1. A method comprising: a database server maintaining a database, said database server comprising one or more central processing units (CPUs); wherein said database includes a plurality of database tables stored in a plurality of data blocks on block-addressable persistent storage devices; said database server storing, in a hierarchy of random-access memories (RAMs), sets of mirror format (MF) data produced by converting data in said plurality of data blocks to a mirror format, each RAM in said hierarchy of RAMs being byte addressable memory having an address space referenceable by software instructions executed by said one or more CPUs, each RAM in said hierarchy of RAMs having an associated latency different from a latency associated with any other RAM in said hierarchy of RAMs; wherein each set of MF data of said sets of MF data store, in column-major form, data from a plurality of columns of said plurality of database tables; wherein, for each set of MF data of said sets of MF data, storing includes: selecting based on one or more criteria, by said database server, a respective RAM from said hierarchy to store said each set of MF data; storing, by said database server, said each set of MF data in said respective RAM.