遇到了线上集群online ddl不生效的问题，本应该使用inplace的走了copy. 看了下代码，一开始看的是is_inplace_alter_imppossible,因为从名字来看就是判断是否能使用inplace的，但是看了一圈，夜没发现什么有用的信息，后面退出来看了mysql_alter_table中的流程，发现也有判断是否使用inplace的逻辑
/*
Use copy algorithm if:
- old_alter_table system variable is set without in-place requested using
the ALGORITHM clause.
- Or if in-place is impossible for given operation.
- Changes to partitioning which were not handled by
fast_alter_partition_table() needs to be handled using table copying
algorithm unless the engine supports auto-partitioning as such engines
can do some changes using in-place API.
*/
if ((thd->variables.old_alter_table &&
alter_info->requested_algorithm !=
Alter_info::ALTER_TABLE_ALGORITHM_INPLACE)
|| is_inplace_alter_impossible(table, create_info, alter_info, &alter_ctx)
|| (partition_changed &&
!(table->s->db_type()->partition_flags() & HA_USE_AUTO_PARTITION))
)
{
if (alter_info->requested_algorithm ==
Alter_info::ALTER_TABLE_ALGORITHM_INPLACE)
{
my_error(ER_ALTER_OPERATION_NOT_SUPPORTED, MYF(0),
"ALGORITHM=INPLACE", "ALGORITHM=COPY");
DBUG_RETURN(true);
}
alter_info->requested_algorithm= Alter_info::ALTER_TABLE_ALGORITHM_COPY;
}
检查了下对应的参数，果然是参数设置引起的问题。 为什么相关逻辑不放到同一个函数下统一处理？ 还单拎出来判断。。。
