Aaron BertrandSQL Sentry

T-SQL : Bad Habits & Best Practices

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

sqlblog.com• Twitter: @AaronBertrand

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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

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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

• Pop Quiz : Do these yield the same answer?

DECLARE @x VARCHAR = 'xyz';

SELECT @x, CAST('xyz' AS VARCHAR), CONVERT(VARCHAR, 'xyz');

Specify length for (n)(var)char

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Choose the wrong data type

• All kinds of violations here:• String/numeric types for date/time data• Datetime when 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

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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

schemas7

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

order8

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

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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

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Use safe date formats

• This is not safe at all:mm/dd/yyyy

• Always use:yyyymmdd or yyyy-mm-ddThh:mm:ss.nnn

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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

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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

• 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

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Use efficient cursor options

• Avoid heavy locking / resource usage• My syntax is always:

DECLARE c CURSOR LOCAL FAST_FORWARD FOR …

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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

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Use sp_executesql vs. EXEC()

• Helps thwart SQL injection • Allows use of strongly-typed parameters • Only partial protection, but better than zero

• Promotes better plan re-use

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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

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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 devs write ad hoc queries

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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] = …

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Stay Employed

• Always use BEGIN TRAN on ad hoc updates– SQL Server doesn’t have Ctrl + Z

• Otherwise, keep resume in open transaction

• Grab Mladen Prajdic’s SSMS Tools Pack– Lets you modify the “New Query”

template21

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

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Plenty more…Search for bad habits at sqlblog.com

Please check the slide notes for additional info and links to blog posts and articles

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