Reasons against using local indexes For Oracle 10g and above: except in cases where a local index cannot be used and a global index is needed (e.g. unique constraint), is there any reason to favor a global index over a local index?
There is a point where the queries using the index are accessing enough partitions that a local index will be slower than a global index.  

For example, if your table is partitioned by date, but you are querying only on a status column.