Nếu người lao động được sử dụng cho vòng lặp bên ngoài, mỗi khối NHÂN VIÊN được đọc một lần, và các tập tin cục toàn bộ (mỗi khối của nó) được đọc một lần cho mỗi lần chúng ta đọc trong (NB - 2) các khối tập tin EMPLOYEE. Chúng tôi nhận được như sau: Tổng số của các khối truy cập cho tập tin bên ngoài = Số lần (NB - 2) các khối của tập tin bên ngoài được nạp | 610 I Chapter 18 Concurrency Control Techniques and Bassiouni 1988 . Papadimitriou and Kanellakis 1979 and Bernstein and Goodman 1983 discuss multiversion techniques. Multiversion timestamp ordering was proposed in Reed 1978 1983 and multiversion two-phase locking is discussed in Lai and Wilkinson 1984 . A method for multiple locking granularities was proposed in Gray et al. 1975 and the effects of locking granularities are analyzed in Ries and Stonebraker 1977 . Bhargava and Reidl 1988 presents an approach for dynamically choosing among various concurrency control and recovery methods. Concurrency control methods for indexes are presented in Lehman and Yao 1981 and in Shasha and Goodman 1988 . A performance study of various B tree concurrency control algorithms is presented in Srinivasan and Carey 1991 . Other recent work on concurrency control includes semantic-based concurrency control Badrinath and Ramamritham 1992 transaction models for long running activities Dayal et al. 1991 and multilevel transaction management Hasse and Weikum 1991 . 19 Database Recovery Techniques In this chapter we discuss some of the techniques that can be used for database recovery from failures. We have already discussed the different causes of failure such as system crashes and transaction errors in Section . We have also covered many of the concepts that are used by recovery processes such as the system log and commit points in Section . We start Section with an outline of a typical recovery procedures and a categorization of recovery algorithms and then discuss several recovery concepts including write-ahead logging in-place versus shadow updates and the process of rolling back undoing the effect of an incomplete or failed transaction. In Section we present recovery techniques based on deferred update also known as the NO-UNDO REDO technique. In Section we discuss recovery techniques based on immediate update these include the UNDO REDO and UNDO NO-REDO .
