Archive for December, 2009

Some Hidden Facts: Stored Procedure and Its Optimization

December 26, 2009 2 comments

We create stored procedures in database applications for several reasons and benefits like enhanced performance, security, code maintenance, etc. But with time we may see that these stored procedures are not performing well as expected. There can be many reasons like dependent objects (tables, indexes, execution plans, and data size) changed or stored procedures executing improperly. So we have to be a little bit careful from the beginning of creating stored procedures to execution.

I like to summarize few points about all these things.

A. Stored Procedure Recompilation
When creating a stored procedure, we can specify WITH RECOMPILE option in it. But such stored procedure will never benefit from the cached execution plan as each time it is executed; it forces the cached execution plan to invalidate or flush and create new plan based upon the parameters passed, if any, to it. I do not see any such big benefits of this option. But one may find this useful when such stored procedure will return results or execute only selective part of the stored procedure body based upon supplied input parameters. For example, statements within If-block or Select-Case block based upon input parameters.

But I still feel one should go after the new feature of SQL Server 2008 that helps recompile statement level queries rather than whole stored procedure. But this option will be quite useful as such recompilation is dependent upon input data. Let’s say if you are executing the stored procedure by supplying input parameter like ‘FirstName’ or ‘LastName’ or ‘DateOfBirth’ at a time, then statement-level query recompilation option is better. To use this method, one has to specify that SQL-statement within stored procedure with RECOMPILE query hint.

For example,
CREATE PROC dbo.uspExample
@x_input AS INT,
@y_input AS INT
If @x_input = 1
SELECT x, y, z
FROM dbo.tblXYZ
WHERE xColumn >= @x_input
OPTION(RECOMPILE);               —See here the query hint
If @y_input = 2
SELECT x, y, z
FROM dbo.tblXYZ
WHERE yColumn >= @y_input

Another tool that we can use is sp_recompile system stored procedure. This procedure forces recompile of user defined stored procedure next time it is run.

Let’s look at its syntax first.
Sp_recompile ‘@somedependent_object’;

Here, ‘@somedependent_object’ can be either table or view or another stored procedure or even trigger. If this ‘@somedependent_object’ is table name, then sql server will compile all the stored procedures that references this table. If this object name is some stored procedure, then this stored procedure will recompile next time it is run.
This option is good when the table properties are changed, and this table is in use of many other stored procedures. Instead of recompiling each and every such depending stored procedure, a simple sp_recompile will do enough, and with no server restart!

But there are many other scenarios when stored procedure recompilation can happen automatically. If server is running out of memory, the execution cache will get flushed. If the stored procedure has some session specific SET modifiers (like LOCK TIMEOUT, DATEFIRST, ANSI_WARNINGS, etc), then also stored procedure can recompile.

Situations like when we mix DDL and DML statements together inside stored procedure may also cause stored procedure recompilation. For example, most of us create temporary tables inside stored procedure. Then, do DML operations based upon values in these temporary tables. Even this type of stored procedure is forced to recompile to create new plan according to such new temporary tables. For this type of stored procedure, one should opt for statement-level recompilation by using RECOMPILE hint in that DML statement following DDL statements.

But what is alternative to temporary tables here? We can use table variables instead! Or, if we cannot do without temporary tables, then we should write such DDL statements in the beginning of stored procedure body itself so that multiple recompilations are not happening for a single call to this stored procedure.

So stored procedure recompilation can be harmful and useful both depending upon situations. One has to think twice whether recompilation is required or not.

B. Stored Procedure Name
We should always create a stored procedure with a full naming convention. Schema name should always prefix the stored procedure name. Schema name will help sql server name resolution easily when it is called. This helps sql server in which schema to query the stored procedure.

C. Table Indexes
Tables should have proper indexes and compiled time to time as indexes may be weird off after some time due to huge data insertion or deletion.

This is all about some useful facts about stored procedure.