tailieunhanh - Microsoft SQL Server 2000 Weekend Crash Course phần 6

¼ Serializable - mức cao nhất có thể bị cô lập, trong đó giao dịch hoàn toàn biệt lập với nhau. Bảng 16-1 phác thảo các hành vi trưng bày các giao dịch ở các mức độ khác nhau của sự cô lập. Bảng 16-1 dữ liệu sẵn có tại Phần Mức độ cách ly khác nhau III-phiên Chiều thứ bảy 16 cách ly Cấp đọc | Session 16 Understanding Transactions and Locks 183 Read committed Allows the transaction to see the data after they are committed by any previous transactions. This is the default isolation level for SQL Server 2000. Repeatable read Ensures just that Reading can be repeated. Serializable The highest possible level of isolation wherein transactions are completely isolated from one another. Table 16-1 outlines the behavior exhibited by transactions at the different levels of isolation. Table 16-1 Data Availability at Different Isolation Levels Isolation Level Dirty Read Non-Repeatable Read Phantom Read Read uncommitted Yes Yes Yes Read committed No Yes Yes Repeatable read No No Yes Serializable No No No Dirty read refers to the ability to read records that are being modified since the data are in the process of being changed dirty reading may result in unpredictable results. Phantom read refers to the ability to see records that have already been deleted by another transaction. Part III Saturday Afternoon Session 16 When designing transactions keep them as short as possible as they consume valuable system resources. Introducing SQL Server Locks Locking is there to protect you. It is highly unlikely that you have the luxury of being the only user of your database. It is usually a case of tens hundreds or in case of the Internet thousands of concurrent users trying to read or modify the data sometimes exactly the same data. If not for locking your database would quickly lose its integrity. 184 Saturday Afternoon Consider a scenario wherein two transactions are working on the same record. If locking is not used the final results will be unpredictable because data written by one user can be overwritten or even deleted by another user. Fortunately SQL Server automatically applies locking when certain types of T-SQL operations are performed. SQL Server offers two types of locking control optimistic concurrency and pessimistic concurrency. Use optimistic concurrency when .