Description:
The table events_stages_history_long in PFS can store all the stages after a statement is executed (https://dev.mysql.com/doc/refman/8.0/en/performance-schema-stage-tables.html).
I have used it to search the execute progress of DDL, and I founded that the stages about DDL are wrong after introduced parallel DDL (WL#14283).
The table structure:
```sql
CREATE TABLE `t1` (
`id` bigint NOT NULL AUTO_INCREMENT,
`c1` int DEFAULT NULL,
`c2` text,
`c3` int DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `idx_1` (`c1`),
KEY `idx_3` (`c3`)
) ENGINE=InnoDB;
```
The DDL:
```sql
optimize table t1;
```
The result in MySQL 5.7:
```sql
mysql> select THREAD_ID, EVENT_NAME, TIMER_START, TIMER_END, TIMER_WAIT from performance_schema.events_stages_history_long where EVENT_NAME like "%alter%" order by TIMER_START;
+-----------+------------------------------------------------------+--------------------+--------------------+--------------+
| THREAD_ID | EVENT_NAME | TIMER_START | TIMER_END | TIMER_WAIT |
+-----------+------------------------------------------------------+--------------------+--------------------+--------------+
| 81 | stage/sql/preparing for alter table | 251701856368837000 | 251701859868915000 | 3500078000 |
| 81 | stage/sql/altering table | 251701859868915000 | 251701859870071000 | 1156000 |
| 81 | stage/innodb/alter table (read PK and internal sort) | 251701859870071000 | 251702150226287000 | 290356216000 |
| 81 | stage/innodb/alter table (merge sort) | 251702150226287000 | 251702157141860000 | 6915573000 |
| 81 | stage/innodb/alter table (insert) | 251702157141860000 | 251702175641217000 | 18499357000 |
| 81 | stage/innodb/alter table (merge sort) | 251702175641217000 | 251702181858863000 | 6217646000 |
| 81 | stage/innodb/alter table (insert) | 251702181858863000 | 251702201963443000 | 20104580000 |
| 81 | stage/innodb/alter table (flush) | 251702201963443000 | 251702930840563000 | 728877120000 |
| 81 | stage/innodb/alter table (log apply table) | 251702930840563000 | 251702930878908000 | 38345000 |
| 81 | stage/sql/committing alter table to storage engine | 251702930878908000 | 251702930881347000 | 2439000 |
| 81 | stage/innodb/alter table (end) | 251702930881347000 | 251702930895480000 | 14133000 |
| 81 | stage/innodb/alter table (log apply table) | 251702930895480000 | 251703086731347000 | 155835867000 |
+-----------+------------------------------------------------------+--------------------+--------------------+--------------+
12 rows in set (0.00 sec)
```
The result in MySQL 8.0:
```sql
mysql> select THREAD_ID, EVENT_NAME, TIMER_START, TIMER_END, TIMER_WAIT from performance_schema.events_stages_history_long where EVENT_NAME like "%alter%" order by TIMER_START;
+-----------+------------------------------------------------------+------------------+------------------+--------------+
| THREAD_ID | EVENT_NAME | TIMER_START | TIMER_END | TIMER_WAIT |
+-----------+------------------------------------------------------+------------------+------------------+--------------+
| 59 | stage/sql/preparing for alter table | 2912076608653000 | 2912114419697000 | 37811044000 |
| 59 | stage/sql/altering table | 2912114419697000 | 2912114470811000 | 51114000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912114470811000 | 2912114873229000 | 402418000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912114873229000 | 2912115486035000 | 612806000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912115486035000 | 2912671572611000 | 556086576000 |
| 59 | stage/innodb/alter table (merge sort) | 2912671572611000 | 2912671824880000 | 252269000 |
| 61 | stage/innodb/alter table (merge sort) | 2912671588316000 | 2912671847201000 | 258885000 |
| 59 | stage/innodb/alter table (insert) | 2912671824880000 | 2912891400387000 | 219575507000 |
| 59 | stage/innodb/alter table (end) | 2912891400387000 | 2912891404485000 | 4098000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912891404485000 | 2912891405400000 | 915000 |
| 59 | stage/innodb/alter table (end) | 2912891405400000 | 2912891406074000 | 674000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912891406074000 | 2912891406372000 | 298000 |
| 59 | stage/innodb/alter table (merge sort) | 2912891406372000 | 2912891406657000 | 285000 |
| 59 | stage/innodb/alter table (insert) | 2912891406657000 | 2912891407666000 | 1009000 |
| 59 | stage/innodb/alter table (end) | 2912891407666000 | 2912891407990000 | 324000 |
| 59 | stage/innodb/alter table (read PK and internal sort) | 2912891407990000 | 2912891408209000 | 219000 |
| 59 | stage/innodb/alter table (merge sort) | 2912891408209000 | 2912891408426000 | 217000 |
| 59 | stage/innodb/alter table (insert) | 2912891408426000 | 2912891666599000 | 258173000 |
| 59 | stage/innodb/alter table (end) | 2912891666599000 | 2912891817823000 | 151224000 |
| 59 | stage/innodb/alter table (end) | 2912891817823000 | 2912891969724000 | 151901000 |
| 59 | stage/innodb/alter table (end) | 2912891969724000 | 2912891977100000 | 7376000 |
| 59 | stage/innodb/alter table (log apply table) | 2912891977100000 | 2912892125696000 | 148596000 |
| 59 | stage/sql/committing alter table to storage engine | 2912892125696000 | 2912892128569000 | 2873000 |
| 59 | stage/innodb/alter table (end) | 2912892128569000 | 2912892193906000 | 65337000 |
| 59 | stage/innodb/alter table (log apply table) | 2912892193906000 | 2913057696669000 | 165502763000 |
+-----------+------------------------------------------------------+------------------+------------------+--------------+
25 rows in set (0.00 sec)
```
I can find each stages and the time cost clearly in MySQL 5.7, but it becomes a mess in MySQL 8.0. For example, the stage named 'stage/innodb/alter table (read PK and internal sort)' appears only once in MySQL 5.7, but it appears 6 times in MySQL 8.0. You may say that MySQL 8.0 introduces parallel DDL, and it uses multiple threads to do the DDL, but the THREAD_ID in above result is the same.
And I have searched the code of MySQL 8.0, for optimize table, the parallel_reader will still use the sync mode to do the scan. So, I think the stages about DDL in PFS.events_stages_history_long are wrong.
How to repeat:
No need.
