T-SQL: Bad Habits… and Best PracticesAaron Bertrand, Senior Consultant

SQL Sentry, Inc.

@AaronBertrandhttp://sqlsentry.com/http://sqlperformance.com/

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Agenda

I’m going to talk about ways you might currently be “doing it wrong.”

• Don’t take offense• I learned many of these things the hard way

• Most slides have links to blog posts in the notes• Many have more details, more background, more demos

• I want everyone to take away at least one thing

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DO NOT: SELECT * / Omit column listThe problems are not:

• metadata overhead• the use of the * explicitly

Real problem is needless lookups, network, I/O

Also change management:

• Views do not magically update• INSERT dbo.table SELECT * FROM ^ ^ problem problem

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DO: Specify lengths for (n)(var)charDo these yield the same answer?

DECLARE @x VARCHAR = 'aaron';

SELECT [variable] = @x, [concat] = CONCAT(@x,'bertrand'), [cast] = CAST('aaron' AS VARCHAR), [convert] = CONVERT(VARCHAR, 'aaron');

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DO NOT: Choose the wrong data typeAll 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• VARCHAR for proper names• MAX types for URL & e-mail address• TIMESTAMP because people think it involves date/time

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DO: Always use the schema prefix

When creating, altering, and 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

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DO NOT: 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, subquery, CTE etc. does not do this• TOP here dictates the rows to include, not how to order

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DO: Use SET NOCOUNT ON

Eliminates DONE_IN_PROC messages

• Chatter can be interpreted as resultsets by app• Even in SSMS, this chatter can slow processing• Also can make finding errors and warnings tough

BUT : Test your applications!

• Some older providers may rely on this info

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DO NOT: Abuse date / range queries• Non-sargable expressions• YEAR(), CONVERT(), DATEADD() against columns

• Date/time shorthand• GETDATE() + 1• Spell it out! n, ns, m, mi, mm, mcs, ms, w, wk, ww, y, yyyy

• BETWEEN / calculating “end” of period• Open-ended date range is safer

• Non-safe, regional date formats• m/d/y & d/m/y instead of yyyymmdd or yyyy-mm-ddThh:mm:ss

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DO NOT: Compare to DATEDIFF

SQL Server gets DATEDIFF(DAY, 0, GETDATE()) wrong

• Leads to really bad cardinality estimates• The bug was “fixed” under trace flag 4199• Which most of you, hopefully, aren’t running everywhere

• There are many ways to skin this cat, let’s use a different way

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DO: Use MERGE wisely

Yes, it turns multiple statements into one

• But it only prevents race conditions with explicit HOLDLOCK

There are many unresolved bugs and other issues:

• http://bit.ly/merge-with-caution

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DO NOT: Use old-style joins

Old-style outer joins (*= / =*)

• Deprecated syntax• Unpredictable results

Old-style inner joins (FROM x, y)

• Easy to mix up join and filter criteria • Easier to accidentally derive Cartesian product• Not deprecated, but not recommended either

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DO: Use sensible naming conventionsProcedures from a real customer system:

dbo.GetCustomerDetails dbo.Customer_Update dbo.Create_Customer dbo.usp_updatecust

Styles vary; even your own changes over time

• Convention you choose is not the point; consistency is• Just don’t use the sp_ prefix (link in notes)

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DO NOT: 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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DO: Use efficient cursor options

Defaults are slow and heavy-handed• Global, updateable, dynamic, scrollable

My syntax is always:

DECLARE c CURSOR

LOCAL FAST_FORWARD

FOR …

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DO NOT: Default to dynamic SQL

Like cursors, not always evil – can be best

Be aware of:• Potential cache bloat (turn on “optimize for ad hoc workloads”)• “Sea of red”• Concatenating different data types into a string• SQL injection

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DO: Use sp_executesql, not EXEC()

sp_executesql helps thwart SQL injection

• Allows use of strongly-typed parameters • But not for things like table/column names

• Only partial protection, but better than zero

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DO NOT: CASE/COALESCE in subqueryInner SELECT is evaluated twice. This…

SELECT COALESCE((SELECT …), 0) …;

…expands to this…

SELECT CASE WHEN (SELECT …) > 0 THEN (SELECT …) ELSE 0 END;

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DO: Use consistent case / formattingFor 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, ORMs/vendors change

tactics

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DO NOT: Abuse COUNT

For filtered count, use EXISTS:

IF (SELECT COUNT(*) FROM dbo.table WHERE …) > 0

IF EXISTS (SELECT 1 FROM dbo.table WHERE …)

For total count, use sys.partitions:

SELECT COUNT(*) FROM dbo.table;

SELECT SUM(rows) FROM sys.partitions WHERE index_id IN (0,1) AND [object_id] = …

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DO: Stay Employed

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

Otherwise, keep your resume in an open transaction

Grab Mladen Prajdic’s SSMS Tools Pack• Modify the “New Query” template• Use more reliable custom connection coloring• Not free for SSMS 2012, but worth every penny

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DO NOT: Overuse NOLOCK

The magic, pixie-dust “turbo button” …if you’re okay with 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