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John Sansom - SQL Server DBA in the UK

John Sansom (Blog | Twitter) is a Microsoft Certified Master (MCM) of SQL Server and publisher of the free SQL community ebook DBA JumpStart, an inspiring collection of advice for Data Professionals, written by 20 SQL Server experts. Awarded the Microsoft Community Contributor(MCC) award, John is a prolific blogger and can be found regularly writing about SQL Server and Professional Development over at www.johnsansom.com.

Implicit Conversions and Avoiding Them With Computed Columns

I recently encountered an interesting performance issue (due to implicit conversions) that I was able to solve using a lesser known technique and I wanted to share it with you here.

A customer environment was suddenly receiving high CPU alarms and I was called in to take a look.

Having identified the query responsible for the high CPU consumption (courtesy of sp_WhoIsActive), I then inspected it’s execution plan (using SQL Sentry’s Plan Explorer) in an effort to establish a possible cause for the poor performance.

Production Execution plan showing table scan from implicit conversion

From the execution plan for the query we can see that the majority of the cost (83%) is incurred by the Index Scan operator.

Reviewing the properties for this operator showed that an Implicit Conversion was being performed. The query had a parameter with a data type of NVARCHAR and was performing a comparison to a column with data type of VARCHAR.

The table in question had 9 million records and SQL Server was having to perform the implicit conversion for every one of them. This operation requires CPU and when you consider that in this particular case the query is executed hundreds of times a second, it’s easy to see how the cumulative CPU costs can very quickly get out of hand.

What To Do When You Cannot Edit the T-SQL Source

Implicit conversions are a well known source of performance issues. No problem you say, let’s just edit the T-SQL:

  • Find the stored procedure and cast the parameter - No can do,  the code is called directly from the application as an ad-hoc batch.
  • Change the parameter data type on the application side - Sure but we have a problem in production right now! The instance is pegged and you’re customer is loosing money. Where is the code within the application? What if you don’t have access to the application source, maybe it’s a third party product.

What we need here is a way to change the data type of the column and index, without touching the existing T-SQL code or adversely affecting the current schema configuration. In other words, the changes need to be completely transparent to the application.

Fortunately these requirements can be met by using an Indexed Computed Column.

Using an Indexed Computed Column to Avoid Implicit Conversions

Let’s go through an example to see this behavior in action.

(You can use the following build script to create a simple test database with a single table and some data should you wish.)

Consider the following simple table schema and query:

CREATE TABle TableA
(
	ID		INT identity(1,1) not null,
	SomeData	varchar(50) null,
	SomeDate	datetime
);

declare @p1 nvarchar(50)
set @p1 = N'Username 4'

select
	A.ID,
	A.SomeData,
	A.SomeDate
from TableA A
Where A.SomeData = @p1

The table TableA has a column SomeData with a data type of VARCHAR(50) and a parameter @p1 of NVARCHAR(50). This data type mismatch comes in to play during a comparison in the WHERE clause.

The execution plan looks like this:

Example execution plan with implicit conversionsExecution plan properties showing implicit conversions

As you can see, the Index Seek operator on TableA.ncl_SomeData performs an implicit conversion.

Now we’ll create a computed column SomeData_comp and add a non-clustered index to it.

--Create a computed column using the preferred data type
ALTER TABLE TableA ADD SomeData_comp AS (convert(nvarchar(50),SomeData));

--Create a non-clustered index on the computed column
CREATE NONCLUSTERED INDEX ncl_SomeData_comp on TableA(SomeData_comp);

Let’s now execute our example query once more:

Example execution plan using computed column to avoid implicit conversion

Example exec plan properties using indexed computed column

As you can see, SQL Server is able to determine that although we have requested the column SomeData (a varchar) be compared to @p1 (an nvarchar), there is now an index available that provides the same data but in the desired data type, via the computed column.

Implementing this change means that SQL Server is now able to take advantage of this new Index, that is of the preferred data type for the query, without the need to modify the query itself.

(Note: If the application is performing inserts without an explicit field list, they will fail under this solution. A hat-tip to Brent Ozar(Blog|Twitter) for reminding me of this.)

Business As Usual is “the” Priority

In the production scenario I described earlier, this solution meant that a temporary fix could be deployed immediately in order to remove the excessive CPU consumption. Addressing the implicit conversion meant that an Index Seek could be used, rather than the expensive Scan operation, and subsequently a more suitable join operation was chosen by optimizer (Nested Loops rather than Merge) for the query.

The indexed computed column solution enabled business operations to return an acceptable level swiftly, allowing for a more suitable permanent fix to be pursued in a timely fashion.

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