Top10stepstooptimizedataaccessinSQLServer.PartII

Introduction Remember we ware in a mission? Our mission was to optimize the performance of an SQL Server database. We had an application that was built on top of that database. The application was working pretty fine while tested, but, soo

Introduction

Remember we ware in a mission? Our mission was to optimize the performance of an SQL Server database. We had an application that was built on top of that database. The application was working pretty fine while tested, but, soon after deployment at production, it started to perform slowly as the data volume was increased in the database. Within a very few months, the application started performing so slowly that, the poor developers (including me) had to start this mission to optimize the database and thus, optimize the application.

Please have a look at the previous article to know how it started and what did we do to start the optimization process.

Top 10 steps to optimize data access in SQL Server. part I (Use Indexing)

Well, in the first 3 steps (Discussed in the previous article), we had implemented indexing in our database. That was because; we had to do something that improves the database performance in a quick amount of time, with a least amount of effort. But, what if we wrote the data access codes in an inefficient way? What if we wrote the TSQLs poorly?

Applying indexing will obviously improve the data access performance, but, at the most basic level in any data access optimization process, you have to make sure that you have written your data access codes and TSQLs in the most efficient manner, applying the best practices.

So, in this article, we are going to focus on writing or refactoring the data access codes using the best practices. But, before we start playing the game, we need to prepare the ground first. So let’s do the groundwork at this very next step:

Step4: Move TSQL codes from application into the database server

I know you may not like this suggestion at all. You might have used an ORM that does generate all the SQLs for you on the fly. Or, you or your team might have a “principle” of keeping SQLs in your application codes (In the Data access layer methods). But, still, if you need to optimize the data access performance, or, if you need to troubleshoot a performance problem in your application, I would suggest you to move your SQL codes into your database server (Using Stored procedure, Views, Functions and Triggers) from your application. Why? Well, I do have some strong reasons for this recommendation:

Despite the fact that indexing (In Step1 to Step3) will let you troubleshoot the performance problems in your application in a quick time (if properly done), following this step 4 might not give you a real performance boost instantly. But, this will mainly enable you to perform other subsequent optimization steps and apply different other techniques easily to further optimize your data access routines.

If you have used an ORM (Say, NHibernate) to implement the data access routines in your application, you might find your application performing quite well in your development and test environment. But, if you face performance problem in a production system where lots of transactions take place each second, and where too many concurrent database connections are there, in order to optimize your application’s performance you might have to re-think with your ORM based data access logics. It is possible to optimize an ORM based data access routines, but, it is always true that if you implement your data access routines using the TSQL objects in your database, you have the maximum opportunity to optimize your database.

If you have come this far while trying to optimize your application’s data access performance, come on, convince your management and purchase some time to implement a TSQL object based data operational logic. I can promise you, spending one or two man-month doing this might save you a man-year in the long run!

OK, let’s assume that you have implemented your data operational routines using the TSQL objects in your database. So, having done this step, you are done with the “ground work” and ready to start playing. So, let’s move towards the most important step in our optimization adventure. We are going to re-factor our data access codes and apply the best practices.

Step5: Identify inefficient TSQLs, re-factor and apply best practices

No matter how good indexing you apply in your database, if you use poorly written data retrieval/access logic, you are bound to get slow performance.

We all want to write good codes, don’t we? While we write data access routines for a particular requirement, we really have lots of options to follow for implementing particular data access routines (And application’s business logics). But, most of the cases, we have to work in a team with members of different calibers, experience and ideologies. So, while at development, there are strong chances that our team members may write codes in different ways and some of them miss following the best practices. While writing codes, we all want to “get the job done” first (Most of the cases). But, while our codes run in production, we start to see the problems.

Time to re-factor those codes now. Time to implement the best practices in your codes.

I do have some SQL best practices for you that you can follow. But, I am sure that you already know most of them. Problem is, in reality, you just don’t implement these good stuffs in your code (Of course, you always have some good reasons for not doing so). But what happens, at the end of the day, your code runs slowly and your client becomes unhappy.

So, knowing the best practices is not enough at all. The most important part is, you have to make sure that you follow the best practices while writing TSQLs. This is the most important thing.

Some TSQL Best practices

Don’t use “SELECT*" in SQL Query

Avoid unnecessary columns in SELECT list and unnecessary tables in join conditions

Use EXISTS and IN instead of NOT EXISTS and NOT IN (If possible)

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IF NOT EXISTS (SELECT * FROM table WHERE...)
BEGIN
 /* Statement Group 1 */
END
ELSE
BEGIN
 /* Statement Group 2 */
END 

It can be rewritten as:

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IF EXISTS (SELECT * FROM table WHERE...)
BEGIN
 /* Statement Group 2 */
END
ELSE
BEGIN
 /* Statement Group 1 */
END 

Do not use

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SELECT column_list FROM table WHERE 0 < (SELECT count(*) FROM table2 WHERE ..) 

Instead, use

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SELECT column_list FROM table WHERE EXISTS (SELECT * FROM table2 WHERE ...) 

Try to avoid joining between two types of columns

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SELECT column_list FROM small_table, large_table WHERE
smalltable.float_column = large_table.int_column 

In this case, SQL Server converts the integer column to float, because int is lower in the hierarchy than float. It cannot use an index on large_table.int_column, although it can use an index on smalltable.float_column.

Try to avoid deadlocks

Write TSQLs using “Set based approach” rather than using “Procedural approach”

-Use inline sub queries to replace User Defined Functions.
-Use correlated sub queries to replace Cursor based codes.
-If procedural coding is really necessary, at least, use a table variable Instead of a
cursor to navigate and process the result set.

For more info on "set" and "procedural" SQL , see SetAndProceduralSQL.aspx

Do not use COUNT(*) to obtain the record count in the table

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SELECT COUNT(*) FROM dbo.orders 

This query will perform full table scan to get the row count.

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SELECT rows FROM sysindexes 
WHERE id = OBJECT_ID('dbo.Orders') AND indid < 2 

Try to avoid dynamic SQLs

Instead of LIKE search, Use Full Text Search for searching textual data

Full text search always outperforms the LIKE search.

Try to use UNION to implement “OR” operation

Implement a lazy loading strategy for the large objects

Use VARCHAR(MAX), VARBINARY(MAX) and NVARCHAR(MAX)

Implement following good practices in User Defined Function

Implement following good practices in Stored Procedure:

Implement following good practices in Triggers:

Implement following good practices in Views:

Implement following good practices in Transactions:

And, that’s not the end. There are lots of best practices out there! Try finding some of them clicking on the following URL:

Remember, you need to implement the good things that you know, otherwise, you knowledge will not add any value to the system that you are going to build. Also, you need to have a process for reviewing and monitoring the codes (That are written by your team) whether the data access codes are being written following the standards and best practices.

How to analyze and identify the scope for improvement in your TSQLs?

In an ideal world, you always prevent diseases rather than cure. But, in reality you just can’t prevent always. I know your team is composed of brilliant professionals. I know you have good review process, but still bad codes are written, still poor design takes place. Why? Because, no matter what advanced technology you are going to use, your client requirement will always be way much advanced and this is a universal truth in the Software development. As a result, designing, developing and delivering a system based on the requirement will always be a challenging job for you.

So, it’s equally important that you know how to cure. You really need to know how to troubleshoot a performance problem after it happens. You need to learn the ways to analyze the TSQLs, identify the bottlenecks and re-factor those to troubleshoot the performance problem. To be true, there are numerous ways to troubleshoot database and TSQL performance problems, but, at the most basic levels, you have to understand and review the execution plan of the TSQLs that you need to analyze.

Understanding the query execution plan

Whenever you issue an SQL in the SQL Server engine, the SQL Server first has to determine the best possible way to execute it. In order to carry this out, the Query optimizer (A system that generates the optimal query execution plan before executing the query) uses several information like the data distribution statistics, index structure, metadata and other information to analyze several possible execution plans and finally selects one that is likely to be the best execution plan most of the cases.

Did you know? You can use the SQL Server Management Studio to preview and analyze the estimated execution plan for the query that you are going to issue. After writing the SQL in the SQL Server Management Studio, click on the estimated execution plan icon (See below) to see the execution plan before actually executing the query.

(Note: Alternatively, you can switch the Actual execution plan option “on” before executing the query. If you do this, the Management Studio will include the actual execution plan that is being executed along with the result set in the result window)

Figure: Estimated execution plan in Management Studio

Understanding the query execution plan in detail

Each icon in the execution plan graph represents one action item (Operator) in the plan. The execution plan has to be read from right to left and each action item has a percentage of cost relative to the total execution cost of the query (100%).

In the above execution plan graph, the first icon in the right most part represents a “Clustered Index Scan” operation (Reading all primary key index values in the table) in the HumanResources Table (That requires 100% of the total query execution cost) and the left most icon in the graph represents a SELECT operation (That requires only 0% of the total query execution cost).

Following are the important icons and their corresponding operators you are going to see frequently in the graphical query execution plans:

(Each icon in the graphical execution plan represents a particular action item in the query. For a complete list of the icons and their corresponding action item, go to http://technet.microsoft.com/en-us/library/ms175913.aspx )

Note the “Query cost” in the execution plan given above. It has 100% cost relative to the batch. That means, this particular query has 100% cost among all queries in the batch as there is only one query in the batch. If there were multiple queries simultaneously executed in the query window, each query would have its own percentage of cost (Less than 100%).

To know more detail for each particular action item in the query plan, move the mouse pointer on each item/icon. You will see a window that looks like the following:

This window provides detailed estimated information about a particular query item in the execution plan. The above window shows the estimated detailed information for the clustered index scan and it looks for the row(s) which have/has Gender = ‘M’ in the Employee table in HumanResources schema in the AdventureWorks database. The window also shows estimated IO, CPU number of rows with size of each row and other costs that is uses to compare with other possible execution plans to select the optimal plan.

I found an article that can help you further understanding and analyzing the TSQL execution plans in detail. You can take a look at it here: http://www.simple-talk.com/sql/performance/execution-plan-basics/

What information do we get by viewing the execution plans?

Whenever any of your query performs slowly, you can view the estimated (And, actual if required) execution plan and can identify the item that is taking the most amount of time (In terms of percentage) in the query. When you start reviewing any TSQL for any optimization, most of the cases, the first thing you would like to do is to view the execution plan. You will most likely to quickly identify the area in the SQL that is creating the bottlenecks in the overall SQL.

Keep watching for the following costly operators in the execution plan of your query. If you find one of these, you are likely to have problems in your TSQL and you need to re-factor the TSQL to try to improve performance.

Table Scan: Occurs when the corresponding table does not have a clustered index. Most likely, creating clustered index or defragmenting indexes will enable you to get rid of it.

Clustered Index Scan: Sometimes considered equivalent to Table Scan. Takes place when non-clustered index on an eligible column is not available. Most of the cases, creating non-clustered index will enable you to get rid of it.

Hash Join: Most expensive joining methodology. This takes place when the joining columns between two tables are not indexed. Creating indexes on those columns will enable you to get rid of it.

Nested Loops: Most cases, this happens when a non-clustered index does not include (Cover) a column that is used in the SELECT column list. In this case, for each member in the non-clustered index column the database server has to seek into the clustered index to retrieve the other column value specified in the SELECT list. Creating covered index will enable you to get rid of it.

RID Lookup: Takes place when you have a non-clustered index, but, the same table does not have any clustered index. In this case, the database engine has to look up the actual row using the row ID which is an expensive operation. Creating a clustered index on the corresponding table would enable you to get rid of it.

TSQL Refactoring-A real life story

Knowledge comes into values only when applied to solve real-life problems. No matter how knowledgeable you are, you need to utilize your knowledge in an effective way in order to solve your problems.

Let’s read a real life story. In this story, Mr. Tom is one of the members of the development team that built the application that we have mentioned earlier.

When we started our optimization mission in the data access routines (TSQLs) of our application, we identified a Stored Procedure that was performing way below the expected level of performance. It was taking more than 50 seconds to process and retrieve sales data for one month for particular sales items in the production database. Following is how the stored procedure was getting invoked for retrieving sales data for ‘Caps’ for the year 2009:

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exec uspGetSalesInfoForDateRange ‘1/1/2009’, 31/12/2009,’Cap’ 

Accordingly, Mr. Tom was assigned to optimize the Stored Procedure.

Following is a stored procedure that is somewhat close to the original one (I can’t include the original stored procedure for proprietary issue you know).

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ALTER PROCEDURE uspGetSalesInfoForDateRange
 @startYear DateTime,
 @endYear DateTime,
 @keyword nvarchar(50)
AS
BEGIN
 SET NOCOUNT ON;
 SELECT
 Name,
 ProductNumber,
 ProductRates.CurrentProductRate Rate,
	ProductRates.CurrentDiscount Discount,
	OrderQty Qty,
	dbo.ufnGetLineTotal(SalesOrderDetailID) Total,
	OrderDate,
	DetailedDescription
 FROM
	Products INNER JOIN OrderDetails
	ON Products.ProductID = OrderDetails.ProductID
	INNER JOIN Orders
	ON Orders.SalesOrderID = OrderDetails.SalesOrderID
	INNER JOIN ProductRates
	ON
	Products.ProductID = ProductRates.ProductID
 WHERE
	OrderDate between @startYear and @endYear
	AND
	(
	ProductName LIKE '' + @keyword + ' %' OR
	ProductName LIKE '% ' + @keyword + ' ' + '%' OR
	ProductName LIKE '% ' + @keyword + '%' OR
	Keyword LIKE '' + @keyword + ' %' OR
	Keyword LIKE '% ' + @keyword + ' ' + '%' OR
	Keyword LIKE '% ' + @keyword + '%'
	)
 ORDER BY
	ProductName
END

GO 

Analyzing the indexes

As a first step, Mr. Tom wanted to review the indexes of the tables that are being queried in the Stored Procedure. He had a quick look into the query and identified the fields that the tables should have indexes on (For example, fields that have been used in the join queries, WHERE conditions and ORDER BY clause). Immediately he found that, several indexes are missing on some of these columns. For example, indexes on following two columns were missing:

OrderDetails.ProductID

OrderDetails.SalesOrderID

He created non-clustered indexes on those two columns and executed the stored procedure as follows:

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exec uspGetSalesInfoForDateRange ‘1/1/2009’, 31/12/2009 with recompile

The Stored Procedure’s performance was improved now, but still below the expected level (35 seconds). (Note the “with recompile” clause. It forces the SQL Server engine to recompile the stored procedure and re-generate the execution plan to take advantage of the newly built indexes).

Analyzing the query execution plan

Mr. Tom’s next step was to see the execution plan in the SQL Server Management Studio. He did this by writing the ‘exec’ statement for the stored procedure in the query window and viewing the “Estimated execution plan”. (The execution plan is not included here as it is quite a big one that is not going to fit in screen).

Analyzing the execution plan he identified some important scopes for improvement

Being curious about the table scan issue, Mr. Tom wanted to know if any index fragmentation took place or not (Because, all indexes were properly implemented). He ran a TSQL that reports the index fragmentation information on table columns in the database (He collected this from a CodeProject article on Data access optimization) and was surprised to see that, 2 of the existing indexes (In the corresponding tables used in the TSQL in the Stored Procedure) had fragmentation that were responsible for the Table scan operation. Immediately, he defragmented those 2 indexes and found out that the table scan was not occurring and the stored procedure was taking 25 seconds now to execute.

In order to get rid of the “Nested loop join”, he implanted a “Covered index” in the corresponding table including the column in the SELECT list. As a result, when selecting the column, the database engine was able to retrieve the column value in the non-clustered index node. Doing this reduced the query performance up to 23 seconds now.

Implementing some best practices

Mr. Tom now decided to look for any piece of code in the stored procedure that did not conform to the best practices. Following were the changes that he did to implement some best practices:

Getting rid of the “Procedural code”

Mr. Tom identified that, a UDF ufnGetLineTotal(SalesOrderDetailID) was getting executed for each row in the result set and the UDF simply was executing another TSQL using a value in the supplied parameter and was returning a scalar value. Following was the UDF definition:

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ALTER FUNCTION [dbo].[ufnGetLineTotal]
(
	@SalesOrderDetailID int
)
RETURNS money
AS
BEGIN

	DECLARE @CurrentProductRate money
	DECLARE @CurrentDiscount money
	DECLARE @Qty int
	
	SELECT
	@CurrentProductRate = ProductRates.CurrentProductRate,
	@CurrentDiscount = ProductRates.CurrentDiscount,
	@Qty = OrderQty
	FROM
	ProductRates INNER JOIN OrderDetails ON
	OrderDetails.ProductID = ProductRates.ProductID
	WHERE
	OrderDetails.SalesOrderDetailID = @SalesOrderDetailID
	
	RETURN (@CurrentProductRate-@CurrentDiscount)*@Qty
END 

This seemed to be a “Procedural approach” for calculating the order total and Mr. Tom decided to implement the UDF’s TSQL as an inline SQL in the original query. Following was the simple change that he had to implement in the stored procedure:

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dbo.ufnGetLineTotal(SalesOrderDetailID) Total -- Old Code

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(CurrentProductRate-CurrentDiscount)*OrderQty Total -- New Code 

Immediately after executing the query Mr. Tom found that the query was taking 14 seconds now to execute.

Getting rid of the unnecessary Text column in the SELECT list

Exploring for further optimization scopes Mr. Tom decided to take a look at the column types in the SELECT list in the TSQL. Soon he discovered that one Text column (Products.DetailedDescription) were included in the SELECT list. Reviewing the application code Mr. Tom found that this column values were not being processed by the application immediately. Few columns in the result set were being displayed in a listing page in the application, and, when user clicks on a particular item in the list, a detail page was appearing containing the Text column value.

Excluding that Text column from the SELECT list dramatically reduced the query execution time from 14 seconds to 6 seconds! So, Mr. Tom decided to apply a “Lazy loading” strategy to load this Text column using a Stored Procedure that accepts an “ID” parameter and selects the Text column value. After implementation he found out that, the newly created Stored Procedure executes in a reasonable amount of time when user sees the detail page for an item in the item list. He also converted those two “Text” columns to “VARCHAR(MAX) columns and that enabled him to use the len() function on one of these two columns in the TSQLs in other places (That also allowed him to save some query execution time because, he was calculating the length using len(Text_Column as Varchar(8000)) in the earlier version of the code.

Optimizing further : Process of elimination

What’s next? All the optimization steps so far reduced the execution time to 6 seconds. Comparing to the execution time of 50 seconds before optimization, this is a big achievement so far. But, Mr. Tom thinks the query could have further improvement scopes. Reviewing the TSQLs Mr. Tom didn’t find any significant option left for further optimization. So, he indented and re-arranged the TSQL (So that each individual query statement (Say, Product.ProductID = OrderDetail.ProductID) is written in a particular line) and starts executing the Stored Procedure again and again by commenting out each line that he suspects for having improvement scope.

Surprise! Surprise! The TSQL had some LIKE conditions (The actual Stored procedure basically performed a keyword search on some tables) for matching several patterns against some column values. When he commented out the LIKE statements, suddenly the Stored Procedure execution time jumped below 1 second. Wow!

It seemed that, having done with all the optimizations so far, the LIKE searches were taking the most amount of time in the TSQL. After carefully looking at the LIKE search conditions, Mr. Tom became pretty sure that the LIKE search based SQL could easily be implemented using the Full Text search. It seemed that two columns needed to be full text search enabled. These were: ProductName and Keyword.

It just took 5 minutes for him to implement the FTS (Creating the Full text catalog, making the two columns full text enabled and replacing the LIKE clauses with the FREETEXT function) and the query started executing now within a stunning1 second!

Great achievement, isn’t it?