1. A computer system for staging data record changes from a faster storage medium to a slower storage medium using data query rewriting, the computer system comprising: a bus system; a storage device connected to the bus system, wherein the storage device stores computer readable program code; and a processor unit connected to the bus system, wherein the processor unit executes the computer readable program code to rewrite a data query to perform a particular operation on a particular data record based on a pre-defined relational algebra rule associated with the particular operation to reflect that stable data records are stored on a hard disk drive of the computer system and that recent changes to data records are stored on a solid-state drive of the computer system in response to receiving the data query, wherein the pre-defined relational algebra rule associated with the particular operation is one of a plurality of different pre-defined relational algebra rules, and wherein each different one of the plurality of different pre-defined relational algebra rules is associated with a different corresponding data query operation in a plurality of data query operations; and issue the rewritten data query based on the pre-defined relational algebra rule associated with the particular operation to a stable data store on the hard disk drive of the computer system and to a delta change data store on the solid-state drive of the computer system; wherein data record changes are deferred by storing recent data record changes entirely on the solid-state drive and only periodically merging the data record content of the solid-state drive with the data record content of the hard disk drive in batches; and wherein the plurality of different pre-defined relational algebra rules are selected from a group consisting of: 
 R ( R−π R ( R ′))∪π R (σ isDeleted=False ( R ′)), 
 R←R∪I R′←R′∪π R,isDeleted=False ( I )−( I R ), 
 R←R−D R ′←( R′∪π R,isDeleted=True ( D ))−( D R ) and 
 R←R∪U R ′←(( R′−π R,isDeleted=False ( U )).