Slow in the Application, Fast in SSMS? An SQL text by Erland Sommarskog, SQL Server MVP. Last revision: 2. Copyright applies to this text.

See here for font conventions used in this article. This article is also available in Russian, translated by Dima Piliugin. Introduction. When I read various forums about SQL Server, I frequently see questions from. They have identified a slow query or slow stored procedure. They take the SQL batch from the application and run it in SQL Server. Management Studio (SSMS) to analyse it, only to find that the.

At this point. they are inclined to think that SQL Server is all about. A similar mystery is when a developer has extracted a query in his stored. No, SQL Server is not about magic. But if you don't have a good understanding. SQL Server compiles queries and maintains its plan cache, it may seem so.

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In this article, I will try to straighten out why you get this seemingly inconsistent behaviour. I explain how SQL Server uses the cache, and why there may be. Once you have come this far, you. SSMS. To understand how to address that performance problem in your application, you. I first make a short break from the theme of parameter sniffing.

This is followed by two chapters how to deal with. The first is about.

In the second chapter I discuss some scenarios. Next comes a chapter where I discuss how dynamic SQL. SSMS and the application with. SQL. In the last chapter I look at how you can use the Query Store that was introduced in SQL 2.

At the end there is a. Microsoft white papers and similar documents in this area. Presumptions. The essence of this article applies to all versions of SQL Server, but the focus is on SQL 2. SQL 2. 00. 0 and earlier versions had far less instrumentation in this regard. This. database shipped with SQL 2. For later versions of SQL Server you can. Microsoft's web site.

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This is not a beginner- level article, but I assume that the reader has a. SQL programming. You don't need to have any prior experience of.

I will. not explain the basics in depth, as my focus is a little beyond that point. This article will not. Caveat: In some places, I give links to the online version of Books Online. Beware that the URL may lead to Books Online for a. SQL. Server than you are using. On the topic pages, there is a link Other versions, so that you easily can go to. SQL Server. you are using.

If your application does not use stored procedures, but. SQL statements directly, most of what I say this chapter is still applicable.

But there are further. SQL, and since the facts about stored procedures are confusing enough I have deferred the discussion on dynamic. SQL to a separate chapter. What is a Stored Procedure? That may seem like a silly question, but the question I am getting at is What objects. SQL. Server builds query plans for these types of objects: Stored procedures.

Scalar user- defined functions. Multi- step table- valued functions. Triggers. With a more general and stringent terminology I should talk about modules. Bigfix Manage Software Distribution Packages From Home.

I prefer to. talk about stored procedures to keep it simple. For other types of objects than the four listed above, SQL Server does not build query plans.

Queries like: SELECT abc, def FROM myview. SELECT a, b, c FROM mytablefunc(9)are no different from ad- hoc queries that access the tables directly. When. compiling the query, SQL Server expands the view/function into the query, and. There is one more thing we need to understand about what constitutes a stored.

Say that you have two procedures, where the. CREATE PROCECURE Outer. The execution plan for Outer. However, there is a very similar situation where. I've noticed that posters on SQL forums often have a.

SQL: CREATE PROCEDURE Some. The generated. SQL string is not. Some. This applies, no matter if the dynamic. SQL is executed through EXEC() or sp. For each query, SQL Server looks at the distribution statistics it.

From. this, it makes an estimate of what may be best way to execute the query. This. phase is known as optimisation. While the procedure is compiled in one go.

This has a very important ramification: the optimizer has no idea. However, it does know what values. Parameters and Variables. Consider the Orders table in the Northwind database, and these three procedures: CREATE PROCEDURE List. The plan for the third execution is different.

In this case, SQL Server scans the table. To understand why the optimizer makes certain decisions, it is always a good idea to look at what. If you hover with the mouse over the two Seek operators and the Scan operator, you will see the pop- ups similar to those below. Note: the exact appearance depends on which version of SSMS you are using. The samples above were captured with SSMS 2.

SQL 2. 00. 8. If you use a later version of SSMS, you may see the items in a somewhat different order, and there are also more items listed. None of those extra items are relevant for the example, which is why I have preferred to keep my original screen shots. The interesting element is Estimated Number of Rows. For the first two procedures, SQL Server estimates that one row will be returned, but for. List. This difference in estimates explains the different choice of plans.

Index Seek + Key Lookup is a good strategy to return a. But if more rows match the seek.

SQL. Server will need to access the same data page more than once. In the extreme case where all rows.

With a. SQL Server has to read every data page exactly once, whereas with. The Orders table in Northwind has 8.

SQL Server estimates that as many. Where Do These Estimates Come From? Now we know why the optimizer arrives at different execution plans: because the estimates are different. But that only leads to the next question: why. That is the key topic of this article. In the first procedure, the date is a constant, which means that the SQL Server only needs to consider exactly this case.

It interrogates the statistics for the. Orders table, which indicates that there are no rows with an Order. Date in the. third millennium. When performing the optimisation, SQL Server knows that the. Since it does not any perform flow analysis, it can't say.

This strategy of looking at the values of the input. The input value is copied to a. SQL Server builds the plan, it has no understanding of. I don't know what the value of this variable will.

Because of this, it applies a standard assumption, which for an inequality. Here is a variation of the theme: CREATE PROCEDURE List.

Say that the user invokes the procedure. EXEC List. That is, Index Seek + Key Lookup, despite.

If you. look at the pop- up for the Index Seek operator, you will see that it is identical to the pop- up for List. Since all comparisons with NULL yield. UNKNOWN, the query cannot return any rows at all, if @fromdate still. If SQL Server would take the input value as the final. Constant Scan that does not access the table at all (run the query SELECT * FROM Orders WHERE Order. Date >. NULL to see an example of this). But SQL Server must generate a plan which.

On. the other hand, there is no obligation to build a plan which is the best for all. Thus, since the assumption is that.

SQL Server settles for the Index Seek. This is. because SQL Server never uses an estimate of 0 rows.)This is an example of when parameter sniffing backfires, and in this.

CREATE PROCEDURE List. For a local variable, SQL Server has no idea at all of the run- time value, and applies standard assumptions. More about this later. Putting the Query Plan into the Cache. If SQL Server would compile a stored procedure – that is optimise and build a query plan – every time the procedure is executed, there is a big risk that SQL Server would.

CPU resources it would take. I immediately need to qualify. In a big data warehouse where a. But in an OLTP database where plenty of users run stored. For this reason, SQL Server caches the query plan for a stored procedure, so.

The plan will stay in the cache, until some. Examples of such events are: SQL Server's buffer cache is fully utilised, and SQL Server needs to age.