T-SQL : Bad Habits & Best Practices

1. T-SQL : Bad Habits&Best Practices Aaron Bertrand SQL Sentry

2. Who Am I? • Senior consultant at SQL Sentry • Microsoft MVP since 1997 • Blog: sqlperformance.com / sqlblog.com • Twitter: @AaronBertrand 2

3. Before we start: • Don’t take offense • I’ve learned many of these things the hard way • I want everyone to take away at least one thing • Most slides have links to blog posts in the notes 3

4. Use SELECT * / omit columns • Metadata overhead is NOT the problem here • Can cause needless lookups, network, I/O • Change management: • Views do not magically update • INSERT dbo.table SELECT * FROM • Also, don’t just create an index because the plan, DMV or Tuning Advisor tells you to… 4

5. Specify length for (n)(var)char • Pop Quiz : Do these yield the same answer? DECLARE @x VARCHAR = 'xyz'; SELECT @x, CAST('xyz' AS VARCHAR), CONVERT(VARCHAR, 'xyz'); 5

6. Choose the wrong data type • All kinds of violations here: • String/numeric types for date/time data • Datetimewhen date/smalldatetime will do • Time in place of an interval • MONEY/FLOAT because they sound appropriate • NVARCHAR for postal code • MAX types for URL & e-mail address • VARCHAR for proper names 6

7. Always use the schema prefix • When creating, altering, referencing objects • Being explicit prevents confusion or worse • Object resolution can work harder without it • Can yield multiple cached plans for same query • Even if all objects belong to dbo, specify • Eventually, you or 3rd parties will use schemas 7

8. Abuse ORDER BY • ORDER BY [ordinal] • OK for ad hoc, not for production • Query or underlying structure can change • Popular myth: table has “natural order” • Without ORDER BY, no guaranteed order • TOP + ORDER BY in a view does not do this • TOP here is which rows to include, not how to order 8

9. Use SET NOCOUNT ON • Eliminates DONE_IN_PROC messages • Chatter can interpreted as resultsets by app code • Even in SSMS, this chatter can slow processing • BUT : Test your applications! • Some older providers may rely on this info 9

10. Abuse date / range queries • Avoid non-sargable expressions • YEAR(), CONVERT(), DATEADD() against columns • Avoid date/time shorthand • GETDATE() + 1 • Spell it out! n, ns, m, mi, mm, mcs, ms, w, wk, ww, y, yyyy • Avoid BETWEEN / calculating “end” of period • Open-ended date range is safer • Don’t “Chop off time” with a serrated edge 10

11. Use safe date formats • This is not safe at all: mm/dd/yyyy • Always use: yyyymmddoryyyy-mm-ddThh:mm:ss.nnn 11

12. Use old-style joins • Old-style inner joins (FROM x, y) • Easy to muddle join and filter criteria • Easy to accidentally derive Cartesian product • Not deprecated, but not recommended either • Old-style outer joins (*= / =*) • Deprecated syntax with unpredictable results 12

13. Use sensible naming conventions • Procedures from a real customer system: dbo.GetCustomerDetails dbo.Customer_Update dbo.Create_Customer dbo.sp_updatecust • Styles vary; even your own changes over time • Convention you choose isn’t the point; consistency is • Just don’t use the sp_ prefix (link in notes) 13

14. Default to cursors • Can be difficult to think set-based • For maintenance tasks, maybe not worth it • Not always possible to go set-based • Cursors are often “okay” but rarely optimal • Most common exception : running totals 14

15. Use efficient cursor options • Avoid heavy locking / resource usage • My syntax is always: DECLARE c CURSOR LOCAL FAST_FORWARD FOR … 15

16. Default to dynamic SQL • Like cursors, not always evil – can be best • However, be aware of: • Potential cache bloat • Use “optimize for ad hoc workloads” setting • “Sea of red” – maintenance is tough • SQL injection 16

17. Use sp_executesqlvs. EXEC() • Helps thwart SQL injection • Allows use of strongly-typed parameters • Only partial protection, but better than zero • Promotes better plan re-use 17

18. Use subqueries in CASE / COALESCE • SELECT is evaluated twice: SELECT CASE WHEN (SELECT …) > 0 THEN (SELECT …) ELSE -1 END; SELECT COALESCE((SELECT …), -1) …; • One case where ISNULL() is better 18

19. Use consistent case / formatting • For readability, be liberal with: • BEGIN / END • Carriage returns • Indenting • Use semi-colons to future-proof code • Case/spacing differences yield different plans • A concern if devswrite ad hoc queries 19

20. Abuse COUNT • Use EXISTS instead of this common pattern: IF (SELECT COUNT(*) FROM dbo.table WHERE …) > 0 IF EXISTS (SELECT 1 FROM dbo.table WHERE …) • And for total count, use sys.partitions rather than: SELECT COUNT(*) FROM dbo.table; SELECT SUM(rows) FROM sys.partitions WHERE index_id IN (0,1) AND [object_id] = … 20

21. Stay Employed • Always use BEGIN TRAN on ad hoc updates • SQL Server doesn’t have Ctrl + Z • Otherwise, keep resume in open transaction • Grab MladenPrajdic’s SSMS Tools Pack • Lets you modify the “New Query” template 21

22. Overuse NOLOCK • The magic, pixie-dust “turbo button” …if you like inaccuracy • There are times it is perfectly valid • Ballpark row counts • Usually, though, better to use RCSI • Test under heavy load – can hammer tempdb • Use scope-level setting, not table hint 22

23. Plenty more…Search for bad habitsat sqlblog.comPlease check the slide notes for additional info and links to blog posts and articles 23

24. Coming up… Followed by… • #SQLBITS

25. Coming up… • #SQLBITS