Data Access (Cont.) ■ Each transaction T has its private work-area in which local copies of all data items accessed and updated by it are kept. Ti's local copy of a data item X is called x;. Transferring data items between system buffer blocks and its private work- area done by: read(X)assigns the value of data item X to the local variable x,. write(X)assigns the value of local variable x;to data item {X in the buffer block. Note:output(B)need not immediately follow write(X).System can perform the output operation when it deems fit. ■Transactions Must perform read(X)before accessing X for the first time(subsequent reads can be from local copy) write()can be executed at any time before the transaction commits Database System Concepts-7th Edition 19.9 @Silberschatz,Korth and SudarshanDatabase System Concepts - 7 19.9 ©Silberschatz, Korth and Sudarshan th Edition Data Access (Cont.) ▪ Each transaction Ti has its private work-area in which local copies of all data items accessed and updated by it are kept. • Ti 's local copy of a data item X is called xi . ▪ Transferring data items between system buffer blocks and its private work￾area done by: • 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. • Note: output(BX) need not immediately follow write(X). System can perform the output operation when it deems fit. ▪ Transactions • Must perform read(X) before accessing X for the first time (subsequent reads can be from local copy) • write(X) can be executed at any time before the transaction commits