There's a massive difference between the statements

Select 1 where exists(select * from dbo.WordIndex where Term='inc')

and

select * from dbo.WordIndex where Term='inc'

Exists is only checking for the existence of a row. It can use whatever indexes are appropriate based on the where clause and return as soon as a row is found. It doesn't need to retrieve any actual values from the table in question

The select * , however has to retrieve all the columns that match the where clause. It also, if it was using a NC index, has to lookup the rest of the columns in the table from the clustered index.

All Conor was talking about was the difference between

IF EXISTS (SELECT 1 FROM SomeTable WHERE SomeCondition)

and

IF EXISTS (SELECT * FROM SomeTable WHERE SomeCondition)