Not using the specified OPTIMIZER_MODE

Question

Hi,
We set the OPTIMIZER_MODE on our 10g db as FIRST_ROWS_100. However there are a couple of queries from application are using the optimizer mode ALL_ROWS instead. If we run those queries alone, directly in TOAD or SQLPLUS the explain plan shows they are using FIRST_ROWS_100. And that is how we want them to use.
Why the optimizer chooses to use ALL_ROWS?
How to force it to use FIRST_ROWS_100? Note the "troubled" queries are executed dynamically from an application.
Thanks for all suggestions.

Answer 1

That would be the expected result, if i understand your last post. If the optimizer knows that only twenty rows are going to be returned from the query then it might use that information to modify drive the execution plan and choose to ignore the hint.
For example, imagine that a query might return about 10,000 rows. A FIRST_ROWS hint might prompt the use of an index instead of a full table scan in order to find the first rows quickly, even though a higher cost is incurred that way. However, if you put a restriction on the query that says "return only the first 20 of the rows" then that part of the query might drive a change to use indexes rather than a FTS, in which case your "first 100 rows" hint becomes redundant and is discarded.
This is 100% speculation on my part, but you might test the theory (since you already might have data + queries that are sensitive to this situation) by examining the optimizer's plans for executing these queries with and without different combinations of FIRST_ROWS hints and ROWNUM predicates. See if the presence of the ROWNUM predicate and the FIRST_ROWS hint modify the plan in the same way when used independently of each other.