导读：如何查看DDL的进度？

使用MariaDB/Percona版本的一个便利之处就是可以及时查看DDL的进度，进而预估DDL耗时。
其实，在官方版本里也是可以查看DDL进度的，认真看手册的同学就能发现手册中有提到过：

You can monitor ALTER TABLE progress for InnoDB tables using Performance Schema.

如何开启

需要启用performance_schema，并设置2个地方：

UPDATE performance_schema.setup_instruments SET ENABLED = 'YES' WHERE NAME LIKE 'stage/innodb/alter%';
 
UPDATE performance_schema.setup_consumers SET ENABLED = 'YES' WHERE NAME LIKE '%stages%';

测试

• ddl(copy方式)

mysql> select count(1) from sbtest2;
+----------+
| count(1) |
+----------+
|  9998951 |
+----------+
1 row in set (2.10 sec)

mysql> alter table sysbench.sbtest2 ALGORITHM=COPY, add test7 varchar(20) not null default 'test7';

另一个窗口查看进度:

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+-----------------------------+----------------+----------------+
| EVENT_NAME                  | WORK_COMPLETED | WORK_ESTIMATED |
+-----------------------------+----------------+----------------+
| stage/sql/copy to tmp table |         126959 |        9842742 |
+-----------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+-----------------------------+----------------+----------------+
| EVENT_NAME                  | WORK_COMPLETED | WORK_ESTIMATED |
+-----------------------------+----------------+----------------+
| stage/sql/copy to tmp table |        4494926 |        9842742 |
+-----------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+-----------------------------+----------------+----------------+
| EVENT_NAME                  | WORK_COMPLETED | WORK_ESTIMATED |
+-----------------------------+----------------+----------------+
| stage/sql/copy to tmp table |        8398679 |        9842742 |
+-----------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
Empty set (0.00 sec)

• ddl(inplace方式)

mysql> alter table sysbench.sbtest2 add test8 varchar(20) not null default 'test8';                 

另一个窗口查看

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+------------------------------------------------------+----------------+----------------+
| EVENT_NAME                                           | WORK_COMPLETED | WORK_ESTIMATED |
+------------------------------------------------------+----------------+----------------+
| stage/innodb/alter table (read PK and internal sort) |         226282 |         703674 |
+------------------------------------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+-----------------------------------+----------------+----------------+
| EVENT_NAME                        | WORK_COMPLETED | WORK_ESTIMATED |
+-----------------------------------+----------------+----------------+
| stage/innodb/alter table (insert) |         988006 |        1210970 |
+-----------------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+----------------------------------+----------------+----------------+
| EVENT_NAME                       | WORK_COMPLETED | WORK_ESTIMATED |
+----------------------------------+----------------+----------------+
| stage/innodb/alter table (flush) |        1184776 |        1184776 |
+----------------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
+----------------------------------+----------------+----------------+
| EVENT_NAME                       | WORK_COMPLETED | WORK_ESTIMATED |
+----------------------------------+----------------+----------------+
| stage/innodb/alter table (flush) |        1205192 |        1205192 |
+----------------------------------+----------------+----------------+
1 row in set (0.00 sec)

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_current;
Empty set (0.00 sec)

最后，也可以查看 events_stages_history 里记录的完整过程：

mysql> SELECT EVENT_NAME, WORK_COMPLETED, WORK_ESTIMATED FROM performance_schema.events_stages_history; 
+------------------------------------------------------+----------------+----------------+
| EVENT_NAME                                           | WORK_COMPLETED | WORK_ESTIMATED |
+------------------------------------------------------+----------------+----------------+
| stage/innodb/alter table (merge sort)                |         896692 |        1210970 |
| stage/innodb/alter table (insert)                    |        1179932 |        1179932 |
| stage/innodb/alter table (flush)                     |        1219397 |        1219397 |
| stage/innodb/alter table (log apply table)           |        1219781 |        1219781 |
| stage/innodb/alter table (end)                       |        1219781 |        1219781 |
| stage/innodb/alter table (log apply table)           |        1220165 |        1220165 |
| stage/innodb/alter table (end)                       |        1154899 |        1154899 |
| stage/innodb/alter table (log apply table)           |        1155283 |        1155283 |
| stage/sql/copy to tmp table                          |        9998951 |        9842742 |
| stage/innodb/alter table (read PK and internal sort) |         566352 |         703674 |
+------------------------------------------------------+----------------+----------------+
10 rows in set (0.00 sec)

一个ddl的执行过程(inplace)

从上面的结果我们也能看到，一个DDL执行过程包括下面几个主要阶段：

1. stage/innodb/alter table (read PK and internal sort)，读取主键（聚集索引），计算需要处理的data page数；

2. stage/innodb/alter table (merge sort)，处理ALTER TABLE影响的索引，每个索引跑一次（含主键索引）；

3. stage/innodb/alter table (insert)，同上；

4. stage/innodb/alter table (log apply index)，将执行DDL期间新增的DML操作应用到index上；

5. stage/innodb/alter table (flush)，flush阶段；

6. stage/innodb/alter table (log apply table)，将执行DDL期间新增的DML操作应用到table上；

7. stage/innodb/alter table (end)，收尾阶段。

局限

• 只支持MySQL 5.7+的版本
• 只支持InnoDB引擎表；
• 不支持spatial indexes。

查看当前所有ALTER TABLE的进度及其对应的ddl sql

mysql>  SELECT ec.THREAD_ID, ec.EVENT_NAME, ec.WORK_COMPLETED, ec.WORK_ESTIMATED, pt.STATE, pt.INFO FROM performance_schema.events_stages_current ec left join performance_schema.threads th on ec.thread_id = th.thread_id left join information_schema.PROCESSLIST pt on th.PROCESSLIST_ID = pt.ID where pt.INFO like 'ALTER%'\G
*************************** 1. row ***************************
     THREAD_ID: 1258
    EVENT_NAME: stage/innodb/alter table (read PK and internal sort)
WORK_COMPLETED: 11366
WORK_ESTIMATED: 737631
         STATE: altering table
          INFO: alter table sysbench.sbtest2 add test9 varchar(20) not null default 'test9'
*************************** 2. row ***************************
     THREAD_ID: 1314
    EVENT_NAME: stage/innodb/alter table (read PK and internal sort)
WORK_COMPLETED: 22970
WORK_ESTIMATED: 197835
         STATE: altering table
          INFO: alter table sbtest3 add test1 varchar(257) not null default 'test1'
2 rows in set (0.00 sec)