When I'm talking to someone about database optimization, I'm referring to an index-friendly condition as index-friendly condition. Could it be any harder than that? But lo and behold, there's a handy word for that programming discipline, it's called SARGable
In relational databases, a condition (or predicate) in a query is said to be sargable if the DBMS engine can take advantage of an index to speed up the execution of the query. The term is derived from a contraction of Search ARGument ABLE.
A query failing to be sargable is known as Non-Sargable query and has an effect in query time, so one of the steps in query optimization is convert them to be sargable.
The typical thing that will make a sql query non-sargable is to include a function in left part of a condition of a Where clause.
Read the rest of the article at: http://en.wikipedia.org/wiki/Sargable