pt-osc原理

1、檢查設定環境

測試db是否可連通,并且驗證database是否存在
SET SESSION innodb_lock_wait_timeout=1 //InnoDB事務等待行鎖的逾時時間
SET SESSION lock_wait_timeout=60  //設定擷取中繼資料瑣逾時為60s
SET SESSION wait_timeout=10000   //互動逾時時間
權限驗證 show grants for current_user()
           

2、建立臨時表_tablename_new并修改臨時表結構

3、在源表建立三個觸發器

--建立delete觸發器
CREATE TRIGGER `pt_osc_darren_t_user_del` AFTER DELETE ON `darren`.`t_user` FOR EACH ROW DELETE IGNORE FROM `darren`.`_t_user_new`
WHERE `darren`.`_t_user_new`.`id` <=> OLD.`id`

--建立update觸發器
CREATE TRIGGER `pt_osc_darren_t_user_upd` AFTER UPDATE ON `darren`.`t_user` FOR EACH ROW 
BEGIN 
    DELETE IGNORE FROM `darren`.`_t_user_new` WHERE !(OLD.`id` <=> NEW.`id`) AND `darren`.`_t_user_new`.`id` <=> OLD.`id`;
    REPLACE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
    VALUES (NEW.`id`, NEW.`name`, NEW.`phone`, NEW.`gender`, NEW.`type`, NEW.`birth`, NEW.`is_delete`, NEW.`c1`, NEW.`c2`, NEW.`c3`);
END;

--建立insert觸發器
CREATE TRIGGER `pt_osc_darren_t_user_ins` AFTER INSERT ON `darren`.`t_user` 
  FOR EACH ROW 
     REPLACE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
     VALUES (NEW.`id`, NEW.`name`, NEW.`phone`, NEW.`gender`, NEW.`type`, NEW.`birth`, NEW.`is_delete`, NEW.`c1`, NEW.`c2`, NEW.`c3`)
           

4、開始遷移資料：

1）估算表的大小，僅僅用于計算進度：EXPLAIN SELECT * FROM `darren`.`t_user` WHERE 1=1；
   2）最小值：SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) ORDER BY `id` LIMIT 1;
   3) 根據chunk-size，計算本次chunk最大值及下一個chunk開始值：SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) 
                     ORDER BY `id` LIMIT 2, 2；
   4) 插入資料：
         INSERT LOW_PRIORITY IGNORE INTO `darren`.`_t_user_new` (`id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3`) 
         SELECT `id`, `name`, `phone`, `gender`, `type`, `birth`, `is_delete`, `c1`, `c2`, `c3` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) 
         WHERE ((`id` >= '1')) AND ((`id` <= '3')) LOCK IN SHARE MODE /*pt-online-schema-change 4670 copy nibble*/
   5）開始下一輪疊代......
   6) 何時copy結束？
       根據SELECT `id` FROM `darren`.`t_user` FORCE INDEX(`PRIMARY`) WHERE ((`id` >= '1')) ORDER BY `id` LIMIT 2, 2；
       如果擷取不到下一個chunk開始值，視為表copy完成，相關源碼片段如下：
					sub _get_bounds {
					   my ($self) = @_;
					
					   if ( $self->{one_nibble} ) {
					      if ( $self->{resume} ) {
					         $self->{no_more_boundaries} = 1;
					      }
					      return;
					   }
					
					   my $dbh = $self->{Cxn}->dbh();
					
					   $self->{first_lower} = $dbh->selectrow_arrayref($self->{first_lb_sql});
					   PTDEBUG && _d('First lower boundary:', Dumper($self->{first_lower}));
					
					   if ( my $nibble = $self->{resume} ) {
					      if (    defined $nibble->{lower_boundary}
					           && defined $nibble->{upper_boundary} ) {
					         my $sth = $dbh->prepare($self->{resume_lb_sql});
					         my @ub  = split ',', $nibble->{upper_boundary};
					         PTDEBUG && _d($sth->{Statement}, 'params:', @ub);
					         $sth->execute(@ub);
					         $self->{next_lower} = $sth->fetchrow_arrayref();
					         $sth->finish();
					      }
					   }
					   else {
					      $self->{next_lower}  = $self->{first_lower};
					   }
					   PTDEBUG && _d('Next lower boundary:', Dumper($self->{next_lower}));
					
					   if ( !$self->{next_lower} ) {
					      PTDEBUG && _d('At end of table, or no more boundaries to resume');
					      $self->{no_more_boundaries} = 1;
					
					      $self->{last_upper} = $dbh->selectrow_arrayref($self->{last_ub_sql});
					      PTDEBUG && _d('Last upper boundary:', Dumper($self->{last_upper}));
					   }
					
					   return;
					}

           

5、rename表

ANALYZE TABLE `darren`.`_t_user_new`;
RENAME TABLE `darren`.`t_user` TO `darren`.`_t_user_old`, `darren`.`_t_user_new` TO `darren`.`t_user`;

           

6、清理戰場

DROP TABLE IF EXISTS `darren`.`_t_user_old`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_del`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_upd`
DROP TRIGGER IF EXISTS `darren`.`pt_osc_darren_t_user_ins`
SHOW TABLES FROM `darren` LIKE '_t_user_new'
           

【copy rows和觸發器實作資料遷移和增量寫入原理：】

DDL功能	映射執行的SQL語句
insert觸發器	replace into
update觸發器	delete ignore + replace into
delete觸發器	delete ignore
copy rows	insert ignore into