Understanding query execution plans is something many IT professionals either don’t know about or don’t fully understand. SQL Server’s programming language, T-SQL, is different from most other programming languages. In most languages, you tell the language what you want it to do, but in T-SQL, you tell it what you want. This means you are at the mercy of the query optimizer, which will take the logical instruction and convert it into a physical plan. The query execution plan is a map of what the optimiser thinks will be a good way of returning the data you have asked for. Being able to understand query plans and what they show is critical for both DBAs and developers in uncovering query performance problems. To view the webcast go to: http://software-dell-com/webcast-ondemand/understanding-sql-server-query-execution-plans-826629

1. Understanding Query
Execution Plans
Richard Douglas

2. Agenda
• Introductions
• The Query Optimizer
• Commonly used operators
• Blocking and non-blocking operators
• Reading Query Execution Plans
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3. Your host
• Richard Douglas
• Systems Consultant
• SQL Server MCITPro
• Maidenhead SQL User Group Leader
• Blog: http://SQL.RichardDouglas.co.uk
• Twitter: @SQLRich
• Email:
Richard.Douglas@Software.Dell.com
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4. The Query Optimizer
Why do we need an optimizer?
• T-SQL is a “What” not “how” language.
• We write “logical” requests.
• SQL Optimizer Engine converts
logical requests into physical
plans.
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5. The Query Optimizer
What is the goal of the Optimizer?
The job of the
SQL Optimizer is
to find “the best
plan possible”.
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6. Query optimization explained simply
1.
Query submitted
2. Magic happens
3. Shedload of data returned
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7. Query Optimization (in a bit more detail)
Optimizer steps
Parse
Bind
Optimize
Execute
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8. Parse
Builds a tree structure based upon the logical operators in the query.
For example:
Logical Operations
SELECT
SSOD.[SalesOrderID],
PP.[Name],
PP.[Weight],
SSOD.[UnitPrice]
FROM [Sales].[SalesOrderDetail] SSOD
INNER JOIN [Production].[Product] PP
ON SSOD.ProductID = PP.ProductID
WHERE PP.Weight > 100
Nodes
Project
Filter
Join
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Sales
Order
Detail
Product
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9. Bind
• Series of validation steps
• Schema validation
• Table validation
• Attribute validation
• Permission validation
SELECT
SSOD.[SalesOrderID],
PP.[Name],
PP.[Weight],
SSOD.[UnitPrice]
FROM [Sales].[SalesOrderDetail] SSOD
INNER JOIN [Production].[Product] PP
ON SSOD.ProductID = PP.ProductID
WHERE PP.Weight > 100
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10. Optimize
Works though many rules and heuristics.
These Include:
• Commutativity
• Substitution rules
• Exploration rules
• Implementation rules
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11. Execute
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12. Statistics
• SQL uses a cost based optimizer
• Costs influenced by statistics
*Trace flag 2371 can change these thresholds.
It is undocumented and was included in 2008 R2 SP1.
http://blogs.msdn.com/b/saponsqlserver/archive/2011/09/07/c
hanges-to-automatic-update-statistics-in-sql-server-traceflag2371.aspx
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13. Commonly used operators
SELECT
UPDATE
INSERT
DELETE
Table scan
Clustered
Index Scan
NonClustered
Index Scan
Clustered
Index Seek
NonClustered
Index Seek
Key Lookup
Nested Loop
Join
Merge Join
Hash Join
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14. Blocking and Non-blocking Operators
• Operators / Iterators can be put in two categories:
1. Blocking
2. Non-blocking
• Having a blocking operator in your plan means other operators
further down the line are sitting idle.
This will reduce the overall performance of your query
• Some examples…
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15. Blocking and Non-blocking operators
Non-blocking example
• An example using a Compute Scalar function
?
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Compute
Scalar
Function
Row 1
Row 2
Row 3
Row 4
Row 5
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16. Blocking and Non-blocking operators
Blocking example
• An example using the sort operator:
?
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Sort
Desc
Row 1
Row 2
Row 3
Row 4
Row 5
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17. Demo’s

18. SQL Server Tools
SQL Server
Experience
Monitoring and
Diagnostics
Spotlight on SQL server
Enterprise
FREE TOOLS
Healthcheck,
Performance
Analytics,
Freemium
Monitoring
Free Tools at
www.SpotlightEssentials.com
NetVault LiteSpeed for SQL
Server
Innovation
Backup
Leadership
Administration
Toad for SQL Server
and Development
Business
Intelligence
Toad BI Suite
Application
Performance
Monitoring
Foglight for SQL Server

19. SpotlightEssentials.com
Free SQL Server Tools
Download at SpotlightEssentials.com
You can now monitor all your SQL Server Connections for FREE within SQL Server Management
Studio.
Monitoring features include a Heatmap, Alarms List and detailed analytics for each connection
Alarms
Real Time
Notifications

20. Any questions?
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21. Thank you for attending
Richard Douglas
Richard.Douglas@Software.Dell.com
@SQLRich