If the subsume_row_locks option is On (the default) then whenever a table t is locked exclusively with LOCK TABLE t IN EXCLUSIVE MODE, the database server no longer acquires individual row locks for t.
This can result in a significant performance improvement if extensive updates are made to t in a single transaction, especially if t is large relative to cache size. It also allows for atomic update operations that are larger than the lock table can currently
handle (approximately 2-4 million rows).
When this option is On, keyset cursors over a table locked in this fashion will return row changed warnings for every row
in the cursor, if any row in the database has been modified. Note that the database server could turn an updatable cursor
with an ORDER BY into a keyset cursor as a result.