The term SARG (or search argument) is used to describe whether a Transact SQL WHERE clause will be able to take advantage of any indexes present on columns in a table.
For example, this can use an index:
SELECT SomeColumn FROM MyTable
WHERE SomeColumn LIKE ‘Whea%’
But this can not:
SELECT SomeColumn FROM MyTable
WHERE SomeColumn LIKE ‘%eat’
This important topic is covered in some detail in the following references:
pgs 351 – 368, Guru’s Guide to Transact-SQL, Ken Henderson
pgs 318 – 326, SQL Server Performance Tuning Distilled, Sajal Dam
SARGs are also mentioned in this more general performance tuning article.