Abstract: A database management system receives a request to perform an operation on a set of target data and, at some point after it begins executing that operation, places a lock on the target data to prevent concurrent execution of other operations on the target data. In some embodiments, the system receives an instruction from a user to perform a data-definition operation on a set of target data; places an initial lock on the target data at a level that allows at least one concurrent operation on the target data; begins executing the operation; and then places a final lock on the target data at a level that excludes all other concurrent operations on the target data.