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

2. 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

3. 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

4. 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

5. 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!

6. (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

7. 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

8. 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

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

10. 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