如下测例中,没有违反引用约束的插入失败。
- create database `a-b`;
- use `a-b`;
- SET FOREIGN_KEY_CHECKS=0;
- create table t1(c1 int primary key, c2 int) engine=innodb;
- create table t2(c1 int primary key, c2 int) engine=innodb;
- alter table t2 add foreign key(c2) references `a-b`.t1(c1);
- SET FOREIGN_KEY_CHECKS=1;
- insert into t1 values(1,1);
- select * from t1;
- c1 c2
- 1 1
- select * from t2;
- c1 c2
- insert into t2 values(1,1);
- ERROR 23000: Cannot add or update a child row: a foreign key constraint fails (`a-b`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`c2`) REFERENCES `a-b`.`t1` (`c1`))
- insert into t2 values(1,1); //预期应该成功实际失败了。子表插入任何数据都会报违反引用约束。
首先我们会检查表结构是否正常
- show create table t2;
- Table Create Table
- t2 CREATE TABLE `t2` (
- `c1` int(11) NOT NULL,
- `c2` int(11) DEFAULT NULL,
- PRIMARY KEY (`c1`),
- KEY `c2` (`c2`),
- CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`c2`) REFERENCES `a-b`.`t1` (`c1`)
- ) ENGINE=InnoDB DEFAULT CHARSET=latin1
查看 innodb_sys_foreign 表
- select * from information_schema.innodb_sys_foreign where id='a@002db/t2_ibfk_1';
- +-------------------+------------+----------+--------+------+
- | ID | FOR_NAME | REF_NAME | N_COLS | TYPE |
- +-------------------+------------+----------+--------+------+
- | a@002db/t2_ibfk_1 | a@002db/t2 | a-b/t1 | 1 | 0 |
- +-------------------+------------+----------+--------+------+
- select * from information_schema.innodb_sys_tables where name='a@002db/t1';
- +----------+------------+------+--------+-------+-------------+------------+---------------+
- | TABLE_ID | NAME | FLAG | N_COLS | SPACE | FILE_FORMAT | ROW_FORMAT | ZIP_PAGE_SIZE |
- +----------+------------+------+--------+-------+-------------+------------+---------------+
- | 530 | a@002db/t1 | 1 | 5 | 525 | Antelope | Compact | 0 |
- +----------+------------+------+--------+-------+-------------+------------+---------------+
表结构正常,表面上看外键在系统表中元数据库信息正常。仔细比较发现 innodb_sys_foreign 的REF_NAME字段”a-b/t1”实际应为”a@002db/t2”。
MySQL 内部用 my_charset_filename 字符集来表名和库名。
以下数组定义了 my_charset_filename 字符集需要转换的字符。数组下标为 ascii 值,1代表不需要转换。可以看到字母数字和下划线等不需要转换,同时字符’-‘是需要转换的, 转换函数参见
。
- my_wc_mb_filename
- static char filename_safe_char[128]=
- {
- 1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* ................ */
- 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* ................ */
- 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, /* !"#$%&'()*+,-./ */
- 1,1,1,1,1,1,1,1,1,1,0,0,0,0,0,0, /* 0123456789:;<=>? */
- 0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, /* @ABCDEFGHIJKLMNO */
- 1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,1, /* PQRSTUVWXYZ[\]^_ */
- 0,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1, /* `abcdefghijklmno */
- 1,1,1,1,1,1,1,1,1,1,1,0,0,0,0,0, /* pqrstuvwxyz{|}~. */
- };
由上节可知,字符’-‘作为库名或表名是需要转换的。innodb_sys_foreign 中 FOR_NAME 值是转换过的,只有 REF_NAME 未转换,而系统表 innodb_sys_tables 中存储的表名是转换后的。
根据未转换的表名 a-b/t1 去系统表 SYS_TABLES 查找会查找不到记录。于是会导致
- dict_get_referenced_table
- foreign->referenced_table==NULL
因此对子表的任何插入都会返回错误 DB_NO_REFERENCED_ROW,如下代码
- row_ins_check_foreign_constraint:
- if (check_ref) {
- check_table = foreign->referenced_table;
- check_index = foreign->referenced_index;
- } else {
- check_table = foreign->foreign_table;
- check_index = foreign->foreign_index;
- }
- if (check_table == NULL
- || check_table->ibd_file_missing
- || check_index == NULL) {
- if (!srv_read_only_mode && check_ref) {
- ……
- err = DB_NO_REFERENCED_ROW;
- }
- goto exit_func;
经过进一步调试分析发现,函数
中主表的库名和表名都没有经过转换,而是直接使用系统字符集。
- innobase_get_foreign_key_info
回过头再看看bug的触发条件:
这里强调下第3条, 如果上面的测例中去掉了SET FOREIGN_KEY_CHECKS=0,那么结果 REF_NAME会正常转换
- SET FOREIGN_KEY_CHECKS=1;
- create table t1(c1 int primary key, c2 int) engine=innodb;
- create table t2(c1 int primary key, c2 int) engine=innodb;
- alter table t2 add foreign key(c2) references `a-b`.t1(c1);
- select * from information_schema.innodb_sys_foreign where id='a@002db/t2_ibfk_1';
- +-------------------+------------+------------+--------+------+
- | ID | FOR_NAME | REF_NAME | N_COLS | TYPE |
- +-------------------+------------+------------+--------+------+
- | a@002db/t2_ibfk_1 | a@002db/t2 | a@002db/t1 | 1 | 0 |
- +-------------------+------------+------------+--------+------+
MySQL 5.6 online DDL 是支持建索引的。而对于建外键索引同样也是支持的,条件是SET FOREIGN_KEY_CHECKS=0。
- ha_innobase::check_if_supported_inplace_alter:
- if ((ha_alter_info->handler_flags
- & Alter_inplace_info::ADD_FOREIGN_KEY)
- && prebuilt->trx->check_foreigns) {
- ha_alter_info->unsupported_reason = innobase_get_err_msg(
- ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_FK_CHECK);
- DBUG_RETURN(HA_ALTER_INPLACE_NOT_SUPPORTED);
- }
SET FOREIGN_KEY_CHECKS=0时,
为false。
- prebuilt->trx->check_foreigns
我们再来看出问题的函数
,只有online DDL的代码路径才会调用此函数:
- innobase_get_foreign_key_info
- #0 innobase_get_foreign_key_info
- #1 ha_innobase::prepare_inplace_alter_table
- #2 handler::ha_prepare_inplace_alter_table
- #3 mysql_inplace_alter_table
- #4 mysql_alter_table
- ......
而非online DDL的路径如下,函数
会对表名和库名进行转换:
- dict_scan_id
- #0 dict_scan_id
- #1 dict_scan_table_name
- #2 dict_create_foreign_constraints_low
- #3 dict_create_foreign_constraints
- #4 row_table_add_foreign_constraints
- #5 ha_innobase::create
- #6 handler::ha_create
- #7 ha_create_table
- #8 mysql_alter_table
- ......
bug系统中虽然没有相关的bug信息,但从MySQL 5.6.26中我们看到官方Bug#21094069已经进行了修复,在
中对库名和表名进行转换。
- innobase_get_foreign_key_info
参考commit:1fae0d42c352908fed03e29db2b391a0d2969269
来源: http://mysql.taobao.org/monthly/2015/11/06/