Understanding query execution plans
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Understanding Query Execution Plans Richard Douglas
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Understanding query execution plans is something that a lot of IT professionals either don't know about or don't 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, in T-SQL you tell it what you want. This means you are at the mercy of the query optimiser 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 DBA's and developers in uncovering query performance problems. To view the webcast:http://dell.to/17k8zWK
Transcript of Understanding query execution plans
Understanding Query Execution Plans
Richard Douglas
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Agenda
• Introductions
• The Query Optimizer
• Commonly used operators
• Blocking and non-blocking operators
• Reading Query Execution Plans
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Your host
• Richard Douglas
• Systems Consultant
• SQL Server MCITPro
• Maidenhead SQL User Group Leader
• Blog: http://SQL.RichardDouglas.co.uk
• Twitter: @SQLRich
• Email: [email protected]
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Why do we need an optimizer?
The Query 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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The job of the SQL Optimizer is to find “the best plan possible”.
The Query Optimizer
XWhat is the goal of the Optimizer?
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Query optimization explained simply
1. Query submitted
2. Magic happens
3. Shedload of data returned
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Optimizer steps
Query Optimization (in a bit more detail)
Bind
Execute
Optimize
Parse
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Parse
Builds a tree structure based upon the logical operators in the query.For example:
SELECT SSOD.[SalesOrderID], PP.[Name], PP.[Weight], SSOD.[UnitPrice]
FROM [Sales].[SalesOrderDetail] SSODINNER JOIN [Production].[Product] PP
ON SSOD.ProductID = PP.ProductIDWHERE PP.Weight > 100
Project
Filter
Join
ProductSalesOrderDetail
Log
ical O
pera
tion
s
Nod
es
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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] SSODINNER JOIN [Production].[Product] PP
ON SSOD.ProductID = PP.ProductIDWHERE PP.Weight > 100
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Optimize
Works though many rules and heuristics.
These Include:
• Commutativity
• Substitution rules
• Exploration rules
• Implementation rules
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Execute
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Statistics
• SQL uses a cost based optimizer
• Costs influenced by statistics
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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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Blocking and Non-blocking Operators
• Operators / Iterators can be put in two categories:1. Blocking2. 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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Non-blocking example
Blocking and Non-blocking operators
• An example using a Compute Scalar function
Compute Scalar Function
Row 1Row 2Row 3Row 4Row 5
?
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Blocking example
Blocking and Non-blocking operators
• An example using the sort operator:
Row 1Row 2Row 3Row 4Row 5
? Sort Desc
Demo’s
Pg. 22© 2012 Quest Software Inc. All rights reserved.
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Any questions?
