This document provides an overview of indexing in SQL Server. It begins with the basics of index types including clustered, non-clustered, unique, included column and filtered indexes. It then discusses index design best practices for helping queries, joins and sort operations. Common index issues are also covered such as fragmentation, duplicate indexes, and having too many indexes. The document demonstrates index recommendations through examples and provides diagnostic tools for analyzing indexes.

1. Index the obvious and not so obvious

2. About myself:
• Seasoned data architect/DBA with 15 years of SQL Server experience
• Independent consultant, currently Technical Delivery Manager at
Government of Alberta
• Microsoft Certified System Engineer: MCSE
• Oracle Certified Professional: Oracle DBA
• IBM Certified Solution Expert: DB2 UDB
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3. Session agenda
1. SQL Server index basics
2. Index Design
3. Common index issues
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4. SQL Server index basics
Why do we need index?
• Reduce rows selected in a query as early as possible
• Help join
• Support sort
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5. SQL Server index basics
Index Type:
• Clustered index
• Non-clustered index
• Unique
• Index with included columns
• Filtered index
Not covered in this session
• Columnstore
• Spatial index
• XML index
• Full-text index
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6. SQL Server index basics
B-Tree Index structure:
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7. SQL Server index basics
Get Index Information
• exec sp_helpindex
• select * from sys.indexes
• select * from sys.index_columns
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8. SQL Server index basics
Index Statistics Info
• DBCC SHOW_STATISTICS ('demo.person', 'IX_Person_BusinessIdentityID');
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9. SQL Server index basics
Index Statistics Info
• The statistics allow the optimizer to determine the selectivity of an
index
• A unique, single-column index always has a selectivity of 1
• One index entry points to exactly one row
• Density is the inverse of selectivity
• Density values range from 0 to 1
• A selective index has a density of 0.10 or less
• A unique, single-column index always has a density of 0.0
• When the index is composite
• SQL Server maintains detailed statistics only on the leftmost column
• It does compute density for each column
• Assume there is an index on (col1, col2, col3)
• Density is computed for
• Col1
• Col1 + Col2
• Col1 + Col2 + Col3
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10. SQL Server index basics
Clustered index:
• Leaf level is the actual data page
• Non-leaf levels contains the index key columns
• Clustered index scan = table scan on Heap
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11. SQL Server index basics
Clustered index - Primary key:
• Alter table Demo.Person add constraint PK_Person primary key
(BusinessEntityID);
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12. SQL Server index basics
Non-clustered index:
• Leaf level contains the key and include columns
• Non-leaf levels contains the index key columns
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13. SQL Server index basics
Unique index:
• Create unique index IX_Person_BusinessIdentityID on
demo.Person(BusinessEntityID);
• Is clustered index always unique?
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14. SQL Server index basics
Index with included columns - Covering index:
Create index IX_Person_LastName_FirstName
on demo.Person ( LastName,FirstName)
include (ModifiedDate);
Covered query:
select FirstName, LastName, ModifiedDate
from Demo.Person
where LastName = 'Miller'
and FirstName = 'Dylan'
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15. SQL Server index basics
Filtered index:
Create index IX_Person_persontype
on demo.Person ( persontype)
where persontype = 'EM';
Covered query:
select * from Demo.Person
where persontype = 'EM'
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16. Index Design
General design rule:
• Clustered index:
• Narrow
• Unique
• Static
• Ever-increasing
• Non-clustered index:
• Wider often more useful than narrow
• Decided by the queries that are run against the table
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17. Index Design
Help Where clause:
• Queries can seek on index Where the columns in the Where clause
are a left based subset of the index key
• Columns used in equality predicates before inequality
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18. Index Design
Help Where clause:
Case 1:
Select * from Demo.Person
Where FirstName = 'Dylan'
and LastName = 'Miller'
How to index?
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19. Index Design
Help Where clause:
Case 1:
Select * from Demo.Person
Where FirstName = 'Dylan'
and LastName = 'Miller'
How to index?
Create index IX_Person_FirstName_LastName
on Demo.Person
( FirstName, LastName);
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20. Index Design
Help Where clause:
Case 2 Session 1:
Select * from Demo.Person
Where FirstName = 'Dylan'
and LastName = 'Miller'
Case 2 Session 2:
Select * from Demo.Person
Where LastName = 'Miller'
How to index?
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21. Index Design
Help Where clause:
Case 2 Session 1:
Select * from Demo.Person
Where FirstName = 'Dylan'
and LastName = 'Miller'
Case 2 Session 2:
Select * from Demo.Person
Where LastName = 'Miller'
How to index?
Create index IX_Person_LastName_FirstName
on demo.Person ( LastName, FirstName);
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22. Index Design
Help Where clause:
Session 3:
Select * from Demo.Person
Where LastName = 'Miller'
and ModifiedDate > '2006-09-01‘
How to index?
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23. Index Design
Help Where clause:
Session 3:
Select * from Demo.Person
Where LastName = 'Miller'
and ModifiedDate > '2006-09-01‘
How to index?
Create index IX_Person_LastName_ModifiedDate
on demo.Person ( LastName, ModifiedDate);
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24. Index Design
Help Where clause:
Case 4:
Select * from Demo.Person
Where LastName = 'Miller'
and FirstName > 'Dylan'
and ModifiedDate > '2006-09-01‘
How to index?
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25. Index Design
Help Where clause:
Case 4:
Select * from Demo.Person
Where LastName = 'Miller'
and FirstName > 'Dylan'
and ModifiedDate > '2006-09-01‘
How to index?
Create index IX_Person_LastName_FirstName_ModifiedDate
on demo.Person ( LastName,FirstName, ModifiedDate);
Create index IX_Person_LastName_ModifiedDate_FirstName
on demo.Person ( LastName, ModifiedDate,FirstName);
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26. Index Design
Help Where clause:
Case 5:
Select * from Demo.Person
Where LastName = 'Miller'
Or FirstName = 'Dylan‘
How to index?
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27. Index Design
Help Where clause:
Case 5:
Select * from Demo.Person
Where LastName = 'Miller'
Or FirstName = 'Dylan‘
How to index?
Create index IX_Person_FirstName on demo.Person ( FirstName);
Create index IX_Person_LastName on demo.Person ( LastName);
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28. Index Design
Help join:
Case 6:
Select A.AddressLine1,A.City, SP.Name as StateProvince
From Demo.Address A
Inner Join Demo.StateProvince SP
on SP.StateProvinceID = A.StateProvinceID
How to index?
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29. Index Design
Help join:
Case 6:
Select A.AddressLine1,A.City, SP.Name as StateProvince
From Demo.Address A
Inner Join Demo.StateProvince SP
on SP.StateProvinceID = A.StateProvinceID
How to index?
Create index IX_Address_StateProvinceID
on demo.Address ( StateProvinceID);
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30. Index Design
Help Where clause:
Case 7:
Select A.AddressLine1,A.City, SP.Name as StateProvince
From Demo.Address A
Inner Join Demo.StateProvince SP
on SP.StateProvinceID = A.StateProvinceID
Where A.City = 'Bothell‘
How to index?
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31. Index Design
Help Where clause:
Case 7:
Select A.AddressLine1,A.City, SP.Name as StateProvince
From Demo.Address A
Inner Join Demo.StateProvince SP
on SP.StateProvinceID = A.StateProvinceID
Where A.City = 'Bothell‘
How to index?
Create index IX_Address_City_StateProvinceID
on demo.Address ( City, StateProvinceID);
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32. Index Design
Help order by clause:
Case 8:
Select * from Demo.Person
Where LastName = 'Miller'
Order by FirstName
How to index?
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33. Index Design
Help Order by clause:
Case 8:
Select * from Demo.Person
Where LastName = 'Miller'
Order by FirstName
How to index?
Create index IX_Person_LastName_FirstName
on Demo.Person ( LastName, FirstName);
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34. Index Design
Help Order by clause:
Case 9:
Select FirstName, LastName, ModifiedDate from Demo.Person
Where LastName = 'Miller'
and ModifiedDate > '2006-09-01'
Order by FirstName
How to index?
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35. Index Design
Help Order by clause:
Case 9:
Select FirstName, LastName, ModifiedDate from Demo.Person
Where LastName = 'Miller'
and ModifiedDate > '2006-09-01'
Order by FirstName
How to index?
1.Create index IX_Person_LastName_ModifiedDate on demo.Person (
LastName, ModifiedDate);
2.Create index IX_Person_LastName_ModifiedDate_FirstName on
demo.Person ( LastName, ModifiedDate,FirstName);
3.Create index IX_Person_LastName_ModifiedDate on demo.Person (
LastName, ModifiedDate) include (FirstName);
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36. Common index issues
Drawbacks of index:
• Slow down DML
• Disk Space
• Fragmentation – page split – low page density
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37. Common index issues
Index Fragmentation:
• DBCC SHOWCONTIG
DBCC SHOWCONTIG scanning 'Person' table...
Table: 'Person' (615673241); index ID: 2, database ID: 17
LEAF level scan performed.
- Pages Scanned................................: 25
- Extents Scanned..............................: 5
- Extent Switches..............................: 4
- Avg. Pages per Extent........................: 5.0
- Scan Density [Best Count:Actual Count].......: 80.00% [4:5]
- Logical Scan Fragmentation ..................: 12.00%
- Extent Scan Fragmentation ...................: 20.00%
- Avg. Bytes Free per Page.....................: 107.2
- Avg. Page Density (full).....................: 98.68%
DBCC execution completed. If DBCC printed error messages, contact your system administrator.
Defrag:
• DBCC DBREINDEX
• DBCC INDEXDEFRAG
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38. Common index issues
Misconception:
• Index is wonderful, create index to cover every query
• How about create a separate index for each commonly
used columns?
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39. Common index issues
Disorder:
1. Duplicate indexes
• Will SQL Server use both indexes?
2. Index Hoarder
• Lots of Non-Clustered indexes on a single table
• >5% of Non-Clustered indexes unused
• Non-Clustered indexes with 0 reads
• Indexes with 7 or more columns
• Clustered indexes with > 1 column
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40. Common index issues
Disorder:
3. Feature Phobia
• Included columns SQL 2005+
• Filtered indexes – SQL 2008+ Enterprise Edition
4. Self-Loathing indexes
• Low fill factor, disabled indexes
5. Index-A-Phobia
• Afraid to add index
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41. Common index issues
Diagnostics Tools:
• sp_BlitzIndex
• http://www.brentozar.com/blitzindex/
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42. Common index issues
Don’t:
• Don’t index for the sake of indexing
• Don’t create duplicated indexes
• Don’t create very wide indexes
• Don’t just trust SSMS suggestion
Discussion:
• Should we always create indexes for FK columns?
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43. Common index issues
DMVs for Index Tune
• sys.dm_db_index_usage_stats
• sys.dm_db_index_operational_stats
• sys.dm_db_index_physical_stats
• sys.dm_db_missing_index_groups
• sys.dm_db_missing_index_details
• sys.dm_db_missing_index_group_stats
• sys.dm_db_missing_index_columns
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44. Reference
PASS Summit 2012 Sessions:
• What, Where, Why and How of Indexes (DBA-210)
• Index Psychiatry Diagnose and Treat the Top 5 Disorders (AD-204)
• http://sqlinthewild.co.za/index.php/2010/09/14/one-wide-index-or-multiple-narrow-indexes/
• http://www.sqlservercentral.com/articles/Indexing/68439/
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