The Database is Slow

Transcription

1 The Database is Slow SQL Server Performance Tuning Starter Kit Calgary PASS Chapter, 19 August 2015 Randolph West, Born SQL

2 Basic Internals Data File Transaction Log File Data Page Indexes Statistics

3 Basic Internals: Data File Transaction Log File (LDF: Log Data File) Database File Group Data File (MDF: Main Data File) (NDF: Non-Primary Data File) 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8KB 8 x 8KB pages = 1 extent (64KB)

4 Basic Internals: Transaction Log File Group Transaction Log File (LDF: Log Data File) Database Commit On Write VLF VLF VLF VLF VLF VLF

5 Basic Internals: Page 8KB Page Header Rows ID 1: ID 2: ID 3: ID 4: ID 6: ID 5: Paul Randal (SQLskills.com): Anatomy of a Page Slot Array

6 Basic Internals: Tables IndexID PageID 1 2 Page IndexID PageID 1 1 Page 2 IndexID PageID 7 5 Page IndexID Province IndexID Province IndexID Province IndexID Province 1 AB 4 ON 7 SK 10 QC 2 ON 5 AB 8 YT 11 SK 3 ON 6 SK 9 YT 12 NU Page 1 Page 4 Page 5 Page 7

7 Basic Internals: Indexes Province PageID AB 2 Page 3 QC 6 Province PageID AB 1 Page 2 Province PageID QC 5 Page 6 ON 4 SK 5 Province IndexID Province IndexID Province IndexID Province IndexID AB 1 ON 2 QC 10 SK 11 AB 5 ON 3 SK 6 YT 8 NU 12 ON 4 SK 7 YT 9 Page 1 Page 4 Page 5 Page 7

8 Basic Internals: Indexes (Contd.) Heap No B-tree, no links all info stored in Index Allocation Map Clustered Index Non-Clustered Index Unique Indexes and Constraints Filtered Index Indexed View Fill Factor

9 Basic Internals: Statistics Distribution of Data (up to 200 steps) Density of Data (rows per step) Tipping Point (Selectivity of Data) Auto updated after 20% plus 500 rows changed All query plan costs are based on these values DBCC SHOW_STATISTICS Read more by Gail Shaw and Kimberly L Tripp

10 Where Do I Begin? Hardware Configuration Operating System SQL Server Configuration Monitoring Tools

11 Hardware CPU (SMP or NUMA?) RAM (Memory pressure?) Storage (Lots of things) Network (SAN? TCP offloading?)

12 Operating System Physical or Virtual? Version and Edition Drive allocation Also cluster size, partition alignment Instant File Initialization Power Saving What Else Is Running On This Server?

13 SQL Server Configuration Version, Edition, Build, and Licence Part of a Cluster, Mirror or Availability Group? Trace Flags (1118, 1117, 3226, and possibly 4199) sp_configure Cost Threshold for Parallelism? Appropriate to workload Max Degree of Parallelism (MAXDOP)? Cores in NUMA node Optimize for Ad-Hoc Queries? Turn it on Max Server Memory? Appropriate to environment Backup Compression? Turn it on Lightweight Pooling and Priority Boost? Turn it off Agent Jobs Maintenance tasks System Databases

14 User Databases Number of Databases Data and Transaction Log file allocation File growth, free space, location, VLFs I/O Performance Latency < 10ms Recovery Model Compatibility Level SQL Server 2014 Page Verify Option Always be CHECKSUM Auto-Update Statistics Always be on

15 Monitoring Tools WHAT IS THE BASELINE? Dynamic Management Views Missing Indexes: sys.dm_db_missing_index_details sys.dm_db_missing_index_groups Wait Statistics: sys.dm_os_wait_stats Sessions: sys.dm_exec_sessions Free Tools: sp_whoisactive (Adam Machanic) Diagnostic Scripts (Glenn Berry, SQLskills.com) sp_blitz (BrentOzar.com) sp_blitzindex (BrentOzar.com) sp_askbrent (BrentOzar.com)

16 Tuning Process tempdb performance Indexes Unique clustered indexes on all tables Non-clustered covering indexes (avoid key lookups) Included columns (2005+) Filtered indexes (2008+) (WHERE clause) Indexed views (materialised views) Statistics 20% plus 500 rows changed Query Tuning (the last resort)

17 Query Tuning Examine the longest running queries Memory consumers Sort, hash aggregate, hash join Blocking operators Sort, scalar aggregates, hash join, eager spool, any operation that can spill to disk (e.g. worktable) Types of join (nested loop, merge, hash) Key / Bookmark Lookups (try a covering index) Cursors vs CROSS APPLY Temp tables vs table variables Collation Implicit conversions Craig Freedman:

18 Tips and Tricks Stored procedures beat ad-hoc queries any day Statistics are updated at 20% plus 500 rows Avoid query hints at all costs Parameterised dynamic T-SQL can be good Tipping point (scans vs seeks) Parameter sniffing can be bad OPTIMIZE FOR UNKNOWN could be worse It depends