MySQL从库的列类型不一致导致的复制异常问题
官方文档：https://dev.mysql.com/doc/refman/5.6/en/replication-features-differing-tables.html
slave_type_conversions 这个参数在mysql5.5.3 引入，目的是启用row 格式的bin-log 的时候，如果主从的column 的数据类型不一致，会导致复制失败，mysql5.5.3 之后支持，主库是int 从库是bigint 这种类型的复制，
这个参数的意义就是控制些类型转换容错性。
如果从库的字段类型范围比主库类型大，那么设置slave_type_conversions=ALL_NON_LOSSY后复制没有问题的。
如果从库类型比主库类型小，比如从int 复制到tinyint ，虽然可以通过设置slave_type_conversions=ALL_LOSSY,ALL_NON_LOSSY让主从不出问题，但是实际上会造成数据丢失的风险。
几种值的设置：
ALL_LOSSY：允许数据截断
ALL_NON_LOSSY：不允许数据截断，如果从库类型大于主库类型，是可以复制的，反过了，就不行了，从库报复制错误，复制终止。
ALL_LOSSY,ALL_NON_LOSSY: 所有允许的转换都会执行，而不管是不是数据丢失。
空值（不设置）：要求主从库的数据类型必须严格一致，否则都报错。
Mode | Effect |
|---|---|
ALL_LOSSY | In this mode, type conversions that would mean loss of information are permitted.This does not imply that non-lossy conversions are permitted, merely that only cases requiring either lossy conversions or no conversion at all are permitted; for example, enabling only this mode permits an INT column to be converted to TINYINT (a lossy conversion), but not a TINYINT column to an INT column (non-lossy). Attempting the latter conversion in this case would cause replication to stop with an error on the slave. |
ALL_NON_LOSSY | This mode permits conversions that do not require truncation or other special handling of the source value; that is, it permits conversions where the target type has a wider range than the source type. 【确保从库的列类型更宽泛些也不会导致复制报错】Setting this mode has no bearing on whether lossy conversions are permitted; this is controlled with the ALL_LOSSY mode. If only ALL_NON_LOSSY is set, but not ALL_LOSSY, then attempting a conversion that would result in the loss of data (such as INT to TINYINT, or CHAR(25) to VARCHAR(20)) causes the slave to stop with an error. |
ALL_LOSSY,ALL_NON_LOSSY | When this mode is set, all supported type conversions are permitted, whether or not they are lossy conversions. |
ALL_SIGNED | Treat promoted integer types as signed values (the default behavior). |
ALL_UNSIGNED | Treat promoted integer types as unsigned values. |
ALL_SIGNED,ALL_UNSIGNED | Treat promoted integer types as signed if possible, otherwise as unsigned. |
[empty] | When slave_type_conversions is not set, no attribute promotion or demotion is permitted; this means that all columns in the source and target tables must be of the same types.This mode is the default. |
生产环境一个案例：
从库是有个oracle系统去查数据的，原先从库的cp_shop_activity表示utf8mb4字符集，oracle不支持。因此在这个从库上手动修改了mode字段的字符类型为varchar(200) utf8，修改后的字段类型如下图：
但是没几天后，发现主从复制报错了，错误码1677。从库上记录的错误日志如下：
我们看下主库的mode字段是varchar(50)，从库的mode字段是varchar(200)。
在从库设置：
stop slave;
set global slave_type_conversions=ALL_NON_LOSSY;
# 默认slave_type_conversions为空，表示强制从库和主库的字段类型一致，不然就停止复制。设置为ALL_NON_LOSSY就稍微宽泛些同时确保不会造成类型转换的问题。
start slave;
show slave status\G
这样即可解决这个问题。
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