4---可重复读级别(Repeatable read isolation): 防止脏读与可重复读 8---可串行化级别(Serializable isolation): 防止脏读、可重复读与幻读, 事务串行化,严重影响效率.
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比如，读取只读的查找表不需要可重复读级别，未提交读级别就足够了。
For example, reading from a read - only look up table doesn t require repeatable read, and uncommitted read is sufficient.
支持表级和行级锁以及4个隔离级别：RR(可重复读)、RS(读可靠)、CS(默认——游标可靠)和UR(未提交读)。
Supports table and row level locking and 4 isolation levels, RR (repeatable read), rs (read stability), CS (default - cursor stability) and ur (uncommitted read).
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