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Posted Jan 21, 2004

Benchmarking Performance of a Query - Part 1 Elapsed Time

By Gregory A. Larsen

How many times have you had more than one way to do something and wanted to determine which method was faster and/or which one used more resources? I am going to guess that most developers have run across this dilemma more than once. So how do you decide with method is faster--or which method requires more resources? In this two part series, I will discuss some of the methods I use in Query Analyzer (QA) to help me decide which coding method might be faster and use less resources.

So when we talk about how fast a chunk of code runs, what do we really mean? Are we talking about how much CPU the code consumes to accomplish its task, or are we talking about the number of I/O's the code takes to get all the data needed to return the result set? Alternatively, are we talking about the elapsed time it takes the code to run? In my mind, elapsed time is really the yardstick to use for how fast a piece of code runs. Elapsed time is the time customers have to wait while a piece of code runs. However, you still need to consider the cost of the code, and that is where CPU and I/O come into play. I consider CPU and I/O to be the resources consumed when running code. This two part series will look at techniques that can be used to identify the elapsed time, and resources used when you run a piece of code. This first article will discuss the methods I used to determine how fast code runs, or what I call the elapsed time of a query or set of T-SQL statements.

Elapsed Time

Elapsed time is the clock time it takes to run a piece of code. Basically, the elapsed time is the number of hours, minutes, second, and/or milliseconds it takes to run a chuck of code. Normally when we talk about how fast some code runs we are really talking about the overall elapsed time. Let's review a couple of different methods I use to review the elapsed time of a query.

Prior to performing an elapsed time benchmark, I first get my hands on the T-SQL code I want to benchmark. I then paste the code into QA on a stand-alone SQL Server machine, which is where I gather all my benchmark information. Using a stand-alone machine isolates my testing from performance issues that might arise in a multi-user environment. With QA, there are a couple of different ways to measure the elapsed time of a chunk of code.

The first method I use to get my initial benchmark for elapsed time is to use the Execution Time that is displayed in QA. The screen print below shows where you can find QA's Execution Time display. I have circled the display in red.

Above I executed a single SELECT statement 1,000,000 times. As you can see by using the Execution Time display, which is displayed in the lower right hand corner of the QA window, this particular set of T-SQL statements took 3 minutes, and 54 seconds to process.

Now, sometimes you need more than just the total execution time for a T-SQL batch, or you would like to know the milliseconds associated with elapsed time. Suppose you have a T-SQL batch that contains a number of different T-SQL statements and you would like to know how long each T-SQL statement runs, or how long a subset of the statements run. In this case, you would not be able to use the QA Execution Time display since it is for the entire batch. So in order to display execution time for each command or subset of commands, you would need to use a different method.

One of the methods I use to determine how long, a single T-SQL statement, or a series of statements in a batch takes, is to capture the start and end time for each command or series of statements in a batch for which I am interested in measuring elapsed time. This allows me to calculate the elapsed time for a single command, or a group of T-SQL statements. To demonstrate what I am talking about here is a chuck of code that displays the elapsed time for two different while loops.

declare @output varchar(100)
declare @i int
declare @st datetime

set @i = 0
set @st = getdate()
while @i < 2000
  select @output=OrderId from Northwind.dbo.Orders
  set @i = @i + 1
print 'First batch completed in: ' + 
      rtrim(cast(datediff(ss,@st,getdate()) as char(10))) +
      ' seconds!'
set @i = 0
set @st = getdate()
while @i < 4000
  select @output=OrderId from Northwind.dbo.[Order Details]
  set @i = @i + 1
print 'Second batch completed in: ' + 
      rtrim(cast(datediff(ss,@st,getdate()) as char(10))) +
      ' seconds!'

Below are the elapsed times calculated for each while loop when I ran this code against my server. These elapsed times were produced by the calculation contained in the print statements in the above code.

First batch completed in: 3 seconds!
Second batch completed in: 17 seconds!

By looking at the code you can see I am using a local variable @st to capture the start time for each while loop. At the completion of the while loop I calculate the time it took to process the entire while loop, and display the calculated elapsed time by using a PRINT statement. Using this method, allows me to determine the elapsed time for each while loop. As you can see the first while loop completed faster than the second one.

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