My Query is slow, now what?
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How many times did we have to spend countless hours looking for a T-SQL solution for the fancy requests of our users, to later discover our code doesn’t perform acceptably? What can we do to improve the performance of our code? Is there a methodology to follow in order to deliver better performance? What are the mistakes to avoid?
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My Query is slow, now what?
- 1. Gianluca Sartori Owner sqlconsulting.it My query is slow, now what?
- 2. 2 Gianluca Sartori • Independent SQL Server consultant • Working with SQL Server since version 7 • MCTS, MCITP, MCT • DBA @ Formula 1 team Blog: spaghettidba.com Twitter: @spaghettidba
- 3. 3 Agenda My query is slow! The performance tuning pyramid Schema design Code optimization Indexing
- 5. 5
- 7. 7 Schema Design Normalization 1NF: Must have a key, atomic attributes only 2NF: Each attribute depends on the whole key 3NF: Each attribute depends only on the key «The key, the whole key and nothing but the key, so help me Codd»
- 8. 8 Schema Design Denormalization clues • Data repeated redundancy • Inconsistent data anomalies • Data separated by «,» eg: john@gmail.com, john@business.com • Structured data in «notes» columns • Column names with a numeric suffix eg: Area1, Area2 , Area3…
- 9. 9 Schema Design Worst Practices No Primary Key or surrogate keys only «Id» is not the only possible key! No Foreign Keys They’re «difficult» to deal with No CHECK constraint The application will enforce consistency… Wrong data types VAT number, Telephone number Dates stored as strings Use of NULL where inappropriate Use of «dummy» values (eg: ‘.’ , 0)
- 11. 11 Query optimization RBAR Code Re-Use Sort / DISTINCT A query to rule them all Redundant subqueries
- 12. 12 Query optimization • Row By Agonizing Row • Cursors • WHILE loops • App-side cursors • Scalar and multi-statement functions http://www.sqlservercentral.com/Authors/Articles/Jeff_Moden/80567/ Jeff Moden
- 13. 13 RBAR - Cursors • Procedural code • Fixed execution strategy doesn’t scale! • Use lots of memory and CPU • Use lots of tempdb space • Execute a huge amount of statements • Can (almost) always be replaced with set-based code
- 14. 14 RBAR
- 15. 15 Code Reuse • Stored procedures, functions and views encapsulate the complexity • Code reuse works as far as it doesn’t hurt performance • What to avoid: • Stored procedure invoked inside cursor loops • Scalar functions with data access • Multi-statement table-valued functions • Views on views on views…
- 16. 16 Code Reuse Stored procedures encapsulate complex logic • Different from OO • No inheritance, polymorfism etc… Impossible to combine with other constructs • Require cursors to be applied on a set • App-side cursors are no better! • The same applies to ORMs If no data modification happens, better use a function (ITVF if possible)
- 17. 17 Code Reuse – Cursors and stored procedures
- 18. 18 Functions Scalar functions work well for complex calculations with NO data access • Invoked for each row in the input set • «hidden» RBAR Better use inline table-valued functions • Multi-statement table-valued functions return table variables estimated cardinality = 1 • Merged in the outer execution plan
- 19. 19 Code Reuse – Scalar functions
- 20. Might look like a brilliant idea at first • You can end up losing control • Unneeded multiple accesses to the same tables • Unnecessary JOINs Views on views on views…
- 21. 21 Code Reuse – Views on views on views…
- 22. 22 SORT / DISTINCT If not necessary, don’t sort results DISTINCT is the refugium peccatorum for missed JOIN predicates EXISTS often helps avoiding CROSS JOINs for filter predicates UNION is always DISTINCT Use UNION ALL whenever possible
- 23. One query to rule them all • Set-based is ok, everything in one query is too much • The optimizer is good, not perfect • Too complex: where do we start from? • Divide et impera • Break the code into pieces • CTEs • Temporary tables • Table Variables • Functions • Identify redundant pieces • Re-assemble • http://spaghettidba.com/2012/03/15/how-to-eat-a-sql-elephant/
- 24. 24 Divide et impera Redundant Subqueries
- 25. Indexing
- 26. 26 Indexing SARGAbility Deciding which indexes to create? Common mistakes Included columns Execution plan analysis How to read an execution plan What to look for
- 27. 27 SARGAbility SARGABLE = Search ARGument ABLE A predicate is «sargable» when it can be evaluated by means of an index • Non-Sargable in general is every predicate that requires transforming the column before evaluation eg: functions WHERE YEAR(order_date) = YEAR(GETDATE())
- 28. 28 SARGabilty - examples WHERE YEAR(SellStartDate) = YEAR(GETDATE()) WHERE SellStartDate >= DATEADD(yy, DATEDIFF(yy,0,getdate()), 0) AND SellStartDate < DATEADD(yy, DATEDIFF(yy,0,getdate())+1, 0) WHERE ISNULL(ProductLine,'M') = 'M' WHERE ProductLine = 'M' OR ProductLine IS NULL WHERE LEFT(ProductNumber,2) = 'BK' WHERE ProductNumber LIKE 'BK%'
- 29. 29 Deciding which indexes to create Create indexes to support the following predicates • WHERE • JOIN • GROUP BY • ORDER BY Indexes are used effectively when the leading column appears in the predicate Execution plans suggest missing indexes • Watch out from overly «aggressive» suggestions
- 30. 30 Common mistakes • One index for each column • Missing indexes on Foreign Keys • Accepting all suggestions from the DB Tuning Advisor • Duplicate Indexes • Sub-optimal clustered index • Unique • Small • Invariable • Ever-increasing • Not testing on the whole workload
- 31. 31 Included columns Columns not present in filter predicates but only in the SELECT list can be included Included columns are not for free • Present at leaf level in the B-tree • Contribute to index size Clustered index always includes all columns Nonclustered indexes always include the clustering key
- 32. 32 Execution plans Output of the query optimizer T-SQL statement Parser Algebrizer Algebrizer Tree Optimizer Statistics Execution Plan
- 33. 33 Execution plans How to read an execution plan
- 34. 34 Execution plans What to look for: Implicit conversions Scans Missing JOIN predicate alerts Spools Sorts Parallelism Wrong cardinality estimates
- 35. 35 Recap Simplify code RBAR is the root of all evil Remove redundant accesses Divide et impera Execution plans Adding indexes may help Unused indexes? SARGability Included columns Schema design mistakes?
- 36. Q&A.