Until recently concurrency control theory has focussed on one-level systems, I. 8. regarding schedules simply as sequences of read and write operatlons on objects of one particular abstractlon level (/Pa79/,/BSW79/). usually pages or tuples. Nevertheless. real data base systems sometlmes go beyond this classlcal view. a well-known eXarI’tple being System R. To Increase concurrency, locking Is applied twice within thls prototype. on tuples until end-of-transactlon and on pages for the scope of each tuple operatlon. Since page locks are released before the commitment of the complete transaction. the approach Is called ‘open nested transactlon’(/Tr83/), A typical schedule acting on two layers would look like: