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Improving my SQL BI Skills

Improving my SQL BI Skills

Daily Archives: June 25, 2015

How to find latest order Id from Orders table for each product? (Top n orders for each product) Let’s write a better query.

25 Thursday Jun 2015

Posted by Pawan Kumar Khowal in SQL Performance Tuning

≈ Leave a comment

Tags

26 performance tuning questions and solutions, How to tune SQL queries, Interview questions for SQL Server Performance Tuning, Looking for SQL Optimization Interview Questions, performance sql server, Performance tips for faster SQL queries, Performance Tuning, Performance Tuning for SQL Server, Query Optimization, Query Performance Tuning, 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 Overview, Tips for SQL Database Tuning and Performance, Top 10 performance tuning tips for relational databases


How to find latest order Id from Orders table for each product? (Top n orders for each product) Let’s write a better query.

Download PDF – How to find latest order Id from Orders table for each product? (Top n orders for each product) Let’s write a better query.

Let’s say I wanted to find out the maximum sales order id for each project. Connect to Adventure works 2012 database. Now for that let’s say we wrote below query.


--

SELECT s.ProductID,MAX(SalesOrderID)
FROM [Sales].[SalesOrderDetail] s
GROUP BY s.ProductID

--

Well the above query works fine and returns 266 records. Now let’s examine the execution plan. Here we have a nonclustered index scan and a stream Aggregate. The Stream Aggregate is used to group some rows by one or more columns, and to calculate any aggregation expressions that are specified in the query. The common types of aggregation are: SUM, COUNT, AGV, MIN, and MAX.

Pawan Kumar Khowal - BadPlan1

Now problem with approach is that we are reading all the pages from leaf level and if we have million or billion row this approach will slow down the performance of the query. A more appropriate and better approach is given below.


--

SELECT p.ProductID,tr.SalesOrderID
FROM [Production].[Product] p
CROSS APPLY
	(
	  SELECT MAX(s.SalesOrderID) SalesOrderID
	  FROM [Sales].[SalesOrderDetail] s
	  WHERE s.ProductID = p.ProductID
	) tr

--

Now let’s put both the queries in the query window and execute. Let’s compare the execution plan and the cost taken by each query.

Pawan Kumar Khowal - Plan2

What we are doing in the second query is we are reading all product ids from products master table and then for each product id we are hitting orders table to find out the latest salesorderid which works perfectly in our case since we need only 1 salesorderid per product that too we are getting with the help of nonclustered index seek. Also check the cost each query took. The first query is taking 82% of the most and the second query is taking 18% cost only. Thus it is clearly evident that second query excels over first one.

Summary

Always execute your query and check what is there in the execution plan and see if you can tweak the query to get better performance. Apply operator is one of the most efficient tool to improve your queries.

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.

That’s all folks; I hope you’ve enjoyed learning about how we can write better that will perform better, and I’ll see you soon with more “Performance Tuning” articles.

Thanks!

Pawan Kumar Khowal

MSBISKills.com

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How to properly read an Execution Plan/Query Plan?

25 Thursday Jun 2015

Posted by Pawan Kumar Khowal in SQL Performance Tuning

≈ Leave a comment

Tags

26 performance tuning questions and solutions, How to properly read an Execution Plan/Query Plan?, How to properly read an Execution Plan?, How to read an Execution Plan?, How to tune SQL queries, Interview questions for SQL Server Performance Tuning, Looking for SQL Optimization Interview Questions, performance sql server, Performance tips for faster SQL queries, Performance Tuning, Performance Tuning for SQL Server, Query Optimization, Query Performance Tuning, SQL Complex Queries, SQL Execution plan, 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 Overview, Tips for SQL Database Tuning and Performance, Top 10 performance tuning tips for relational databases


How to properly read an Execution Plan/Query Plan?

Download PDF – How to properly read execution plan?

Data flows from RIGHT (TOP) to LEFT in the query plan. That is it data centric view of the query plan.

Logic in the query plan flows from LEFT to RIGHT. This is logic centric view of the query plan.

Well there is nothing like correct or incorrect way of reading the plan. It’s just that we have two different ways of reading the plan.

Pawan Kumar Khowal - How to properly read execution plan

All the operators used in the execution plan are called physical operators.

Let’s go through an example. We know that select and top operator cannot have rows from their own. They are dependent on Clustered Index scan. So when the execution starts the “Select” operator asks the “Top” operator that hey do you have any rows for me, Top operator says that I don’t have rows let me ask “Clustered Index Scan”. Hey “Clustered Index scan” do you have any rows for me and “Clustered index scan” says yes I have rows for you.

That is how the logic flows from left to right and data flows from right to left.

Summary

By reading the execution plan both the ways we can easily track down issues and bottlenecks very quickly. Basically we will easily understand what’s going on in the execution plan.

That’s all folks; I hope you’ve enjoyed learning about how we can read execution plan more effectively and I’ll see you soon with more “Performance Tuning” articles.

Thanks!

Pawan Kumar Khowal

MSBISKills.com

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