SQL Server 2012 Query. Performance Tuning. Grant Fritchey. Apress*

SQL Server 2012 Query Performance Tuning Grant Fritchey Apress*

Contents J About the Author About the Technical Reviewer Acknowledgments Introduction xxiii xxv xxvii xxix Chapter 1: SQL Query Performance Tuning 1 The Performance Tuning Process 2 The Core Process 2 Iterating the Process 4 Performance vs. Price 5 Performance Targets 5 "Good Enough" Tuning 8 Performance Baseline 8 Where to Focus Efforts 9 SQL Server Performance Killers 10 Poor Indexing 11 Inaccurate Statistics 11 Poor Query Design 11 Poor Execution Plans 12 Excessive Blocking and Deadlocks 12 Non-Set-Based Operations 12 Poor Database Design 12 Excessive Fragmentation 13

Nonreusable Execution Plans 13 Frequent Recompilation of Queries 13 Improper Use of Cursors 14 Improper Configuration of the Database Log 14 Excessive Use or Improper Configuration of tempdb 14 Summary 14 Chapter 2: System Performance Analysis...15 Performance Monitor Tool 15 Dynamic Management Objects 17 Hardware Resource Bottlenecks 18 Identifying Bottlenecks 18 Bottleneck Resolution 19 Memory Bottleneck Analysis 19 SQL Server Memory Management 19 Available Bytes 23 Pages/sec and Page Faults/sec 23 Paging File %Usage and Page File %Usage 23 Buffer Cache Hit Ratio 24 Page Life Expectancy 24 Checkpoint Pages/sec 24 Lazy writes/sec 24 Memory Grants Pending 25 Target Server Memory (KB) and Total Server Memory (KB) 25 Additional Memory Monitoring Tools 25 DBCC Memorystatus 25 Dynamic Management Objects 26 Memory Bottleneck Resolutions 27 Optimizing Application Workload 27 Allocating More Memory to SQL Server 29 Increasing System Memory 29 tri

Changing from a 32-bit to a 64-bit Processor 29 Data Compression 29 Enabling 3GB of Process Address Space 29 Disk Bottleneck Analysis 30 Disk Counters 30 % Disk Time 31 Current Disk Queue Length 31 Disk Transfers/sec 31 Disk Bytes/sec 32 Avg. Disk Sec/Read and Avg. Disk Sec/Write 32 Additional I/O Monitoring Tools 32 Sys.dm_io_virtuaLfile_stats 32 Sys.dm_os_wait_stats 33 Disk Bottleneck Resolutions 33 Optimizing Application Workload 33 Using a Faster I/O Path 34 Using a RAID Array 34 RaidO 34 Raid 1 35 Raid 5 35 Raid 6 35 RAID 1+0 (RAID 10) 36 Using a SAN System 36 Using SSD Disks 36 Aligning Disks Properly 36 Adding System Memory 37 Creating Multiple Files and Filegroups 37 Placing the Table and Index on Separate Disks 39 Moving the Log Files to a Separate Physical Disk 40 Partitioning Tables 40 vii

y. CONTENTS Processor Bottleneck Analysis 40 % Processor Time 41 % Privileged Time 41 Processor Queue Length 42 Context Switches/sec 42 Batch Requests/sec 42 SQL Compilations/sec 42 SQL Recompilations/sec 42 Other Tools for Measuring CPU Performance 43 Sys.dm_os_wait_stats 43 Sys.dm_os_workers and Sys.dm_os_schedulers 43 Processor Bottleneck Resolutions 43 Optimizing Application Workload 43 Eliminating Excessive Compiles/Recompiles 44 Using More or Faster Processors 44 Using a Large L2/L3 Cache 44 Running More Efficient Controllers/Drivers 44 Not Running Unnecessary Software 45 Network Bottleneck Analysis 45 Bytes Total/sec 45 % Net Utilization 45 Network Bottleneck Resolutions 46 Optimizing Application Workload 46 SQL Server Overall Performance 46 Missing Indexes 47 Database Blocking 48 Nonreusable Execution Plans 49 General Behavior 49 User Connections 49 Batch requests/sec 50 viii

Considerations for Monitoring Virtual Machines 50 Creating a Baseline 50 Creating a Reusable List of Performance Counters 51 Creating a Counter Log Using the List of Performance Counters 52 Performance Monitor Considerations 54 System Behavior Analysis Against Baseline 55 Summary 57 Chapter 3: SQL Query Performance Analysis 59 Extended Events Wizard 59 Extended Events Sessions 60 Events 61 Global Fields 64 Event Filters 66 Datastorage 66 Finishing the Wizard and Starting the Session 68 Extended Events Automation 70 Creating a Session Script Using the GUI 70 Defining a Session Using Stored Procedures 71 Extended Events Recommendations 72 Set Max File Size Appropriately 72 Avoid Debug Events 72 Partition Memory in the Sessions 72 Avoid Use of No_Event_Loss 73 Other Methods for Query Performance Metrics 73 Costly Queries 74 Identifying Costly Queries 75 Costly Queries with a Single Execution 75 Costly Queries with Multiple Executions 76 Identifying Slow-Running Queries 78 ix

Execution Plans 79 Analyzing a Query Execution Plan 81 Identifying the Costly Steps in an Execution Plan 83 Analyzing Index Effectiveness 85 Analyzing Join Effectiveness 86 Hash Join 87 Actual vs. Estimated Execution Plans 90 Plan Cache 92 Query Cost 93 Client Statistics 93 Execution Time 94 Statistics lo 95 Summary 97 Chapter 4: Index Analysis 99 What Is an Index? 99 The Benefit of Indexes 101 Index Overhead 103 Index Design Recommendations 105 Examine the WHERE Clause and JOIN Criteria Columns 105 Use Narrow Indexes 107 Examine Column Uniqueness 109 Examine the Column Data Type 112 Consider Column Order 112 Consider the Type of Index 115 Clustered Indexes 115 Heap Tables 115 Relationship with Nonclustered Indexes 116 Clustered Index Recommendations 118 When to Use a Clustered Index 120 Poor Design Practices for a Clustered Index 121 x

Nonclustered Indexes 122 Nonclustered Index Maintenance 123 Defining the Bookmark Lookup 123 Nonclustered Index Recommendations 123 Clustered vs. Nonclustered Indexes 124 Benefits of a Clustered Index over a Nonclustered Index 124 Benefits of a Nonclustered Index over a Clustered Index 126 Advanced Indexing Techniques 128 Covering Indexes 128 A Pseudoclustered Index 130 Recommendations 130 Index Intersections 130 Index Joins 132 Filtered Indexes 134 Indexed Views 136 Index Compression 141 ColumnStore Indexes 142 Special Index Types 144 Full-Text 144 Spatial 145 XML 145 Additional Characteristics of Indexes 145 Different Column Sort Order 145 Index on Computed Columns 146 Index on BIT Data Type Columns 146 CREATE INDEX Statement Processed As a Query 146 Parallel Index Creation 146 Online Index Creation 147 Considering the Database Engine Tuning Advisor 147 Summary 147

HCONTENTS Chapter 5: Database Engine Tuning Advisor 149 Database Engine Tuning Advisor Mechanisms 149 Database Engine Tuning Advisor Examples 154 Tuning a Query 154 Tuning a Trace Workload 158 Tuning from the Procedure Cache 161 Database Engine Tuning Advisor Limitations 161 Summary 162 Chapter 6: Lookup Analysis 163 Purpose of Lookups 163 Drawbacks of Lookups 165 Analyzing the Cause of a Lookup 166 Resolving Lookups 168 Using a Clustered Index 168 Using a Covering Index 168 Using an Index Join 172 Summary 174 Chapter 7: Statistics Analysis 175 The Role of Statistics in Query Optimization 175 Statistics on an Indexed Column 176 Benefits of Updated Statistics 176 Drawbacks of Outdated Statistics 179 Statistics on a Nonindexed Column 181 Benefits of Statistics on a Nonindexed Column 181 Drawback of Missing Statistics on a Nonindexed Column 186 Analyzing Statistics 188 Density 190 Statistics on a Multicolumn Index 191 Statistics on a Filtered Index 192 xii

Statistics Maintenance 193 Automatic Maintenance 194 Auto Create Statistics 194 Auto Update Statistics 195 Auto Update Statistics Asynchronously 195 Manual Maintenance 196 Manage Statistics Settings 197 Generate Statistics 198 Statistics Maintenance Status 199 Status of Auto Create Statistics 199 Status of Auto Update Statistics 200 Analyzing the Effectiveness of Statistics for a Query 200 Resolving a Missing Statistics Issue 200 Resolving an Outdated Statistics Issue 202 Recommendations 205 Backward Compatibility of Statistics 205 Auto Create Statistics 206 Auto Update Statistics 206 Automatic Update Statistics Asynchronously 206 Amount of Sampling to Collect Statistics 207 Summary 208 Chapter 8: Fragmentation Analysis 211 Causes of Fragmentation 211 Page Split by an UPDATE Statement 213 Page Split by an INSERT Statement 217 Fragmentation Overhead 218 Analyzing the Amount of Fragmentation 221 Analyzing the Fragmentation of a Small Table 223 xiii

Fragmentation Resolutions 225 Dropping and Re-creating the Index 225 Re-creating the Index with the DROP_EXISTING Clause 226 Executing the ALTER INDEX REBUILD Statement 227 Executing the ALTER INDEX REORGANIZE Statement 229 Significance of the Fill Factor 230 Automatic Maintenance 233 Summary 239 Chapter 9: Execution Plan Cache Analysis 241 Execution Plan Generation 241 Parser 243 Binding 243 Optimization 244 Execution Plan Caching 251 Components of the Execution Plan 251 Query Plan Execution Context 251 251 Aging of the Execution Plan 251 Analyzing the Execution Plan Cache 252 Execution Plan Reuse 253 Ad Hoc Workload 254 Prepared Workload 254 Plan Reusability of an Ad Hoc Workload 255 Plan Reusability of a Prepared Workload 262 Parameter Sniffing 270 Query Plan Hash and Query Hash 273 Execution Plan Cache Recommendations 276 Explicitly Parameterize Variable Parts of a Query 277 Create Stored Procedures to Implement Business Functionality 277 xiv

Code with sp_executesql to Avoid Stored Procedure Maintenance 277 Implement the Prepare/Execute Model to Avoid Resending a Query String 277 Avoid Ad Hoc Queries 278 Prefer sp_executesql over EXECUTE for Dynamic Queries 278 Parameterize Variable Parts of Queries with Care 279 Do Not Allow Implicit Resolution of Objects in Queries 279 Summary 279 Chapter 10: Query Recompilation 281 Benefits and Drawbacks of Recompilation 281 Identifying the Statement Causing Recompilation 284 Analyzing Causes of Recompilation 285 Schema or Bindings Changes 287 Statistics Changes 287 Deferred Object Resolution 289 SET Options Changes 293 Execution Plan Aging 294 Explicit Call to sp_recompile 294 Explicit Use of RECOMPILE 295 Avoiding Recompilations 297 Don't Interleave DDL and DML Statements 297 Avoiding Recompilations Caused by Statistics Change 299 Using the KEEPFIXED PLAN Option 299 Disable Auto Update Statistics on the Table 301 Using Table Variables 301 Avoiding Changing SET Options Within a Stored Procedure 304 Using OPTIMIZE FOR Query Hint 305 Using Plan Guides 307 Summary 311 XV

Chapter 11: Query Design Analysis...313 Query Design Recommendations 313 Operating on Small Result Sets 314 Limit the Number of Columns in selectjist 314 Use Highly Selective WHERE Clauses 315 Using Indexes Effectively 315 Avoid Nonsargable Search Conditions 316 BETWEEN vs. IN/OR 316 Avoid Arithmetic Operators on the WHERE Clause Column 319 Avoid Functions on the WHERE Clause Column 320 Avoiding Optimizer Hints 323 JOIN Hint 323 INDEX Hints 327 Using Domain and Referential Integrity 327 NOT NULL Constraint 328 Declarative Referential Integrity 331 Avoiding Resource-Intensive Queries 333 Avoid Data Type Conversion 333 Use EXISTS over C0UNT(*) to Verify Data Existence 335 Use UNION ALL Instead of UNION 336 Use Indexes foraggregate and Sort Conditions 337 Avoid Local Variables in a Batch Query 338 Be Careful When Naming Stored Procedures 341 Reducing the Number of Network Round-Trips 343 Execute Multiple Queries Together 343 Use SETN0C0UNT 343 Reducing the Transaction Cost 344 Reduce Logging Overhead 344 Reduce Lock Overhead 345 Summary 347 xvi

Chapter 12: Blocking Analysis 349 Blocking Fundamentals 349 Understanding Blocking 350 Atomicity 350 Consistency 353 Isolation 353 Durability 354 Locks 354 Lock Granularity 355 Row-Level Lock 355 Page-Level Lock 357 Extent-Level Lock 358 Heap or B-tree Lock 358 Table-Level Lock 358 Database-Level Lock 359 Lock Operations and Modes 359 Lock Escalation 359 Lock Modes 359 Exclusive (X) Mode 364 Intent Shared (IS), Intent Exclusive (IX), and Shared with Intent Exclusive (SIX) Modes 364 Schema Modification (Sch-M) and Schema Stability (Sch-S) Modes 365 Bulk Update (BU) Mode 365 Key-range Mode 365 Lock Compatibility 366 Isolation Levels 366 Read Uncommitted 366 Read Committed 367 Repeatable Read 368 Serializable 371 Snapshot 376 xvii

Effect of Indexes on Locking 376 Effect of a Nonclustered Index 377 Effect of a Clustered Index 379 Effect of Indexes on the Serializable Isolation Level 380 Capturing Blocking Information 380 Capturing Blocking Information with SQL 381 Extended Events and the blocked_process_report Event 383 Blocking Resolutions 385 Optimize the Queries 386 Decrease the Isolation Level 386 Partition the Contended Data 387 Recommendations to Reduce Blocking 388 Automation to Detect and Collect Blocking Information 389 Summary 392 Chapter 13: Deadlock Analysis 393 Deadlock Fundamentals 393 Choosing the Deadlock Victim 394 Using Error Handling to Catch a Deadlock 394 Deadlock Analysis 395 Collecting Deadlock Information 395 Analyzing the Deadlock 398 Avoiding Deadlocks 403 Accessing Resources in the Same Physical Order 403 Decreasing the Number of Resources Accessed 404 Convert a Nonclustered Index to a Clustered Index 404 Use a Covering Index for a SELECT Statement 404 Minimizing Lock Contention 404 Summary 406 xviii

Chapter 14: Cursor Cost Analysis 407 Cursor Fundamentals 407 Cursor Location 409 Cursor Concurrency 410 Cursor Types 411 Cursor Cost Comparison 414 Cost Comparison on Cursor Location 414 Cost Comparison on Cursor Concurrency 416 Read-Only 416 Cost Comparison on Cursor Type 417 Default Result Set 419 Benefits 420 Multiple Active Result Sets 420 Drawbacks 421 Cursor Overhead 423 Analyzing Overhead with T-SQL Cursors 423 Cursor Recommendations 427 Summary 428 Chapter 15: Database Performance Testing 429 Database Performance Testing 429 A Repeatable Process 430 Distributed Replay 430 Capturing Data with the Server Side Trace 431 Distributed Replay for Database Testing 434 Configuring the Client 435 Running the Distributed Tests 436 Conclusion 436 xix

Chapter 16: Database Workload Optimization.. 437 Workload Optimization Fundamentals 437 Workload Optimization Steps 438 Sample Workload 439 Capturing the Workload 441 Analyzing the Workload 442 Identifying the Costliest Query 444 Determining the Baseline Resource Use of the Costliest Query 445 Overall Resource Use 445 Detailed Resource Use 446 Analyzing and Optimizing External Factors 448 Analyzing the Connection Options Used by the Application 449 Analyzing the Effectiveness of Statistics 449 Analyzing the Need for Defragmentation 450 Analyzing the Internal Behavior of the Costliest Query 453 Analyzing the Query Execution Plan 454 Identifying the Costly Steps in the Execution Plan 456 Analyzing the Processing Strategy 456 Optimizing the Costliest Query 457 Modifying an Existing Index 457 Analyzing the Application of a Join Hint 459 Avoiding the Clustered Index Scan Operation 461 Modifying the Procedure 462 Analyzing the Effect on Database Workload 464 Iterating Through Optimization Phases 466 Summary 468 XX

: CONTENTS Chapter 17: SQL Server Optimization Checklist 469 Database Design 469 Balancing Under- and Ovemormalization 470 Benefiting from Entity-Integrity Constraints 471 Benefiting from Domain and Referential Integrity Constraints 472 Adopting Index-Design Best Practices 474 Avoiding the Use of the sp_ Prefix for Stored Procedure Names 476 Minimizing the Use of Triggers 476 Query Design 476 Use the Command SET NOCOUNT ON 476 Explicitly Define the Owner of an Object 477 Avoid Nonsargable Search Conditions 477 Avoid Arithmetic Expressions on the WHERE Clause Column 477 Avoid Optimizer Hints 478 StayAway from Nesting Views 478 Ensure No Implicit Data Type Conversions 479 Minimize Logging Overhead 479 Adopt Best Practices for Reusing Execution Plans 479 Adopt Best Practices for Database Transactions 480 Eliminate or Reduce the Overhead of Database Cursors 481 Configuration Settings 481 Affinity Mask 481 Memory Configuration Options 482 Cost Threshold for Parallelism 482 Max Degree of Parallelism 482 Optimize for Ad Hoc Workloads 482 Query Governor Cost Limit 482 Fill Factor (%) 482 Blocked Process Threshold 483 xxi

Database File Layout 483 Database Compression 483 Database Administration 483 Keep the Statistics Up-to-Date 484 Maintain a Minimum Amount of Index Defrag mentation 484 Cycle the SQL Error Log File 485 Avoid Database Functions Such As AUTO_CLOSE or AUTO_SHRINK 485 Database Backup 485 Incremental and Transaction Log Backup Frequency 485 Backup Distribution 486 Backup Compression 486 Summary 487 Index. 489 xxii