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T-SQL : Bad Habits & Best Practices. Aaron Bertrand SQL Sentry. Who Am I?. Senior consultant at SQL Sentry Microsoft MVP since 1997 Blog: sqlperformance.com / sqlblog.com Twitter: @AaronBertrand. 2. Before we start: Don’t take offense

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t sql bad habits best practices

T-SQL : Bad Habits&Best Practices

Aaron Bertrand

SQL Sentry

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

2

slide3

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

use select omit columns
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

specify length for n var char
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

choose the wrong data type
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

always use the schema prefix
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

abuse order by
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

use set nocount on
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

abuse date range queries
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

use safe date formats
Use safe date formats
  • This is not safe at all:

mm/dd/yyyy

  • Always use:

yyyymmddoryyyy-mm-ddThh:mm:ss.nnn

11

use old style joins
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

use sensible naming conventions
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

default to cursors
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

use efficient cursor options
Use efficient cursor options
  • Avoid heavy locking / resource usage
  • My syntax is always:

DECLARE c CURSOR

LOCAL FAST_FORWARD

FOR …

15

default to dynamic sql
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

use sp executesql vs exec
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

use subqueries in case coalesce
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

use consistent case formatting
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

abuse count
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

stay employed
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

overuse nolock
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

slide23

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

23

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  • #SQLBITS
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