A Critique of ANSI SQL Isolation Levels

事务隔离级别定义

操作冲突: 两个不同事务对同一个数据项进行操作，且至少一个为write操作，则这两个操作是冲突操作。

等价调度: 不存在冲突的操作可以进行执行顺序的交换，交换后的结果与交换前效果是一致的，称为等价调度。

可串行化: 并发执行的事务操作顺序，通过等价调度，可以得到与顺序执行相同的调度，则称为事务是可串行化的。

隔离级别对应的3个现象:

1. Dirty Read

Transaction T1 modifies a data item. Another transaction T2 then reads that data item before T1 performs a COMMIT or ROLLBACK. If T1 then performs a ROLLBACK, T2 has read a data item that was never committed and so never really existed.

2. Non-repeatable or Fuzzy Read

Transaction T1 reads a data item. Another transaction T2 then modifies or deletes that data item and commits. If T1 then attempts to reread the data item, it receives a modified value or discovers that the data item has been deleted.

3. Phantom

Transaction T1 reads a set of data items satisfying some

. Transaction T2 then creates data items that satisfy T1’s and commits. If T1 then repeats its read with the same , it gets a set of data items different from the first read.

对应的宽泛解释和实际解释：

P1: w1[x]...r2[x]...((c1 or a1) and (c2 or a2) in any order)
A1: w1[x]...r2[x]...(a1 and c2 in any order)
P2: r1[x]...w2[x]...((c1 or a1) and (c2 or a2) in any order)
A2: r1[x]...w2[x]...c2...r1[x]...c1
P3: r1[P]...w2[y in P]...((c1 or a1) and (c2 or a2) any order)
A3: r1[P]...w2[y in P]...c2...r1[P]...c1

隔离级别和现象的对应关系：

串行化的定义前面已经讲过了，排出P1,P2,P3三种现象并不是串行化的实际定义。