SQL Server performance tuning basics (for the developers) Dean Vitner SQL/Dev UG Osijek Osijek, 30.01.2014..

Some like it hot CHECKPOINT DBCC DROPCLEANBUFFERS DBCC FREESYSTEMCACHE Store (sys.dm_os_memory_clerks) Database Pool ALL DBCC FREEPROCCACHE Plan handle DBCC FLUSHPROCINDB DB_ID

Testing code as you go Mock up or real data Baseline Multiple runs SSMS SET STATISTICS IO ON Logical vs physical reads SET STATISTICS TIME ON Graphical execution plan Missing index hint DMVs sys.dm_os_wait_stats sys.dm_exec_query_stats sys.dm_exec_procedure_stats

What to look for? Fat arrows Operators Estimates vs actual values Table scans Lookups Hash joins in OLTP workload Joins on non-selective columns Sort & sort warnings Parallelism Implicit conversions Estimates vs actual values Stale statistics Parameter sniffing UDFs Complicated predicates

Cursors and while loops Row-by-agonizing-row (RBAR) Could be useful: Running totals Calling SPs Administrative tasks Default settings: global, scrollable, writeable, dynamic Use LOCAL FAST_FORWARD sp_MSForEachDB is dangerous!

(NOT) IN and friends NULLS are neither equal nor different NOT IN on columns with NULLS will return incorrect data Large IN lists are a performance problem EXCEPT eliminates dups  sort Anti semi join Personally, I’d always prefer (NOT) EXISTS

Updating data Indexes Referential integrity Cascades Large inserts – disable indexes first Consider the whole workload, not just SELECTs Referential integrity Cascades Triggers and rollback from trigger Use CTEs to minimize human errors

Stored procedures SPs are not faster than ad hoc, but… Security is a plus Easier maintenance and tuning Parameterization Greater chance of plan reuse Parameter sniffing problem Recompilation SET NOCOUNT ON

Dynamic search procedures Freely combined search conditions Static code for every combination Maintenance nightmare Static code with WHERE (col = @param or @param IS NULL) Performance issues, recompilation Dynamic SQL with sp_executesql Plan reuse Strongly-typed parameters Injection

OMGs (common misconceptions) Order of predicates matters Table variables are stored differently from #tables COALESCE is better than ISNULL UDF are good Adding hints makes it run faster