Locking behavior of TRUNCATE PARTITION yyy UPDATE GLOBAL INDEXES clause

 Recently we had issues with statements such as:

ALTER TABLE xxx TRUNCATE PARTITION yyy UPDATE GLOBAL INDEXES

My understanding of the documentation is it is supposed to keep a global index usable

despite the partition truncate clause, at the cost of a DDL statement no longer being a quick

data dictionary operation.

Not much is said regarding locking during the operation. I was expecting a TM lock only

on the partition being truncated, and not held during the index maintenance operation,

but the application seems to be blocked on a 'select for update' query on the same

table during the whole operation.