Local prefixed-index vs non-prefixed

I'm reading the chapter "11 Partitioned Tables and Indexes" of Oracle Concepts (8.1.6).
There is a comparison between local prefixed and local non-prefixed indexes.

It's written :
---

"It is more expensive to probe into a nonprefixed index than to probe into a prefixed
index.
If an index is prefixed (either local or global) and Oracle is presented with a
predicate involving the index columns, then partition pruning can restrict
application of the predicate to a subset of the index partitions."
---

then a bit further :
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"Of course, if there is also a predicate on the partitioning columns, then multiple
(local non-prefixed) index probes might not be necessary. Oracle takes advantage of the fact that a local
index is equipartitioned with the underlying table to prune partitions based on the
partition key...
So for a nonprefixed index, if the partition key is a part of the WHERE clause but
not of the index key, then the optimizer determines which index partitions to probe
based on the underlying table partition."
---

So what i understand is that :
- in the case of prefixed index, partition pruining works (with the index itself),
- in the case of non-prefixed index, partition pruining works TOO (with the underlying table)

I must miss something...
(May be that in the case of a local prefixed index, a predicate involving the index columns doesn't imply that the partition key is part of the where clause.)

Can you show me an example where local prefixed index has a true advantage over local non-prefixed ?

TIA Jean-Michel Received on Thu Nov 21 2002 - 12:17:18 CST