Data Access(Cont.) Transaction transfers data items between system buffer blocks and its private work-area using the following operations read(X)assigns the value of data item X to the local variable Xj. write(X)assigns the value of local variable x;to data item {X)in the buffer block. both these commands may necessitate the issue of an input(Bx) instruction before the assignment,if the block By in which X resides is not already in memory. Transactions Perform read(X)while accessing X for the first time; All subsequent accesses are to the local copy. After last access,transaction executes write(X). output(Bx)need not immediately follow write(X).System can perform the output operation when it deems fit. Database System Concepts,5th Edition,Oct 5,2006 17.9 @Silberschatz,Korth and SudarshanDatabase System Concepts, 5 17.9 ©Silberschatz, Korth and Sudarshan th Edition, Oct 5, 2006 Data Access (Cont.) Transaction transfers data items between system buffer blocks and its private work-area using the following operations : read(X) assigns the value of data item X to the local variable xi . write(X) assigns the value of local variable xi to data item {X} in the buffer block. both these commands may necessitate the issue of an input(BX) instruction before the assignment, if the block BX in which X resides is not already in memory. Transactions Perform read(X) while accessing X for the first time; All subsequent accesses are to the local copy. After last access, transaction executes write(X). output(BX) need not immediately follow write(X). System can perform the output operation when it deems fit