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Understanding SQL Server
Execution Plans
Dr Greg Low
Managing Director and Mentor – SolidQ Australia
Host of SQL Down Under Podcast
Microsoft Regional Director
Microsoft MVP for SQL Server
Organizer of SQL Down Under CodeCamp
Author with MS Press, SQL Server Magazine and Rational Press
Who am I?
Session Prerequisites
Basic understanding of:
T-SQL
Table Design
Indexing
Desire to understand reading execution plans
Module Overview
Core Execution Plan Concepts
Common Execution Plan Elements
Query, Table and Join Hints
Why Query Plans Matter
Common Questions:
Why does my query take so long?
Why does this query take so much longer than a very
similar query?
Why is SQL Server ignoring this perfectly fine index I’ve
built?
Execution Plans can help:
Best use is in verifying expected plans
Query Execution
SQL Server parses T-SQL
Resolves object references
Query Optimizer finds a sufficiently good plan
Plan calculation not required for DDL
Storage Engine executes the plan
Instance details stored in an execution context
Plan is stored in a Plan Cache
Execution Plans
Optimizer is cost-based
Biggest impact CPU and I/O
Finding the best plan might take too long
Might decide on a trivial plan
SELECT * FROM SomeTable;
Index usage is a key decision
Statistics based decisions
Actual vs Estimated Plans
Query Optimizer creates an estimated plan
Actual plan used may differ
Out of date statistics
Parallelism
Incorrect estimated plan
Re-executing Queries
Plan re-use is generally desirable
Parameter sniffing an exception
Plans can become unusable
Correctness (SET options, schema changes, etc.)
Optimality (data modifications)
Options available to force compilation behavior
Plans are evicted based upon a cost algorithm
Execution Plan Formats
Text-based plans
Now deprecated
Graphical Plans
XML-based plans
Capturing Execution Plans
SQL Server Profiler
Can capture query execution details
Options for all query plan types
Can include XML column with query plan
Performance events> Showplan XML event
SQL Server Management Studio
Data Collector
SQL Server 2008 Management Data Warehouse
DMVs
Viewing Execution Plans – SSMS
SQL Server Management Studio
Show estimated execution plan
Include actual execution plan in results
Compare plans between queries
Save/view plans in .sqlplan format
Specifically request plans in code
SET SHOWPLAN_XML ON;
Viewing Execution Plans - DMVs
Dynamic Management Views (and Dynamic Management Functions) Show current server state
Not persisted internally
Many useful views/functions related to execution:sys.dm_exec_connections
sys.dm_exec_sessions
sys.dm_exec_requests
sys.dm_exec_sql_text()
sys.dm_exec_query_plan()
sys.dm_exec_cached_plans
sys.dm_exec_cached_plan_dependent_objects()
View Cached Plans
SELECT cp.objtype AS PlanType,
OBJECT_NAME(st.objectid,st.dbid) AS ObjectName,
cp.refcounts AS ReferenceCounts,
cp.usecounts AS UseCounts,
st.text AS SQLBatch,
qp.query_plan AS QueryPlan
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle) AS qp
CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle) AS st;
Core Execution Plan Concepts
demonstration
Module Overview
Core Execution Plan Concepts
Common Execution Plan Elements
Query, Table and Join Hints
SELECT Queries – Basic Operations
Basic Operations
demonstration
SELECT Queries - Lookups
Lookups
demonstration
Join Operations
Join Operations
demonstration
Aggregation
Aggregation
demonstration
Data Modification
Data Modification
demonstration
Views
Views
demonstration
Module Overview
Core Execution Plan Concepts
Common Execution Plan Elements
Query, Table and Join Hints
Query Hints – Common Operations
HASH | ORDER GROUP
MERGE | HASH | CONCAT UNION
FAST n
FORCE ORDER
EXPAND VIEWS | NOEXPAND
MAXRECURSION
Query Hints - Compilation
MAXDOP
OPTIMIZE FOR
RECOMPILE
PARAMETERIZATION SIMPLE | FORCED
ROBUST PLAN
KEEP PLAN
KEEPFIXED PLAN
USE PLAN
JOIN Hints
LOOP JOIN
MERGE JOIN
HASH JOIN
Table Hints
NOEXPAND
INDEX(n)
INDEX(name)
FASTFIRSTROW
Join Hints
demonstration
Session Summary
Excellent tooling available with SQL Server
Clear visibility into query execution
Hints to use (sparingly) to control behavior
Upcoming Training
Melbourne
Advanced T-SQL – 6th July (5 days)
SQL Server Bootcamp for SharePoint Admins – 1st July (3 days)
Solving Business Problems with MDX – 23rd September (3 days)
Designing and Developing Solutions with Analysis Services –
14th July (4 days)
Working with SQL Server 2008 Integration Services – 3rd August
(2 days)
Working with SQL Server 2008 Reporting Services – 21st August
(2 days)
Rushabh Mehta coming to visit
End to End Business Intelligence
5th October (5 days)
PASS Executive Vice President
Also heading to the SQL Code Camp
Places to be!
PASS Community Connection
www.sqldownunder.com (SDU Code Camp tab)
October 10th and 11th Wagga Wagga
PASS U.S. Summit
www.sqlpass.org
November 2th to 5th Seattle
