Stored procedure tunning

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SELECT statements - Try to use only the required number of columns in the SELECT clause instead of using *. Using * returns all columns, which unnecessarily create a fat recordset.

Don’t use the * when selecting data

SELECT * FROM dbo.MyTable

Explicitly name each column you want to be returned in your SELECT statement

SELECT Name, Description, DateEntered FROM dbo.MyTable

This allows you to add new columns to your table without breaking your code Minimizes network traffic by only returns the columns you need.



SQL-92 - Always try to use ANSI 92 syntax. Till now the old syntax is working the old syntax will be deprecated in the next release of MS SQL server.

As an example, for joining, use

SELECT * FROM employee e1 INNER JOIN employee _dtl e2ON e1.id = e2.id

Instead of

SELECT * FROM employee e1, employee_dtl e2WHERE e1.id = e2.id



CREATE TABLE vs. SELECT INTO - Select * INTO works fine for small tables, but when dealing with large record sets or long-running queries, it creates locks on the system objects within the tempdb database. As a result, other queries and procedures that need to create objects within the tempdb database will have to wait for the long-running query to complete. This is because when an object is created, an exclusive lock is taken against the sysobjects, syscolumns, sysindexes tables.

Update: This problem has been greatly reduced from the MS SQL 7.0.

Also, SELECT INTO not only copies data but also it copies the table structure, hence it performs slower.



Try to use table variables instead of Temporary Tables - Temp tables can cause stored procedures to recompile. (From SQL 2005, using temporary table not always causing recompilations. But adding rows to temporary tables may cause recompilations). But table variables were designed specifically to guard against stored procedure recompiles during execution.

If the result set is not containing a huge number of records then you should stick to table variable, otherwise temp table has its advantages. There is a misconception that temp tables always use the tembdb database but table variable do not. Table variables also use tempdb after a certain size



Use proper indexes - You can use the help of the data tuning advisor, but it does not gives the proper result all the time. Index scans and index seeks are much faster than table scans. So identify the table scans from the execution plans. But when a table returns smaller number rows, then it is better to use a table scan.



Subquery vs JOINs - This is a famous debate in many online forums. In fact most sub queries can be expressed as an equivalent form of JOIN. I have a good rule of thumb: subquery is faster when we have to retrieve data from large number of tables because it becomes tedious to join more tables. JOIN is faster to retrieve data from database when we have less number of tables. But try to avoid correlated sub queries because it makes the query much slower.



Avoid using cursors - Using cursors make the program slower as it works against SET based SQL. Try to use temporary table/table variables with identity column and then iterate all the tables using WHILE loop and a looping counter, which will map with the identity column.



Avoid DISTINCT and ORDER BY - If you don't need the DISTINCT/ORDER BY clause, then try to avoid so. Unnecessary DISTINCT or ORDER BY clauses cause extra work for the database engine. Hence making performance slower. (Sometimes ORDER BY helps to speed up the operation).



CAST and CONVERT - Try to use CAST instead of CONVERT. CAST is ANSI-92 standard but CONVERT works in MS SQL server only. Also, some CONVERT styles may be deprecated in future MS SQL releases. It is better to use CONVERT only when you need to format the DATETIME datatype with the style option. CAST cannot do this.



More WHERE clause hints - Avoid unnecessary conditions in the WHERE Clause. Also try to avoid a function in the WHERE clause as it presents SQL engine to do index seek. Even it forces SQL full index scans or even table scans.

Also, try to avoid IN. While checking the existence of some values, then use EXISTS instead of IN. IN counts the NULL values also, but EXISTS not. EXISTS returns Boolean(Yes/No) but IN returns all values hence result set for IN is heavier than EXISTS.

Here, teacher and student table has 1:M relationship and I want to find the teachers who have students. Both the queries have the same result but the second query will run faster because of EXISTS operator.

SELECT name FROM teacherWHERE teacher_id IN (SELECT teacher_id FROM student)SELECT nameFROM teacher WHERE EXISTS (SELECT 1 FROM student where teacher.teacher_id = student.teacher_id)



Capturing Both @@ROWCOUNT and @@ERROR After an UPDATE Statement

declare @rowcnt int,@error intUPDATE dbo.titles set price = price * 1.10where type = 'fiction'select @rowcnt = @@ROWCOUNT, @error = @@ERRORif @rowcnt = 0print 'no rows updated'if @error <> 0raiserror ('Update of titles failed', 16, 1)return



Variables - Use as few as possible variables. It frees spaces in cache.



Dynamic Queries - Try to minimize the usage of dynamic queries. If you are using a dynamic query like: SELECT * FROM mydb.dbo.emp where empid = @eid then there is no problem. You can supply a value for the @eid parameter and there is no recompilation of the execution plan in the database cache. But if you are using a SQL query like

SELECT * FROM emp where empid = " + @eid and supply a parameter (say 100), then the cache will keep the execution plan for the value of 100 only. If the id changes (to say 101), it will recompile the statement. Hence, this approach is slower than the previous one. (You can get the exact value of the SQL statement from Profiler)



Fully Qualified Names - Always use the fully qualified name when calling stored procedures. This would be the format database_name.schema_name.table_name. For example, use EXEC master.dbo.Your_Proc_name instead of EXEC Your_Proc_name This is a very common mistake, which causes an extra trip to the procedure cache to get the execution plan for execution. Also try to use the schema name while creating a procedure. Like: CREATE PROCEDURE dbo.Your_Proc_name instead of CREATE PROCEDURE Your_Proc_name



SET NOCOUNT ON - This suppresses the message that shows number of rows affected by SQL statement. Otherwise this can cause extra network traffic and can have some serious impact on performance when the procedure is called frequently.Its recommend to use SET NOCOUNT ON for the shake of performance unless there is a very good reason for using it.



Naming Procs

Do NOT use sp_xxx as a naming convention.Causes additional searches and added I/O.

SQL Server will scan the procedure cache for Master, no matter what database the procedure was executed from

SQL Server will then acquire an exclusive COMPILE lock to perform a second search in the local database

If a user stored procedure has same name as an sp_xxx stored procedure in MASTER, then the user procedure will NEVER be used.



The sp_ prefix - Don't use the "sp_" prefix in a stored procedure name as the "sp_" prefix is reserved for system stored procedures. Any stored procedure that has the "sp_" prefix will cause an extra lookup in the MASTER database If a stored procedure uses same name in both the user database and a system database, the stored procedure in the user database will execute but first it will find the procedure in resource database and then the user database (for SQL server 2005/2008/2012) hence causing an extra burden to the server.



sp_executeSQL and the KEEPFIXED PLAN options- Both sp_executesql and the KEEPFIXED PLAN option avoid the recompilation of a stored procedure. If you want to provide parameterized dynamic SQL, then go for sp_executesql instead of EXEC(proc_name). Here the execution plan for the procedure is stored with the variable name in cache memory. When the variable values are supplied, then the values are simply mapped to the query, hence no need for a recompilation.Update: Keep in mind that recompilations are not always bad. Sometimes, using an old and inefficient plan can make the procedure more slower.Use the OPTION KEEPFIXED PLAN hint while selecting records from temporary tables. If the query contains this hint, then its plan is not recompiled CREATE PROCEDURE my_procASCREATE TABLE #t (a int )SELECT * FROM #tINSERT #t SELECT * from retestSELECT COUNT(*) FROM #t WHERE a = 37OPTION (KEEPFIXED PLAN)As an example of sp_executesql, we can write:sp_executesql N'SELECT * FROM mydb.dbo.emp where empid = @eid', N'@eid int', @eid=40



Calling ProcsUse stored procedure calls rather than embedded SQL

EXEC versus SP_EXECUTESQL

same behavior with regard to batches, the scope of names, and database context

EXEC compiles entire SQL at one time

SP_EXECUTE compiles and executes as an execution plan separate from the execution plan of the batch that called sp_executesql itself.

SP_EXECUTESQL executes a Transact-SQL statement or batch that can be reused many times, or that has been built dynamically, using a single execution plan.

Often better than EXEC.



SELECT vs SET - A single SELECT statement can assign values to different variables and is much faster than multiple SET statements assigning values to multiple different variables.

SELECT @Var1 = @Var1 + 1, @Var2 = @Var2 – 1

instead of

SET @Var1 = @Var1 + 1SET @Var2 = @Var2 - 1



WHERE clauses - In a WHERE clause, the various operators used directly affect how fast a query can run. Here are the conditional operators used in the WHERE clause, ordered by their precedence.

=, >, <, >=, <=, <>, !=, !>, !<



Include Column List in INSERT Statement

Explicitly name the columns you are going to insert with your INSERT statement

Do ThisINSERT INTO A (COLA, COLB) VALUE(‘A’, ’B’);

Don’t do thisINSERT INTO A VALUE(‘A’, ’B’)

This allows you change the format of your table by adding or removing columns without breaking your INSERT statement.



Avoid using wildcard at the beginning of your search criteria…WHERE LastName LIKE ‘%sen’ Doing this causes an index scan Place a prefix in front of wildcard to SQL can use index

seek to find the records for your search criteria:…WHERE LastName LIKE ‘L%sen’. Might consider implementing full text search if you doing

lots of string searching



Scalar Function Calls in Column List and WHERE clause

Don’t execute scalar function in column list in a SELECT statement or WHERE clause, if the scalar value function returns dataDoing this causes cursor like performanceFunction will be called for ever row returnedReplace scalar function with:

JOIN logic inline table value function view



Referencing Column Names in Function Calls Within WHERE Clause

Don’t use column names within a function call

SELECT * FROM MyTableWHERE DATEADD(DAY,15,MyDate) = ‘05/01/2009’

Column that are used in functions are seen as expressions instead of columns, therefore indexes can not be usedDoing this causes SQL Server to do an index scan or table scanInstead move the column name outside the function call if possible

SELECT * FROM MyTable WHERE MyDate = DATEADD(DAY,-15,’05/1/2009’)



TransactionsAvoid nested transactions. They aren’t truly nested:

COMMIT only saves data from the outermost transaction ROLLBACK nukes ALL transactions, both innermost and

outermost

Orphaned Transactions Errors don’t usually abort a transaction except for deadlocks Returning from a procedure or batch does NOT commit a

transaction. Once a transaction is started, it remains open until: The transaction is committed The transaction is rolled back The connection ends

Use @@TRANCOUNT or sys.dm_tran_active_transactions to look for orphaned transactions when entering a new routineKeep transactions as short as possible!Keep transactions explicit!Remember lock escalation!



DDL/DML in Stored ProcedureMixing DDL and DML operations causes a recompile

Certain operations on temporary tables cause a recompile Refer to temp tables created locally Don’t declare cursors that reference a temp table Don’t create temp tables while in a loop



Interleaving DML/DDL Statements

Objects that dont's exist at procedure first execution cannot be optimized until statement execution.

Upon execution of a DDL statement the procedure gets recompiled to recompile the plans for the DML.

Don't interleave DDL and DML, Seperate it.

All DDL at the beginning of the proc, All DML later.

Try not to create tables conditionally ( IF create ... ELSE create ...)

Use KEEP PLAN on SELECT statement id Data changes more than 6 times but the plan should not change.



Queries with LIKE

Queries on production systems should NOT use SELECT * FROM…

Main reason is that any time the underlying table is changed, all query plans stored in the cache must be rebuilt

The SQL tools allow very quick scripting – so no excuses!

Queries that use the LIKE clause have two simple rules: LIKE can use indexes if the pattern starts with a character

string, such as WHERE lname LIKE ‘w%’ LIKE cannot use an index if the pattern starts with a leading

wildcard, such as WHERE lname LIKE ‘%alton’



Queries with Functions & Calculations in the WHERE clause

Avoid using functions or calculations on the column in a WHERE clause because it causes SQL Server to ignore any index on the column:

WHERE qty * 12 > 10000

WHERE ISNULL(ord_date, ‘Jan 01,2001’) > ‘Jan 01, 2002 12:00:00 AM’

Instead, move the function or calculation to the SARG:

WHERE qty > 10000/12

WHERE ord_date IS NOT NULL AND ord_date > ‘Jan 01, 2002 12:00:00 AM’



Modifying Procedures

DROP and RECREATE Loses the dependency chain stored in sysdepends Loses the permissions already granted Invalidates all plans

ALTER PROC Loses the dependency chain stored in sysdepends Retains the permissions Invalidates all plans

To retain the dependency chain you must also ALTER all procedures that depend on the procedure being altered.



Temp Table Usage

Temp Table can create excessive recompilations for procedures. Consider creating permanent tables (with indexes) and manipulating data there.

Consider dropping and re-creating or rebuilding indexes as part of the procedure instead!

Try not to create tables conditionally (IF create… ELSE create…)

Use Profiler to see if there are significant recompiles Use KEEP PLAN on SELECT statements if data changes

more than 6 times but the plan should not change



Table Variable Usage

Scope is limited to the local procedure\transactionDoes not cause excessive recompiles due to local

only access No re-resolution on CREATE/ALTER Temp Tables need re-resolution for nested procedures

Only Key Indexes can be created Definition of Table allows PRIMARY KEY/UNIQUE

constraint indexes Use TEMP TABLES if large volumes of data will be

manipulated – create the right indexes for access

Population Does not support INSERT EXEC Does not support SELECT INTO



Community Resources

John Theron - Quest Software Patrick O'Keeffe - Quest Software Arup Chakraborty - SQLServerCentral.com Victor Isakov Kimberly L. Tripp,Solid Quality Learning –

SolidQualityLearning.com /SYSolutions, Inc. – SQLSkills.com



Thank You !

Virendra Yaduvanshihttp://wikidba.wordpress.com/

Email : [email protected]


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Stored procedure tunning