Module 7Reading SQL Server®

2008 R2 Execution Plans

Module Overview• Execution Plan Core Concepts• Common Execution Plan Elements• Working with Execution Plans

Lesson 1: Execution Plan Core Concepts• Why Execution Plans Matter• Query Execution Phases• What is an Execution Plan?• Actual vs. Estimated Execution Plans• What is an Execution Context?• Execution Plan Formats• Demonstration 1A: Viewing Execution Plans in SSMS

Why Execution Plans Matter• Common questions

Why does my query take so long to execute? Why does this query take so much longer than a very similar

query? Why is SQL Server ignoring this perfectly fine index that I

created?• Execution plans can help

Best use is in verifying expected plans

Query Execution Phases

• SQL Server parses the T-SQL• Object references are resolved• Query optimizer finds a sufficiently good plan

Not necessary for DDL statements Some statements lead to trivial plans Aim is to quickly find a plan that is good enough

• Storage and Execution engines execute the plan• Plan may be stored in the Plan Cache

What is an Execution Plan?

• Execution plans detail choices that SQL Server makes on: Types of operations Order of operations Choice of indexes Rowcount estimates based on available statistics

• SQL Server performs cost-based optimization A cost is assigned to each element

An execution plan shows how a query was executed or how it would be executed.

Actual vs. Estimated Execution Plans• An option exists to ask SQL Server how it would execute a

query Known as an estimated execution plan SQL Server does not execute the query, it just works out how it

would execute it Not always possible to get an estimated plan

• An actual plan may differ from an estimated plan Out of date statistics Missing statistics Actual degree of parallelism

• Estimated plan returns estimated counts based on statistics Actual plan returns actual counts returned

What is an Execution Context?• Multiple users may execute a plan concurrently• Execution plan details how the query would be executed

Does not hold data for a specific execution• Execution context holds the data associated with a specific

execution of the plan One execution context is needed for each concurrent execution Execution contexts are also cached Not all execution contexts are identical

Execution Plan Formats• Text-based plans

SET SHOWPLAN_TEXT ON SET SHOWPLAN_ALL ON Now deprecated

• XML-based plans SSMS saves these as .sqlplan files Portable format for execution plans SSMS is associated with this filetype

• Graphical plans Renders XML plans in an easier to read format Does not contain all information from the XML plan

Demonstration 1A: Viewing Execution Plans in SSMSIn this demonstration, you will see how to:• Show an estimated execution plan• Compare execution costs between two queries in a batch• Show an actual execution plan• Save an execution plan

Lesson 2: Common Execution Plan Elements• Table and Clustered Index Scans and Seeks• Nested Loops and Lookups• Merge and Hash Joins• Aggregations• Filter and Sort• Data Modification• Demonstration 2A: Common Execution Plan Elements

Table and Clustered Index Scans and Seeks• Table scan

Reading from a heap• Clustered index scan

Reading from a table with a clustered index• Clustered index seek

Following a clustered index to a specific location within the table

Nested Loops and Lookups• Nested Loops

Commonly used for inner join operations Can be used for left outer join, left semi join and

left anti semi join operations For each row of top data path, perform a lookup

to the bottom data path• RID Lookup

Lookup on a heap using a row ID• Key Lookup

Lookup to a clustered index

Merge and Hash Joins• Merge Join

Commonly used for inner joins Can be used for left outer join, left semi join, left anti semi join,

right outer join, right semi join, right anti semi join and union operations

Requires two inputs to be in the same sorted order• Hash Match

More difficult joins where a hash table is built by computing a hash value for each row from one input

Other input is used to lookup into the hash table

Aggregations• Stream Aggregate

Highly efficient Data already in correct

order for processing the aggregate

• Hash Match Aggregate Hash table used to form

aggregate as data not in the necessary order

Filter and Sort• Filter

Low cost operation Typically used for WHERE clause predicates or

HAVING clause predicates Only pass through rows that match the

required filter criteria• Sort

Used whenever a sort operation is necessary Often used for ORDER BY clauses Can be used for other operations such as

sorting inputs for merge join operations or performing DISTINCT operations

Can be very expensive

Data Modification• INSERT

Used in INSERT operations• UPDATE

Used in UPDATE operations• DELETE

Used in DELETE operations• T-SQL MERGE statement can use

combinations of inserts, updates and deletes

Demonstration 2A: Common Execution Plan Elements• In this demonstration, you will see queries that

demonstrate the most common execution plan elements

Lesson 3: Working with Execution Plans• Methods for Capturing Plans• Demonstration 3A: Capturing Plans in Activity Monitor• Re-executing Queries• Execution Plan Related DMVs• Demonstration 3B: Viewing Cached Plans

Methods for Capturing Plans• SQL Server Management Studio

Estimated and actual Also available in Visual Studio 2010

• SQL Server Profiler Can capture query execution details Options for all query plan types Can include an XML column with query plan

• Dynamic Management Views (DMVs)• Activity Monitor• SQL Server Data Collection

Demonstration 3A: Capturing Plans in Activity Monitor• In this demonstration, you will see how to use Activity

Monitor to view recent expensive queries

Re-Executing Queries• Plan re-use is generally desirable

Parameter sniffing issues are an exception to this• Plans can become unusable or suboptimal

Correctness Optimality

• Plans evicted on a cost algorithm basis Cost reduced over time Reuse re-establishes cost Cost of zero indicates a candidate for eviction

• Options are available to force compilation behavior

Execution Plan Related 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_query_stats 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()

Demonstration 3B: Viewing Cached Plans• In this demonstration you will see how to view cached

execution plans

Lab 7: Reading SQL Server Execution Plans• Exercise 1: Actual vs. Estimated Plans • Exercise 2: Identify Common Plan Elements• Challenge Exercise 3: Query Cost Comparison (Only if time

permits)

Logon information

Estimated time: 45 minutes

Virtual machine 623XB-MIA-SQLUser name AdventureWorks\AdministratorPassword Pa$$w0rd

Lab ScenarioYou have been learning about the design of indexes. To take this learning further, you need to have a way to view how these indexes are used.In the first exercise, you will learn to view both estimated and actual execution plans.Execution plans can contain many types of elements. In the second exercise, you will learn to identify the most common plan elements and see how statements lead to these elements being used.You regularly find yourself trying to decide between different ways of structuring SQL queries. You are concerned that you aren’t always choosing the highest-performing options. If time permits, you will learn to use execution plans to compare the cost of statements in multi-statement batches.

Lab Review• Can two different queries end up with the same execution

plan?• Question: If so, how can that occur? If not, why not?

Module Review and Takeaways• Review Questions• Best Practices