Discuss SQL Server index.

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
http://netdbsolutions.com
https://twitter.com/HarryZheng
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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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