A method comprising: maintaining, on persistent storage , a database that is accessible to a database server ; wherein the database includes persistent-format data , stored in a persistent format; wherein a copy of all data items from the persistent-format data is stored on the persistent storage ; converting at least a portion of the persistent-format data to a mirror format to produce mirror-format data ; wherein the mirror format is different from and independent of the persistent format; maintaining the mirror-format data in volatile memory accessible to the database server ; wherein each data item in the mirror-format data is a copy of a corresponding data item in the persistent-format data ; within the volatile memory , as the database server executes database operations that make changes to the persistent-format data , the database server maintaining the mirror-format data transactionally consistent with the persistent-format data in that any data items provided to a transaction from the mirror-format data will be the same version that would have been provided if the data items were provided to the transaction from the persistent-format data ; determining, by the database server , that a scan operation of a table of the database which has at least a portion of its data in volatile memory as mirror-format data is to be performed; in response to said determination, the database server performing a scan operation on the table in which a first set of data items for the scan operation are obtained from the mirror-format data and a second set of data items for the scan operation are obtained from the persistent-format data . maintaining, on persistent storage , a database that is accessible to a database server ; wherein the database includes persistent-format data , stored in a persistent format; wherein a copy of all data items from the persistent-format data is stored on the persistent storage ; converting at least a portion of the persistent-format data to a mirror format to produce mirror-format data ; wherein the mirror format is different from and independent of the persistent format; maintaining the mirror-format data in volatile memory accessible to the database server ; wherein each data item in the mirror-format data is a copy of a corresponding data item in the persistent-format data ; within the volatile memory , as the database server executes database operations that make changes to the persistent-format data , the database server maintaining the mirror-format data transactionally consistent with the persistent-format data in that any data items provided to a transaction from the mirror-format data will be the same version that would have been provided if the data items were provided to the transaction from the persistent-format data ; determining, by the database server , that a scan operation of a table of the database which has at least a portion of its data in volatile memory as mirror-format data is to be performed; in response to said determination, the database server performing a scan operation on the table in which a first set of data items for the scan operation are obtained from the mirror-format data and a second set of data items for the scan operation are obtained from the persistent-format data . <EOS>