Module 7

Reading 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.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• An actual plan may differ from an estimated plan Out of date statistics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 SSMS

In 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-SQL

User name AdventureWorks\Administrator

Password Pa$$w0rd

Lab Scenario

You 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