A method comprising: maintaining, on persistent storage, a database that is accessible to a database server; wherein the database includes persistent-format data, stored on the persistent storage, in a persistent format; wherein the persistent-format data comprises one or more data items; converting 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; wherein the mirror-format data comprises one or more data items; storing the mirror-format data in volatile memory; wherein the mirror-format data is 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; wherein both the persistent-format data and the mirror-format data have copies of a particular data item; maintaining a bitmap, in volatile memory, that indicates which data items, in the mirror-format data, are no longer valid; wherein maintaining the bitmap includes the database server responding to an update of the particular data item by: updating the copy of the particular data item in the persistent-format data; and without updating the copy of the particular data item in the mirror-format data, setting a particular bit, in the bitmap, that corresponds to the particular data item, to indicate that the copy of the particular data item in the mirror-format data is not valid; and wherein, for a certain query that requires access to certain data items which are validly mirrored in the mirror-format data, the database server has a choice of obtaining those certain data items at least (i) from the mirror-format data, (ii) from the persistent-format data, or (iii) partially from the mirror-format data and partially 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 on the persistent storage, in a persistent format; wherein the persistent-format data comprises one or more data items; converting 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; wherein the mirror-format data comprises one or more data items; storing the mirror-format data in volatile memory; wherein the mirror-format data is 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; wherein both the persistent-format data and the mirror-format data have copies of a particular data item; maintaining a bitmap, in volatile memory, that indicates which data items, in the mirror-format data, are no longer valid; wherein maintaining the bitmap includes the database server responding to an update of the particular data item by: updating the copy of the particular data item in the persistent-format data; and without updating the copy of the particular data item in the mirror-format data, setting a particular bit, in the bitmap, that corresponds to the particular data item, to indicate that the copy of the particular data item in the mirror-format data is not valid; and updating the copy of the particular data item in the persistent-format data; and without updating the copy of the particular data item in the mirror-format data, setting a particular bit, in the bitmap, that corresponds to the particular data item, to indicate that the copy of the particular data item in the mirror-format data is not valid; and wherein, for a certain query that requires access to certain data items which are validly mirrored in the mirror-format data, the database server has a choice of obtaining those certain data items at least (i) from the mirror-format data, (ii) from the persistent-format data, or (iii) partially from the mirror-format data and partially from the persistent-format data.