数据库内核月报 - 2015 / 11

MySQL · 捉虫动态 · MySQL 外键异常分析

外键约束异常现象

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。

异常分析

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内部表名和库名存储格式

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{|}~. */
};

异常分析

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。dict_get_referenced_table 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

 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innobase_get_foreign_key_info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。

回过头再看看bug的触发条件:

  1. 表名或库名包含特殊字符;
  2. 此表作为引用约束的主表;
  3. 增加引用约束是设置了SET FOREIGN_KEY_CHECKS=0;

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 |
+-------------------+------------+------------+--------+------+

online DDL 与 foreign key

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,prebuilt->trx->check_foreigns为false。

我们再来看出问题的函数innobase_get_foreign_key_info,只有online DDL的代码路径才会调用此函数:

#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 dict_scan_id 会对表名和库名进行转换:

#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
......

修复

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innobase_get_foreign_key_info中对库名和表名进行转换。

参考commit:1fae0d42c352908fed03e29db2b391a0d2969269