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Binary_Checksum the Alternate of NewID() to Fetch Random Records in SQL Server

Posted by Amit Choudhary Articles | SQL October 04, 2011
In this article i will demonstrate Binary_Checksum, the Alternate of NewID() to fetch random records in SQL Server.
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Sometimes it is required to fetch random records from a database. The simplest approach is to use NewID() with the order by clause. Well I have no problem with that but when you have a large table to query then it'll start showing issues of performance. Why? I'll try to answer this question with alternative solution.

To start with I have the Adventure works database with around 260000 records in a table and we have a query that will take a random record from it.

SELECT TOP 1 [BusinessEntityID] FROM HumanResources.EmployeePayHistory
 
ORDERBY NEWID()

The problem with this query is, when you look at the execution plan then you'll find it performs a sort operation before it picks a top record.

Binary1.gif

So 91% of the cost is due to the sort operation. Well this will happen if you use NewID() to pick the random records it first sorts the records based on NewID() which is the worst case for sorting based on GUID values.

Now to get rid of Sort operation of NEWID() by- BINARY_CHECKSUM

The best approach is to use the BinaryCheckSum instead of NewID to reduce the sort operation to perform on NEWID column generated in Memory.

SELECT TOP 1 [BusinessEntityID]

FROM HumanResources.EmployeePayHistory

WHERE (ABS(CAST(

(BINARY_CHECKSUM(*) *

RAND()) as int)) % 100) < 10

The BINARY_CHECKSUM function generates a checksum value that is based on the values of the columns that you specify. If two rows are different, they typically will generate different checksum numbers. The BINARY_CHECKSUM function generally is used to verify whether any of the columns in a row in a table have changed. However, for our purposes, it generates a number that looks like a random number for each row.

Another form of using BinaryCheckSum can be:

SELECT TOP 1 [BusinessEntityID]

FROM HumanResources.EmployeePayHistory

WHERE

(ABS(CAST(

(BINARY_CHECKSUM

([BusinessEntityID], NEWID())) as int))

% 100) < 10


Both forms of Binary_Cheksum will have the same query Execution plan with similar cost.

Now let's have a look at the Figures analyzed by the Microsoft team to compare performance of both queries.
 

13 million rows

 

 

 

 

 

NEWID query

253

347,420

13,810,132

14,157,552

422,891

BINARY_CHECKSUM query

21

347,420

0

347,420

49,203

And in our case here I've also did some comparison based on Query plans of Old query of NEWID and new query with BINARY_CHECKSUM

Binary2.gif

Just another tip on improving performance but if you have to pick random records from small tables in SQL then you can still use NEWID() with no issues.

Let me know you rfeedback. Please do comments if you like to.

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