It could also be that in this case, the estimate wasn't large enough. I'd try doing a compute statistics and see if that helps. If that still doesn't force the use of the index, I'd look and see if you need to create histograms on the column(s) that the index references.

When you deleted the statistics it forced the CBO to default back to rule_based optimization.

As was said earlier, you can use the parameter

optimizer_index_cost_adj = ## which will influence how much weight is given to indexes when computing the cost for using indexes. Oracle typically responds to users complaining that the CBO isn't using their indexes correctly will tell users to set the value to 10 and if it works then start adjusting it until it seems to be selecting the best access path.

You could always use the CBO and use a hint to influence the particular query as well.

Joe