Question

I’ve just found out that the execution plan performance between the following two select statements are massively different:

select * from your_large_table where LEFT(some_string_field, 4) = '2505'  select * from your_large_table where some_string_field like '2505%' 

The execution plans are 98% and 2% respectively. Bit of a difference in speed then. I was actually shocked when I saw it.

I’ve always done LEFT(xxx) = ‘yyy’ as it reads well. I actually found this out by checking the LINQ generated SQL against my hand crafted SQL. I assumed the LIKE command would be slower, but is in fact much much faster.

My question is why is the LEFT() slower than the LIKE ‘%..’. They are afterall identical?

Also, is there a CPU hit by using LEFT()?

Answer 1

More generally speaking, you should never use a function on the LEFT side of a WHERE clause in a query. If you do, SQL won’t use an index–it has to evaluate the function for every row of the table. The goal is to make sure that your where clause is ‘Sargable‘

Some other examples:

Bad: Select ... WHERE isNull(FullName,'') = 'Ed Jones' Fixed: Select ... WHERE ((FullName = 'Ed Jones') OR (FullName IS NULL))  Bad: Select ... WHERE SUBSTRING(DealerName,4) = 'Ford' Fixed: Select ... WHERE DealerName Like 'Ford%'  Bad: Select ... WHERE DateDiff(mm,OrderDate,GetDate()) >= 30 Fixed: Select ... WHERE OrderDate < DateAdd(mm,-30,GetDate())   Bad: Select ... WHERE Year(OrderDate) = 2003 Fixed: Select ... WHERE OrderDate >= '2003-1-1' AND OrderDate < '2004-1-1'