The document presents SQL Server query tuning strategies, emphasizing the importance of identifying and optimizing queries that contribute most to performance issues. It outlines various techniques including monitoring wait states, analyzing execution plans, and employing Response Time Analysis (RTA) to diagnose and prioritize optimization efforts. The document also advises a collaborative approach between developers and DBAs to ensure effective tuning and highlights the value of indexing and query rewriting in improving database performance.

1. SQL Server Query Tuning Tips
Get it Right the First Time
Dean Richards
Lead DBA, Confio Software
2/12/2013 1

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Who Am I?
• 20+ Years in Oracle & SQL Server
– DBA and Developer
• Lead DBA for Confio Software
– DeanRichards@confio.com
– Makers of Ignite8 Response Time Analysis Tools
– http://www.ignitefree.com – only free RTA Tool
• Specialize in Performance Tuning

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• Introduction
• Which Query Should I Tune?
• Query Plans
• SQL Diagramming
– Who registered yesterday for Tuning Class
– Check order status
Agenda

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• Most Applications
– Read and Write data to/from database
– Simple manipulation of data
– Deal with smaller amounts of data
• Most Databases
– Examine larger amounts of data, return a little
– Inefficiencies quickly become bottleneck
• Why do SQL tuning?
– Tuning SQL - “Gives the most bang for your buck”
– Changes to SQL are Safer
– ~85% of performance issues are SQL related
Why Query Focused

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Who Should Tune
• Developers?
– Developing applications is very difficult
– Typically focused on functionality
– Not much time left to tune SQL
– Do not get enough practice
– SQL runs differently in Production than Dev/Test
• DBA?
– Do not know the code like developers do
– Focus on “Keep the Lights On”
– Very complex environment
• Need a team approach - DevOps
– But also need someone to lead the effort

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Which SQL
• User / Batch Job Complaints
• Queries Performing Most I/O (LIO, PIO)
• Queries Consuming CPU
• Known Poorly Performing SQL
• Tracing a Session / Process
• Highest Response Times (Ignite)
SELECT QS.query_hash, QS.total_elapsed_time, qs.execution_count,
SUBSTRING(ST.text, (QS.statement_start_offset/2) + 1,
((CASE statement_end_offset
WHEN -1 THEN DATALENGTH(ST.text)
ELSE QS.statement_end_offset END - QS.statement_start_offset)/2) + 1) AS sql_text
FROM sys.dm_exec_query_stats AS QS
CROSS APPLY sys.dm_exec_sql_text(QS.sql_handle) as ST
ORDER BY total_elapsed_time DESC

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SQL Wait States
 Understand the total time a Query spends in Database
 Measure time while Query executes
 SQL Server helps by providing Wait Types
Focus on Response Time

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Wait Interface
dm_exec_sql_text
text
dm_exec_sessions
login_time
login_name
host_name
program_name
session_id
dm_exec_query_stats
execution_count
total_logical_writes
total_physical_reads
total_logical_reads
total_elapsed_time
dm_exec_requests
start_time
status
sql_handle
plan_handle
start/stop offset
database_id
user_id
blocking_session
wait_type
wait_time
dm_exec_query_plan
query_plan
http://msdn.microsoft.com/en-us/library/ms188754.aspx

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Base Monitoring Query
INSERT INTO SessionWaitInfo
SELECT r.session_id, r.sql_handle, r.statement_start_offset,
r.statement_end_offset, r.plan_handle, r.database_id,
r.blocking_session_id, r.wait_type, r.query_hash,
s.host_name, s.program_name, s.host_process_id,
s.login_name, CURRENT_TIMESTAMP cdt
FROM sys.dm_exec_requests r
INNER JOIN sys.dm_exec_sessions s ON s.session_id = r.session_id
WHERE r.status <> 'background'
AND r.command <> 'AWAITING COMMAND‘
AND s.session_id <> @@SPID

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RTA - Proactive

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RTA - Firefighting

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RTA - Correlation

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Sample Wait Types
• WRITELOG
– Waiting for a log flush to complete
• LCK_M_S, LCK_M_U, LCK_M_X…
– Waiting to acquire locks
• ASYNC_NETWORK_IO
– Waiting on data on the network
• PAGEIOLATCH_SH, PAGEIOLATCH_EX…
– Physical disk reads
• WAITFOR (idle event)
– Waiting during a WAITFOR command

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Tracing
• Tracing with waits gathers very good data
• Can be High Overhead – especially via Profiler
• Use Server-Side Tracing
– sp_trace_create – create the trace definition
– sp_trace_setevent – add events to trace
– sp_trace_setfilter – apply filters to trace
– sp_trace_setstatus – start/stop the trace
• Use Profiler to Create Initial Trace
– Use File > Script Trace to Get Script
• Cumbersome to review data
• Set trace file sizes appropriately

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Summary of RTA
• Using Response Time Analysis (RTA) Ensures
you work on the Correct Problem
• Shows Exactly Why Performance is Suffering
• Helps Prioritize Problems
• Do Not Rely Exclusively on Health Stats (CPU
Utilization, Disk IO, Cache Hit Ratio)
• Data Collection
– DMVs – build it yourself
– Tracing – know how to process trace data
– Tools – Ensure they use Wait Time and Health

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Why is SQL Slow - Plans
• SQL Server Management Studio
– Estimated Execution Plan - can be wrong
– Actual Execution Plan – must execute query, can be
dangerous in production and also wrong in test
• SQL Server Profiler Tracing
– Event to collect: Performance : Showplan All
– Works when you know a problem will occur
• DM_EXEC_QUERY_PLAN(@handle,@s,@e)
– Real execution plan of executed query

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DM_EXEC_QUERY_PLAN

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Case Studies
• SQL Diagramming
– Who registered yesterday for Tuning Class
– Check order status

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SQL Statement 1
• Who registered yesterday for SQL Tuning
SELECT s.fname, s.lname, r.signup_date
FROM student s
INNER JOIN registration r ON s.student_id = r.student_id
INNER JOIN class c ON r.class_id = c.class_id
WHERE c.name = 'SQL TUNING'
AND r.signup_date BETWEEN @BeginDate AND @EndDate
AND r.cancelled = 'N'
• Execution Stats – 9,634 Logical Reads

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Database Diagram

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Execution Plan
Recommendation from SSMS
CREATE NONCLUSTERED INDEX [<Name of Missing Index>]
ON [dbo].[registration] ([cancelled],[signup_date])
INCLUDE ([student_id],[class_id])

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SQL Diagramming
registration
student class
37
1
1293
1
.03
.001
select count(1) from registration where cancelled = 'N'
and signup_date between '2010-04-23 00:00' and '2010-04-24 00:00'
54,554 / 1,639,186 = 0.03
select count(1) from class where name = 'SQL TUNING'
2 / 1,267 = .001
• Great Book “SQL Tuning” by Dan Tow
– Great book that teaches SQL Diagramming
– http://www.singingsql.com

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New Plan
CREATE INDEX cl_name ON class(name)
 Execution Stats – 9,139 Logical Reads
 Why would an Index Scan still occur on REGISTRATION?

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Database Diagram

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New Plan
CREATE INDEX reg_alt ON registration(class_id)
 Execution Stats – 621 Logical Reads

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Better Plan
CREATE INDEX reg_alt ON registration(class_id)
INCLUDE (signup_date, cancelled)
 Execution Stats – 20 Logical Reads

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SSMS Plan
CREATE INDEX reg_can ON registration(cancelled, signup_date)
INCLUDE (class_id, student_id)
 Execution Stats – 595 Logical Reads
CREATE NONCLUSTERED INDEX [<Name of Missing Index>]
ON [dbo].[registration] ([class_id],[cancelled],[signup_date])
INCLUDE ([student_id])

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SQL Statement 2
• Lookup order status for caller
SELECT o.OrderID, c.LastName, p.ProductID, p.Description,
sd.ActualShipDate, sd.ShipStatus, sd.ExpectedShipDate
FROM [Order] o
INNER JOIN Item i ON i.OrderID = o.OrderID
INNER JOIN Customer c ON c.CustomerID = o.CustomerID
INNER JOIN ShipmentDetails sd ON sd.ShipmentID = i.ShipmentID
LEFT OUTER JOIN Product p ON p.ProductID = i.ProductID
LEFT OUTER JOIN Address a ON a.AddressID = sd.AddressID
WHERE c.LastName LIKE ISNULL(@LastName,'') + '%'
--AND c.FirstName LIKE ISNULL(@FirstName,'') + '%'
AND o.OrderDate >= DATEADD(day, -30, CURRENT_TIMESTAMP)
AND sd.ShipStatus <> 'C'
• Execution Stats – 10,159 Logical Reads

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Database Diagram

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Plan

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SQL Diagramming
o .08
.03
SELECT COUNT(1)*1.0/(SELECT COUNT(1) FROM Customer) FROM Customer
WHERE LastName LIKE 'SMI%'
.03
SELECT COUNT(1)*1.0/(SELECT COUNT(1) FROM [Order]) FROM [Order]
WHERE OrderDate >= DATEADD(day, -30, CURRENT_TIMESTAMP)
.08
SELECT COUNT(1)*1.0/(SELECT COUNT(1) FROM [Order]) FROM [Order]
WHERE OrderStatus <> 'C'
.005
-- Combined
.005
i c
psd
a
.005

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Data Skew
• Only 0.5% of rows are <> ‘C’
• How about changing the query?
– AND o.OrderStatus = 'I'
• Add an Index on ShipStatus
SELECT OrderStatus, COUNT(1)
FROM [Order]
GROUP BY OrderStatus

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New Plan
CREATE INDEX IX2_OrderStatus ON [Order] (OrderStatus)
INCLUDE (OrderID,CustomerID)
Execution Stats – 3,052 Logical Reads

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Takeaway Points
• Tuning Queries gives more “bang for the buck”
• Make sure you are tuning the correct query
• Use Wait Types and Response Time Analysis
– Locking problems may not be a Query Tuning issue
– Wait types tell you where to start
• Use “Real Execution Plans”
– Get plan_handle from DM_EXEC_REQUESTS
– Pass plan_handle to DM_EXEC_QUERY_PLAN()
• SQL Diagramming - “Get it right the First Time”
– Query Tuner Tools can mislead you

35. Q&A
Thank you for attending.
More questions?
Contact Dean at
DeanRichards@confio.com
Download free trial of Ignite at
http//www.confio.com
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