Natural Born Killers, performance issues to avoid

Transcription

1 Natural Born Killers, performance issues to avoid Richard Douglas

2 Natural Born Killer 2 Natural Born Killers

3 3 Natural Born Killers

4 Source: 4 Natural Born Killers

5 Adhering to best practices? 5 Natural Born Killers

6 Your host Richard Douglas Systems Consultant SQL Server MCITPro Maidenhead SQL User Group Leader Blog: Natural Born Killers

7 Agenda Statistics Table Design Scalar UDF s Indices Key lookups Sargability Table variables Parameter sniffing 7 Natural Born Killers

8 Statistics SQL uses a cost based optimizer Costs influenced by statistics 8 Natural Born Killers

9 Statistics Creating Statistics Automatic Manual CREATE STATISTICS sp_createstats Updating Statistics Permanent Table Sp_UpdateStats UPDATE STATISTICS Temporary Table 1 st record 1 st record - 6 records LT 500 recs, 500 changes GT 500 recs, 500 changes + 20% *Trace flag 2371 can change these thresholds. It is undocumented and was included in 2008 R2 SP1. ve/2011/09/07/changes-to-automatic-updatestatistics-in-sql-server-traceflag-2371.aspx LT 500 recs, 500 changes GT 500 recs, 500 changes + 20% 9 Natural Born Killers

10 Table Design ream/ 10 Natural Born Killers

11 Example of bad design 11 Natural Born Killers

12 Example of a better design 12 Natural Born Killers

13 Table Design Size comparison 13 Natural Born Killers

14 Good table design benefits Minimise the CPU overhead. Increase the number of records in the buffer cache. Reduce the amount of physical disk IO. Reduce the amount of network traffic. Reduce the data file size(s). Reduce the working size of the Transaction Log. Reduce Full/Diff/T-Log backup file size. Thereby increasing your ability to deliver your RTO. Keep transaction time to a minimum. 14 Natural Born Killers

15 Scalar User Defined Functions The Good Re-Usable code The Bad Runs once per record in record set They don t take advantage of parallelism They use Nested Loop joins regardless The Ugly So ugly it s hidden from the query plan and IO statistics 19 Natural Born Killers

16 Scalar UDF Solutions and Alternatives If you have to use them: View the actual IO usage in Profiler If you can replace them: Look at Table Value Functions Look at CLR 20 Natural Born Killers

17 Indexing strategies It s all about DWI knowledge: D W I a t a o r k l o a d n t e r n a l s 22 Natural Born Killers

18 Golden Rules for Clustered Indexes Bad example; Surname, Firstname, Middle Initial Narrow Static Progressive Unique Fixed width Not Null Impacts nonclustered indexes Fragmentation Space impact This example will have 13 bytes of overhead alone; 4 byte uniquifier 2 byte variable offset + 6 bytes for variable length fields 1 byte for NULL values and NULL bitmap 23 Natural Born Killers

19 Golden Rules for NonClustered Indexes Have an optimal clustered key Not narrow Reduce unnecessary overhead Fixed Width Not null Consolidate Index foreign keys 24 Natural Born Killers

20 Key Lookups The silent killer 25 Natural Born Killers

21 Sargability SARGable Search ARGument able T-SQL functions around a predicate can break SARGability Some caveats to remember 26 Natural Born Killers

22 Table Variables Vs. Temp Tables Table Variables Fast when used with small data sets. Have a limited scope Use less locking and logging resources than temp tables Estimated statistics always show 1 record Cannot be altered after they are declared Mythbuster - MAY be memory only, this isn t guaranteed Generally faster with smaller data sets 27 Natural Born Killers

23 Table Variables Vs. Temp Tables Temp Tables Uses statistics Stat population will cause recompiles (statement level in ) Can create better plans Wider scope Can be rolled back Can be altered after creation Generally faster with larger data sets 28 Natural Born Killers

24 Parameter Sniffing What is it? It s all a matter of statistics How do I know when it will affect me? Monitor the IO and CPU This means baselining your environment What are my options? Rewrite dynamic queries Query Hints: WITH RECOMPILE OPTIMIZE FOR Plan Guides 29 Natural Born Killers

25 Demos 30 Natural Born Killers

26 Summary Statistics Table Design Scalar UDF s Indices Key lookups Sargability Table variables Parameter sniffing Blogged here Natural Born Killers

27 Any questions? 32 Understanding Indices Dell Software

28 Thank