1. At a computer system, the computer system including one or more processors, system memory, and durable storage, the computer system maintaining an in-memory database in system memory, a method for updating a checkpoint for the in-memory database, the method comprising: committing a transaction, the results of the transaction modifying the content of the in-memory database, the transaction having a timestamp, the timestamp indicating an associated time the transaction was committed relative to other transactions; generating checkpoint data for the transaction from the results of the transaction, the checkpoint data including versions of one or more inserted portions of data inserted into the in-memory database and including identifiers for one or more deleted portions of data deleted from the in-memory database; appending the checkpoint data to a checkpoint, including: determining that the timestamp is within a specified timestamp range for a data file, the data file configured to store any inserted portions of data inserted into the in-memory database within the specified timestamp range; appending the one or more inserted portions of data to the data file; for each of the one or more deleted portions of data: identifying a corresponding insert operation that inserted the deleted portion of data into the in-memory database; locating a timestamp for a transaction that included the corresponding insert operation; determining that the located timestamp is within a second specified time range for a delta file; appending the identifier for the deleted portion of data to the delta file, the delta file configured to store identifiers for any deleted portions of data deleted from the in-memory database during the second specified time range.