Yet another SQL Server enthusiast

2013-12-09

Non SARG-able predicates #1

Sometimes isn’t enough to create an index. to optimize a query. Even we create the proper index we may discover by examining the execution plan that SQL Server doesn’t use that an Index Seek to find the rows  using instead an Index Scan.

For example, if we want to count the number of sales order for December 2005 a first attempt can use following query and an index on OrderDate:

Code Snippet
CREATE INDEX IX_SalesOrderHeader_OrderDate
ON Sales.SalesOrderHeader (OrderDate);
GO
SET STATISTICS IO ON;
DBCC FREEPROCCACHE;
GO
SELECT COUNT(*)
FROM Sales.SalesOrderHeader h
WHERE YEAR(h.OrderDate) = 2005 AND MONTH(h.OrderDate) = 12
GO
/*
SET STATISTICS IO ON output:
Table 'SalesOrderHeader'. Scan count 1, logical reads 73
*/

Execution.Plan.Index.Scan

We can see that for this query the execution plan includes an Index Scan instead of Index Seek. The reason is these two predicates aren’t SARG-able. A SARG-able predicates allows an Index Seek if an index is available.

The second attempt uses a BETWEEN predicate (which is SARG-able) thus:

Code Snippet
DBCC FREEPROCCACHE;
GO
SELECT    COUNT(*)
FROM    Sales.SalesOrderHeader h
WHERE    h.OrderDate BETWEEN '20051201' AND '20051231 23:59:59.997'
– or WHERE    h.OrderDate >= '20051201' AND h.OrderDate < '20060101'
GO
/*
SET STATISTICS IO ON output:
Table 'SalesOrderHeader'. Scan count 1, logical reads 3
*/

Execution.Plan.Index.Seek

This time the execution plan includes an Index Seek and the performance (Logical Reads) is better: just 3 pages / logical reads (Seek) instead of 73 pages logical reads (Index Scan).

More information on SARG-able predicates can be found here.

Note: For this test I used Adventure Works for SQL Server 2012 (data file) database

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