Skip to main content

SQL Moment of Clairity

I was working on a stored procedure the other day, not one that I wrote originally but one that was causing performance issues in out system. The procedure was taking any where from 40 to 60 seconds to run, which is unacceptable in a web service. This in fact was causing the web service to time out.

An inspection of the execution plan showed that one part of the procedure was doing an index scan and that it was this that was taking up over 85% of the execution time. I was totally baffled, it was hitting the index so why wasn’t it faster?

Then I learned three things all at once:

  • Implicit Conversion

  • non-sargable kills performance

  • Index Seek is preferred of Index Scan


Implicit Conversion

In the stored proc a variable was defined as an NVARCHAR(20) but the field in the table it was being compared to was a CHAR(10). This lead to an implicit conversion of the variable to a CHAR(10), which lead into the next issue:

Non-Sargable Kills Performance

Sargable refers to the pseudo-acronym “SARG” – Search ARGument and refers to a WHERE clause that compares a constant value to a column value. The implicit conversion was causing a non-SARGable condition which means the WHERE clause cannot use an index.

Index Scan

Because of the non-sargable condition an Index Scan was being performed. An Index Scan is just as bad as a Table Scan in the SQL realm and should be avoided at all costs.

The solution was simple: Change the variable to a CHAR(10). After doing that the Index Scan became an Index Seek and the whole stored procedure returned in less than a second. Any time I see an order of magnitude improvement like that from one simple change it just boggles my mind.

Comments

Popular posts from this blog

Json for jqGrid from ASP.Net MVC

jqGrid takes a specific format for its json (taken from jqGrid documentation): [js]{ total: "xxx",page: "yyy", records: "zzz", rows : [ {id:"1", cell:["cell11", "cell12", "cell13"]}, {id:"2", cell:["cell21", "cell22", "cell23"]}, ... ]}[/js] The tags mean the following: total - Total number of Pages. page - Current page Index. records - Total number of records in the rows group. rows - An array with the data plus an identifier. id - The unique row identifier, needs to be an int from what I have found. cell - An array of the data for the grid. The ASP.Net MVC framework has the JsonResult response type which we can use to populate the jqGrid. As an example I created a Person model and a method to return some data: [csharp] public class Person { public int ID { get; set; } public string Name { get; set; } public DateTime Birthday { get; set; } } public I

Changing Typed DataSet Connection String

I was working on a WinForm app that connected to a MS Access database. Yeah, Access sucks but I didn't have a choice in the matter. The app itself is used to import a bunch of CSV files into the Access database. It is more of a utility program and it has going through several variations, from being a simple hand driven command line tool to being GUI driven. The command line version was all hand controlled. I had to go in and update configuration files to point to the CSV files and the MDB database. That got old fast so I decided to make a GUI version that would allow me to pick the MDB file and each of the CSV files to import. Picking and using the CSV files was easy, it was changing the connection string for the MDB that proved to be the hardest. I am using strongly typed datasets in VS2005. If you have ever worked with them you find out soon that the connection string gets saved with the project in the app.config file, even if it is a seperate DAL dll project. My guess is

Remote Controlled RoboTank

This is my version of the ever popular to build RoboTank. It uses an Arduino Mega 2560 with the AdaFruit motor shield and an XBee S1 to communicate to the DFRobot Gamepad. The sketch for the RoboTank makes use of the AFMotor.h to drive the motors and includes a serial parser to read and process the commands coming from the Gamepad. Robotank-Sketch.zip DFRobot Wireless Joystick