What makes a SQL statement sargable?

Question

By definition (at least from what I've seen) sargable means that a query is capable of having the query engine optimize the execution plan that the query uses. I've tried looking up the answers, but there doesn't seem to be a lot on the subject matter. So the question is, what does or doesn't make an SQL query sargable? Any documentation would be greatly appreciated.

For reference: Sargable

Answer 1

The most common thing that will make a query non-sargable is to include a field inside a function in the where clause:

SELECT ... FROM ... WHERE Year(myDate) = 2008 

The SQL optimizer can't use an index on myDate, even if one exists. It will literally have to evaluate this function for every row of the table. Much better to use:

WHERE myDate >= '01-01-2008' AND myDate < '01-01-2009' 

Some other examples:

Bad: Select ... WHERE isNull(FullName,'Ed Jones') = '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())  

Answer 2

Don't do this:

WHERE Field LIKE '%blah%' 

That causes a table/index scan, because the LIKE value begins with a wildcard character.

Don't do this:

WHERE FUNCTION(Field) = 'BLAH' 

That causes a table/index scan.

The database server will have to evaluate FUNCTION() against every row in the table and then compare it to 'BLAH'.

If possible, do it in reverse:

WHERE Field = INVERSE_FUNCTION('BLAH') 

This will run INVERSE_FUNCTION() against the parameter once and will still allow use of the index.