adams,James,29285Martinez,Luis,29214Simon,Paul,25946












SELECT FirstName, LastName, FirstVisit

FROM Customer

Logical Reads?10 Data Pages

Index Seek Adams,James,29285Martinez,Luis,29214Simon,Paul,25946













FROM CustomerWHERE LastName = ‘Dean’


Index Scan…or Seek?














FROM CustomerWHERE LastName > ‘Inez’

AND LastName < ‘Sark’Logical Reads?6 Data Pages














SELECT FirstName, LastName, FirstVisitFROM CustomerWHERE LastName > ‘Inez’

AND LastName < ‘Sark’ORDER BY LastName DESC















FROM CustomerWHERE FirstName = ‘Arthur’


Is SeekingAlways Better?













SELECT FirstName, LastName, FirstVisitFROM CustomerWHERE LastName IN (‘Alan’, ‘Chen’, ‘Mehta’, ‘Sandberg’, ‘Simon’, ‘Turner’)


Full Scan?10 Data Pages

Key Lookup Adams,James,29285Martinez,Luis,29214Simon,Paul,25946












SELECT FirstName, LastName, FirstVisit, City, State

FROM CustomerWHERE LastName = ‘Huang’

Index Operation Recap

• Index Key Order Matters. Can only seek on leading key column

• Add Appropriate Columns as Included Columns to “Cover a Query”

• Seeks are not necessarily better than Scans –Check # of Executions in Properties

How Many Indexes?HEAP

OR

CLUSTERED INDEX

Update OperationHEAP

OR

CLUSTERED INDEX

Insert or Delete OperationHEAP

OR

CLUSTERED INDEX

Too Many Indexes?

• OLTP Database or OLAP Database?

• How much DML vs SELECT in your workload?

• Periodic ETLs? Disable, batch load, then rebuild

• Index Survey Results by Paul Randalhttp://www.sqlskills.com/blogs/paul/over-and-under-indexing-how-bad-is-it-out-there/

http://www.sqlskills.com/blogs/paul/over-and-under-indexing-how-bad-is-it-out-there/

T-SQL Tools&Index Analysis Demo

Parting Thoughts

1. Duplicate & Redundant Indexes are worth cleaning up

2. Missing Index Recommendations can be useful

3. Know your workload to make smart decisions

Thank Youhttps://github.com/SQLBek

Andy Yun@SQLBek

[email protected] / [email protected]://sqlbek.wordpress.com

uncovering duplicate, redundant, & missing indexes · leaf level...

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