Should I use Query Hint Fast number_rows / FASTFIRSTROW?

Question

I was reading over the documentation for query hints: http://msdn.microsoft.com/en-us/library/ms181714(SQL.90).aspx

And noticed this: FAST number_rows Specifies that the query is optimized for fast retrieval of the first number_rows. This is a nonnegative integer. After the first number_rows are returned, the query continues execution and produces its full result set.

So when I'm doing a query like:

Select Name from Students where ID = 444

Should I bother with a hint like this? Assuming SQL Server 2005, when should I?

-- edit --

Also should one bother when limiting results:

Select top 10 * from Students OPTION (FAST 10)

Hermilahermina

Answer 1

When using TOP x, there's no benefit of also using OPTION FAST x. The query optimizer already makes its decisions based on how many rows you are retrieving. Same goes for trivial queries, such as querying for a particular value from a unique index.

Other than that, OPTION FAST x could help when you know the number of results is likely below x, but the query optimizer does not. Of course, if the query optimizer is choosing poor paths for complex queries with few results, your statistics may need to be updated. And if you guess wrong on x, the query may end up taking longer--almost always a risk when giving hints.

The above statement has not been tested--it may be that all queries take just as long to fully execute, if not longer. Getting the first 10 rows fast is great if there are only 8 rows, but theoretically the query still has to execute fully before finishing. The benefit I'm thinking may be there because the query execution takes a different path expecting fewer total records, when in fact it's really trying to get the first x faster. Those two types of optimizations may not be in alignment.

Arsenal

Answer 2

The FAST hint only makes sense on complex queries where there are multiple alternatives the optimizer could choose from. For a simple query like your example it doesn't help with anything, the query optimizer will immediately determine that there is a trivial plan (seek in ID index, lookup Name if not covering) to satisfy the query and go for it. Even if no index exists on ID, the plan is still trivial (probably clustered scan).

To give an example where FAST would be useful consider a join between A and B, with an ORDER BY constraint. Say evaluating the join B first and nested loops A honors the ORDER BY constraint, so will produce fast results (no SORT necessary), but is more costly because of cardinality (B has many records that match the WHERE, while A has few). On the other hand evaluating B first and nested loop A would produce a query that does less IO hence is faster overall, but the result would have to be sorted first and SORT can only start after the join is evaluated, so the first result will come very late. The optimizer would normally pick the second plan because is more efficient overall. The FAST hint would cause the optimizer to pick the first plan, because it produces results faster.

Psalm

Answer 3

For that particular query, certainly not! It's only going to return one row — the row with ID = 444. SQL Server will select that row as efficiently as it can.

FAST 10 might be used in a situation where you could make use of the first 10 rows immediately, even as you continue to wait for further results.

Collector