Consistent Nonlocking Reads

Consistent Nonlocking Reads

关于可重复读隔离级别下幻读：http://my.oschina.net/xinxingegeya/blog/296513

mysql官方文档描述：http://dev.mysql.com/doc/refman/5.6/en/innodb-consistent-read.html

‍A consistent read means that InnoDB uses multi-versioning to present to a query a snapshot of the database at a point in time.‍

在提交读和可重复读隔离级别下默认的就是可重复读，也就是一致性读。

Consistent read is the default mode in which InnoDB processes SELECT statements in READ COMMITTED and REPEATABLE READ isolation levels.

A consistent read does not set any locks on the tables it accesses, and therefore other sessions are free to modify those tables at the same time a consistent read is being performed on the table.

Suppose that you are running in the default REPEATABLE READ isolation level. ‍When you issue a consistent read (that is, an ordinary SELECT statement), InnoDB gives your transaction a timepoint according to which your query sees the database. If another transaction deletes a row and commits after your timepoint was assigned, you do not see the row as having been deleted. Inserts and updates are treated similarly.‍

You can advance（提前） your timepoint by committing your transaction and then doing another SELECT or START TRANSACTION WITH CONSISTENT SNAPSHOT.

This is called multi-versioned concurrency control.

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