Tham khảo tài liệu 'an introduction to database systems 8ed - c j date - solutions manual episode 2 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SELECT FROM P ORDER BY P eof FALSE EXEC SQL START TRANSACTION EXEC sQl open CP DO WHILE NOT eof DO count 1 TO 10 EXEC SQL FETCH CP INTO P . IF SQLSTATE 02000 THEN DO EXEC SQL CLOSE CP EXEC SQL COMMIT eof TRUE END DO ELSE print P . END IF END DO EXEC SQL DELETE FROM P WHERE P EXEC SQL COMMIT AND CHAIN END DO A blow-by-blow comparison of the two solutions is left as a subsidiary exercise. End of Chapter 15 Copyright c 2003 C. J. Date page Chapter 16 C o n c u r r e n c y Principal Sections Three concurrency problems Locking The three concurrency problems revisited Deadlock Serializability Recovery revisited Isolation levels Intent locking Dropping ACID SQL facilities General Remarks Very intuitive introduction Two independently acting agents can get in each other s way . interfere with each other think of . two people both trying to use the bathroom at the same time in the morning. The solution to the problem is to introduce a mechanism door locks and a protocol for using that mechanism lock the bathroom door if you don t want to be disturbed . I m not using the term agent here in any special technical sense in particular not in the formal sense of Chapter 21. By analogy with intuitive examples such as the foregoing concurrency control in transaction processing systems has traditionally been based on a mechanism called locking though of course the locks involved are software constructs not hardware and a protocol the two-phase locking protocol for using that mechanism. Moreover most systems still typically rely on locking right up to this day a fact that explains the emphasis on locking in the body of the chapter. However certain nonlocking schemes are described in the annotation to several of the references in the References and Bibliography section. Copyright c 2003 C. J. Date page Three Concurrency Problems The three classic problems lost updates uncommitted dependencies and inconsistent analysis. The .
