Use the LOCK MODE clause to specify the locking granularity of the index.
When you use the COARSE mode, index-level locks are acquired on the index instead of item-level or page-level locks. This mode reduces the number of lock calls on an index. The COARSE mode offers performance advantages when you know that the index is unlikely to change; for example, when read-only operations are performed on the index.
Use the NORMAL mode to place item-level or page-level locks on the index as necessary. Use this mode when the index gets updated frequently.
When the database server executes the LOCK MODE COARSE option, it acquires an exclusive lock on the table for the duration of the ALTER INDEX statement. Any transactions currently using a lock of finer granularity must complete before the database server switches to the COARSE lock mode.
Related statements: CREATE INDEX and CREATE TABLE
For a discussion of the performance implications of clustered indexes, see your IBM Informix Performance Guide.