Use this query hint when you want to retrieve only the first n rows out of a relatively large result set.
Before using any query hints, run a web search for the latest information on issues with query hints. Try searching on the keywords “SQL Server Query Hints” and look specifically for anything by Craig Freedman, who has written several great blog entries on some of the issues you can encounter when using query hints.
A typical example of this is a website that uses paging to display large sets of rows, whereby the first page shows only the first web page worth of rows, and a page might contain only 20, 30, or maybe 40 rows. If the query returns thousands of rows, then SQL Server would possibly optimize this query using hash joins. Hash joins work well with large datasets but have a higher setup time than perhaps a nested loop join. Nested loop joins have a very low setup cost and can return the first set of rows more quickly but takes considerably longer to return all the rows.
Using the FAST <number_rows> query hint causes the Query Optimizer to use nested loop joins and other techniques, rather than hashed joins, to get the first n rows faster.
Typically, once the first n rows are returned, if the remaining rows are retrieved, the query performs slower than if this hint were not used.