Table schema changes during application execution can cause problems when the application caches column names. Applications may protect themselves, or others, from changes in schema by setting table schema locks.

Lock types

• SE_TABLE_SHARED_SCHEMA_LOCK—A shared schema lock prevents others from modifying the table’s schema. Other users can establish shared schema locks on the same table, but no users can establish an exclusive schema lock until all shared schema locks are freed.
   
• SE_TABLE_EXCLUSIVE_SCHEMA_LOCK—An exclusive schema lock prevents any other user from establishing any kind of schema lock on the table until the exclusive schema lock is released. Users can still read data from the table, but they are not permitted to establish a shared schema lock.