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Improving my SQL BI Skills

Improving my SQL BI Skills

Daily Archives: June 23, 2015

LIKE IS SARGable, How to write better Where Clause –II. Let’s Check?

23 Tuesday Jun 2015

Posted by Pawan Kumar Khowal in SQL Performance Tuning

≈ 1 Comment

Tags

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LIKE IS SARGable, How to write better Where Clause –II. Let’s Check?

Download PDF – [LIKE IS SARGable, How to write better Where Clause – II]

SQL Server has many functions that can be used in your WHERE clause and in the SELECT clause. Now these functions can be user defined or inbuilt. These functions normally provide functionality which would be very difficult to get without these functions.

Now when these functions are used improperly in the WHERE clause these functions can cause major performance issues. Now very few SQL functions are SARGable and most of them are NOT SARGable

Now what is SARgable?

It is Search Argument able. It’s the ability of the query optimizer to use indexes.

Let’s see an example that shows using a function in the WHERE clause can affect performance. Add actual execution plan and statistics IO ON for better understanding.

Connect to AdventureWorks2012 & create a NonClustered Index on [Person].[Person](FirstName). Now we are finding all the persons having name start with Letter “K”. Let’s check two queries to get desired results and their execution plan.


--


SET STATISTICS IO ON 

/****************    QUERY 1       *********************/


SELECT FirstName FROM [Person].[Person]
WHERE LEFT(FirstName,1) = 'K'


/****************    QUERY 2       *********************/


SELECT FirstName FROM [Person].[Person]
WHERE FirstName LIKE 'K%'



--

First let’s check out the messages tab. Both the above queries are working fine and we got 1255 rows in both the case. Now let’s check the logical reads (1 Logical Read = 8KB Pages) for both the queries. First query is reading 66 pages and the second query is reading only 7 pages. It is clearly evident that second query is performing much better than the first one.

Pawan Kumar Khowal - Where clause - 31

Now let’s look out the execution plan for both the queries.

Pawan Kumar Khowal - Where clause - 32

We can clearly check out the Query 1 is having Index Scan means reading all the records of the table from leaf level and it is taking 91% of the cost. On the other hand second query is having nonclustered seek and taking only 9% of the cost. Now if we use second query then our system can become at least 10% more scalable; which is a very good thing.

Summary

When we are checking things for performance always try to find out these small things. These can help you queries performance greatly. We are reducing I/O by reading less data pages. So by not using functions in the WHERE clause we can provide big performance gains. We just have to tweak where clause. That should be easy.

That’s all folks; I hope you’ve enjoyed learning about Like IS SARGable, How to write better Where Clause –II and I’ll see you soon with more “Performance Tuning” articles.

Thanks!

Pawan Kumar Khowal

MSBISKills.com

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CAST IS SARgable, How to write better Where Clause. Let’s Check?

23 Tuesday Jun 2015

Posted by Pawan Kumar Khowal in SQL Performance Tuning

≈ Leave a comment

Tags

26 performance tuning questions and solutions, CAST IS SARgable, How to tune SQL queries, Interview questions for SQL Server Performance Tuning, Looking for SQL Optimization Interview Questions, Optimizer, Pawan, Pawan Khowal, Pawan Kumar, Pawan Kumar Khowal, performance sql server, Performance tips for faster SQL queries, Performance Tuning, Performance Tuning for SQL Server, Query Optimization, Query Performance Tuning, SARgable, SQL Complex Queries, SQL Optimization Interview Questions, sql performance, sql performance and tuning, sql performance explained pdf, sql performance tips, SQL Performance Tuning, sql performance tuning and optimization, sql performance tuning interview questions, sql performance tuning tips, SQL Query Optimizer, SQL Query Tuning or Query Optimization, SQL SERVER Interview questions, SQL server optimization interview questions and answers, sql server performance query, sql server performance slow, SQL Server Performance Tuning, SQL Server Performance Tuning Tips, SQL SERVER Tips, SQL Tuning, SQL Tuning Overview, Tips for SQL Database Tuning and Performance, Top 10 performance tuning tips for relational databases, what is SARgable?


CAST IS SARgable, How to write better Where Clause. Let’s Check?

Download PDF – CAST IS SARgable, How to write better Where Clause. Let’s Check?

SQL Server has many functions that can be used in your WHERE clause and in the SELECT clause. Now these functions can be user defined or inbuilt. These functions normally provide functionality which would be very difficult to get without these functions.

Now when these functions are used improperly in the WHERE clause these functions can cause major performance issues.

Now very few SQL functions are SARgable and most of them are NOT SARgable.

Now what is SARgable?

It is Search Argument able. It’s the ability of the query optimizer to use indexes. Well we can tweak where clause of our queries little bit so that optimizer can use indexes.

Let’s see an example that shows using a function in the WHERE clause can affect performance. Add actual execution plan and statistics IO ON for better understanding.

Let’s say we have a table called CustomerInfo with around 20K records. This Customerinfo table has 3 columns named (CustomerID, CustomerName, and ModifiedDate). This table also has 2 indexes which are given below-

index_name index_description index_keys
Ix_ModifiedDate nonclustered located on PRIMARY ModifiedDate
PK__Customer clustered, unique, primary key located on PRIMARY CustomerID

Now let’s say we wanted to find out below all modified dates when ModifiedDate = ’08/29/1947′. So we wrote the below query-


--

SET STATISTICS IO ON

SELECT ModifiedDate FROM CustomerInfo
WHERE CONVERT(VARCHAR(10),ModifiedDate,101) = '08/29/1947'

--

Now the above query is perfectly fine, it’s give us data that we wanted but the problem here is that we are not using any indexes effectively here. It is reading all the pages from the leaf level of the tree. Let’s check out the execution plan.

In the below execution plan we can clearly see that we are doing nonclustered scan of complete leaf pages even we have index on this columns.

Pawan Kumar Khowal - Where Clause - Part21

So this is not good. Now let’s rewrite this query in a different fashion and check its execution plan.


--

SET STATISTICS IO ON

SELECT ModifiedDate FROM CustomerInfo
WHERE CAST(ModifiedDate AS DATE) = '08/29/1947'

--

Pawan Kumar Khowal - Where Clause - Part22

Now as per the execution plan we are doing index seek which is a better approach in this case. But here we are doing some other physical operations like Nested Loop, Compute Scalar and Constant scan. Although these operators are not taking any cost but the question we have right now is can we remove these extra physical operators.

Let’s rewrite this query in some other fashion and check out its execution plan.


--

SET STATISTICS IO ON

SELECT ModifiedDate FROM CustomerInfo
WHERE ModifiedDate >= '08/29/1947' AND ModifiedDate < '08/30/1947'

--

In the below execution plan we can see that we have only one physical operation that is index seek. This type of seek is called range based seek which is good in this case. Well you choose query 2 or query 3 depending upon your requirement but certainly you would like to change query 1.

Pawan Kumar Khowal - Where Clause - Part23

Summary

When we are checking things for performance always try to find out these small things. These can help you queries performance greatly. We are reducing I/O by reading less data pages. So by not using functions in the WHERE clause we can provide big performance gains. We just have to tweak where clause. That should be easy.

That’s all folks; I hope you’ve enjoyed learning about Avoid functions in the where clause – Part II, and I’ll see you soon with more “Performance Tuning” articles.

Thanks!

Pawan Kumar Khowal

MSBISKills.com

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