Every SQL Programmer Should Know Kevin Kline Director of Engineering Services at SQL Sentry Microsoft MVP since 2003 Facebook, LinkedIn, Twitter at KEKLINE KevinEKline.com, ForITPros.com

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Introductions Test & tuning environment 1. Clearing caches Looking for red flags 2. Reading execution plans Query tuning techniques: 8 more specific examples of widespread approaches that lead to poor performance Summary 3

Code to clear the caches*: o CHECKPOINT o DBCC [FreeProcCache | FreeSystemCache | FlushProcInDB( ) ] o DBCC DropCleanBuffers Code to set measurements: o SET STATISTICS [TIME | IO] o SET SHOWPLAN [TEXT | XML] or Graphic Execution Plans Code for Dynamic Management Views (DMV) checks. o System info – sys.dm_os_performance_counters, sys.dm_os_wait_stats o Query info – sys.dm_exec_requests o Index info – sys.dm_db_index_usage_stats, sys.dm_io_virtual_file_stats

Red Flags Query Operators: o Lookups, Scans, Spools, Parallelism Operations Other Red Flags: o Dissimilar estimated versus actual row counts o High physical reads o Missing statistics alarms o Large sort operations o Implicit data type conversions

I don’t always use cursors… o …but when I do, I avoid the default options o Slow and heavy-handed: Global, updateable, dynamic, scrollable o I use LOCAL FAST_FORWARD o May want to test STATIC vs. DYNAMIC, when tempdb is a bottleneck Blog post:

There are lots of ways to find data existing within subsets: IN, EXISTS, JOIN, Apply, subquery Which technique is best? Blog post:

Big differences between a SELECT and a DML statement that effects the same rows. Shouldn’t blindly create every index the Tuning Advisor or execution plan tells you to! Blog post -

8K pages Leaf pages ARE the data. Non-leaf pages are pointers. Leaf Pages Root Page Level 0 Intermediate Pages Level 1 Level 2

Each change to the leaf pages requires all index structures be updated. Leaf Pages Root Page Level 0 Intermediate Pages Level 1 Level 2 Page Split DML Actual place- ment

Execution Load metadata NO In Memory? compile optimize Execute YES ReComp Execute

Expected: Because we request it: CREATE PROC … WITH RECOMPILE or EXEC myproc … WITH RECOMPILE SP_RECOMPILE foo Expected: Plan was aged out of memory Unexpected: Interleaved DDL and DML Unexpected: Big changes since last execution: Schema changes to objects in underlying code New/updated index statistics Sp_configure

CREATE PROC testddldml AS … ; CREATE TABLE #testdml;-- (DDL) INSERT INTO #testdml;-- (DML + RECOMPILE) ALTER TABLE #testdml;-- (DDL) INSERT INTO #testdml;-- (DML + RECOMPILE) DROP TABLE #testdml;-- (DDL)

Usually see it as a one-query-for-all-queries procedure, or even one-proc-for-for-all-transactions procedure: o Where name starts with S, or placed an order this year, or lives in Texas o Insert AND Update AND Delete AND Select Conflicting optional parameters make optimization impossible o OPTION (RECOMPILE) o Dynamic SQL + Optimize for ad hoc workloads o Specialized procedures Better approach? o Specialize and optimize each piece of code to do ONE THING really effectively

I don’t always use dynamic SQL… o …but when I do, I always use sp_executesql o Less fuss with concatenation and implicit/explicit conversions o Better protection against SQL injection (but not for all things) o At worst case, behavior is the same Can promote better plan re-use Encourages strongly typed parameters instead of building up a massive string

SQL Server has to do a lot of extra work / scans when conversion operations are assumed by the SQL programmer. Happens all the time with data types you’d think wouldn’t need it, e.g. between date types and character types. Very useful data type conversion chart at Data type precedence call also have an impact:

Ian Stirk’s Column Mismatch Utility at / / Jonathan Kehayias’ plan cache analyzer at /01/08/finding-implicit-column-conversions-in-the-plan- cache.aspx. /01/08/finding-implicit-column-conversions-in-the-plan- cache.aspx Jonathan Kehayias’ index scan study at queries/implicit-conversion-costs queries/implicit-conversion-costs

Example: pass a comma-separated list of OrderIDs String splitting is expensive, even using CLR Table-valued parameters are typically a better approach

Which are better, temp tables or temp variables? Temp TableTemp Variable Stored in?Tempdb Statistics?YesNo (1 row) Indexs/Keys?Yes1 UK / PK only Truncate?YesNo Recompiles?YesNo Parallelism?YesNo Metadata Overhead? LowLowest Lock Overhead?NormalLowest

Might sound frivolous, but naming schemes are important o Convention is not important; but rather being consistent and logical Story: dbo.UpdateCustomer vs. dbo.Customer_Update Always specify schema when creating, altering, referencing objects o Object resolution works a little bit harder without it o More importantly, it can get the wrong answer o And will often yield multiple copies of the same plan Do not use the sp_ prefix on stored procedures o This has observable overhead, no matter how specific you are

Your dev machine is usually nothing like production o Build representative data when you can o Build a stats-only database when you can’t (a.k.a. a database clone) Will allow you to see plan issues, but not speed o Make sure settings are the same edition Max memory if possible, sp_configure options Logins (and permissions), tempdb settings Parameterization settings, recovery model, compression, snapshot isolation Compatibility level (usually not an issue when working with a restore) Run a full business cycle workload after a restore o Simulate equivalent hardware: DBCC OPTIMIZER_WHATIF o Use Distributed Replay when you can Not perfect, but more realistic than single-threaded trace replay

Test Harness for your SQL Code to ensure stable results. Remember the red flags in SQL code running against SQL Server. Plus 8 more coding techniques: 1.Writing “good” cursors 2.WHERE IN versus WHERE EXISTS 3.The nature of indexes and Optimizing for Reads versus Writes 4.Inadvertent recompiles 5.EXEC versus SP_EXECUTESQL 6.Data types and Implicit conversion 7.Dissimilar execution plans and tracking plan cache bloat 8.Mimicking the production environment More info? Let’s connect! Facebook, LinkedIn, Twitter at KEKLINE. at Blogs at And

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It is a turbo button …if you’re ok with inaccuracy There are times it is perfectly valid o Ballpark row counts o Please use session-level setting, not table hint Usually, though, better to use SNAPSHOT or RCSI o But test under heavy load