ASA SQL User's Guide
Query Optimization and Execution
Query execution algorithms
Miscellaneous
Lock indicates that there is a lock at a certain isolation level. For example, at isolation level 1, a lock is maintained for only one row at a time. If you are at isolation level 0, no lock is acquired, but the node will still be called Lock. In this case, the lock node verifies that the row still exists.
For more information, see How locking works.