The phenomena which are prohibited at various levels are:
A transaction reads data written by a concurrent uncommitted transaction.
A transaction re-reads data it has previously read and finds that data has been modified by another transaction (that committed since the initial read).
A transaction re-executes a query returning a set of rows that satisfy a search condition and finds that the set of rows satisfying the condition has changed due to another recently-committed transaction.
The result of successfully committing a group of transactions is inconsistent with all possible orderings of running those transactions one at a time.
| Isolation Level | Dirty Read | Nonrepeatable Read | Phantom Read | Serialization Anomaly |
|---|---|---|---|---|
| Read uncommitted | Allowed, but not in PG | Possible | Possible | Possible |
| Read committed | Not possible Possible | Possible | Possible | |
| Repeatable read | Not possible | Not possible | Allowed, but not in PG | Possible |
| Serializable | Not possible | Not possible | Not possible | Not possible |
级别越高，执行效率越低
会发生dirty read（脏读）
公司发工资了，把50000元打到我的账号上，但是该事务并未提交，而我正好去查看账户，看到50000元，非常高兴。但是公司发现金额不对，是2000元，于是回滚事务，修改金额后，将事务提交，最后我实际入账只有2000元。
可以避免dirty read，但会发生nonrepeatable read（不可重复读）
我拿着工资卡去消费，系统读取到卡里有2000元，而此时LP也正好在网上转账，把2000元转到她账户，并在我之前提交了事务，当我扣款时，系统检查到工资卡已经没有钱，扣款失败，十分纳闷，明明卡里有钱
可避免nonrepeatable read，但会发生Phantom read（幻读）
当我拿着工资卡去消费时，一旦系统开始读取工资卡信息（即事务开始），LP就不可能对该记录进行修改，也就是不能在此时转账。
幻读：LP正查询到我当月信用卡的总消费金额select sum(amount) from transaction where month = 本月为80元，而我此时正好在收银台买单，消费1000元，即新增了一条1000元的消费记录insert transaction ...，并提交了事务，随后LP将我的当月信用卡消费的明细打印到A4纸上，却发现消费总额为1080元，很诧异，以为出现了幻觉
避免上述各种状况