1. A method comprising: assigning a timestamp to an operation for making an in-memory copy of a chunk of a database object; after assigning the timestamp, performing the operation without blocking updates to data items that belong in the chunk; identifying a straddling transaction that: affected a particular data item from the chunk, started before the timestamp, and had not committed before the timestamp; and prior to allowing access to the in-memory copy of the chunk, storing data that indicates that the particular data item within the in-memory copy is invalid; wherein the method is performed by one or more computing devices.