MySQL：Innodb 关于Handler_commit每次DML增加2的原因

简单描述一下，也是本人的问的，水平有限，如果有误请谅解。

原问题如下@mysqDBA：

请教一个问题。我每次insert一条语句，查询show global status like 'Handler_commit'; 发现每次增加值是2，难道不应该是1吗？

最简单的insert into table a values(1);

一、问题展示

语句如下：

mysql> flush status;
Query OK, 0 rows affected (0.10 sec)

mysql> set  sql_log_bin=1;
Query OK, 0 rows affected (0.01 sec)
mysql> insert into testm values(16,'gaopeng',34);
Query OK, 1 row affected (0.15 sec)

mysql> show status like '%commit%';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| Com_commit     | 0     |
| Com_xa_commit  | 0     |
| Handler_commit | 2     |
+----------------+-------+
3 rows in set (0.01 sec)

问为什么 Handler_commit是2而不是1。

二、原因分析

其实对于这个问题只要看看这个Handler_commit指标增加的方式就可以看出原因。实际上这个指标出现在ha_commit_low函数中如下：

for (; ha_info; ha_info= ha_info_next)
    {
      int err;
      handlerton *ht= ha_info->ht();
      if ((err= ht->commit(ht, thd, all)))
      {
        my_error(ER_ERROR_DURING_COMMIT, MYF(0), err);
        error=1;
      }
      DBUG_ASSERT(!thd->status_var_aggregated);
      thd->status_var.ha_commit_count++; //此处增加
      ha_info_next= ha_info->next();
      if (restore_backup_ha_data)
        reattach_engine_ha_data_to_thd(thd, ht);
      ha_info->reset(); /* keep it conveniently zero-filled */
    }

可以清楚的看到ha_commit_count实际就是调用ht->commit的次数，由于有多个Handler的存在，因此这里需要调用多次。对于开启binlog+innodb的这种结构来讲分别要做：

• binlog的commit
• Innodb的commit

后面会看到实际binlog的commit什么都没做，但是这是一种协议。
那么如果我们关闭binlog可以发现Handler_commit为1了如下：

mysql> set  sql_log_bin=0;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into testm values(15,'gaopeng',34);
Query OK, 1 row affected (0.10 sec)

mysql>  show status like '%commit%';
+----------------+-------+
| Variable_name  | Value |
+----------------+-------+
| Com_commit     | 0     |
| Com_xa_commit  | 0     |
| Handler_commit | 1     |
+----------------+-------+
3 rows in set (0.01 sec)

三、binlog commit栈帧

#0  binlog_commit (hton=0x3485e30, thd=0x7fff2c014430, all=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:1833
#1  0x0000000000f64104 in ha_commit_low (thd=0x7fff2c014430, all=false, run_after_commit=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:1923
#2  0x000000000185772b in MYSQL_BIN_LOG::process_commit_stage_queue (this=0x2e01c80, thd=0x7fff2c014430, first=0x7fff2c014430)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8647
#3  0x0000000001858f5d in MYSQL_BIN_LOG::ordered_commit (this=0x2e01c80, thd=0x7fff2c014430, all=false, skip_commit=false)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:9318
#4  0x000000000185700c in MYSQL_BIN_LOG::commit (this=0x2e01c80, thd=0x7fff2c014430, all=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8440
#5  0x0000000000f63df8 in ha_commit_trans (thd=0x7fff2c014430, all=false, ignore_global_read_lock=false)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:1818

但是实际上binlog_commit什么都没做，因为在此之前他已经做完了需要做的事情比如flush、sync等

static int binlog_commit(handlerton *hton, THD *thd, bool all)
{
  DBUG_ENTER("binlog_commit");
  /*
    Nothing to do (any more) on commit.
   */
  DBUG_RETURN(0);
}

四、Innodb commit接口

#0  innobase_commit (hton=0x2e9edd0, thd=0x7fff2c014430, commit_trx=false) at /root/mysql5.7.14/percona-server-5.7.14-7/storage/innobase/handler/ha_innodb.cc:4652
#1  0x0000000000f64104 in ha_commit_low (thd=0x7fff2c014430, all=false, run_after_commit=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:1923
#2  0x000000000185772b in MYSQL_BIN_LOG::process_commit_stage_queue (this=0x2e01c80, thd=0x7fff2c014430, first=0x7fff2c014430)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8647
#3  0x0000000001858f5d in MYSQL_BIN_LOG::ordered_commit (this=0x2e01c80, thd=0x7fff2c014430, all=false, skip_commit=false)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:9318
#4  0x000000000185700c in MYSQL_BIN_LOG::commit (this=0x2e01c80, thd=0x7fff2c014430, all=false) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8440
#5  0x0000000000f63df8 in ha_commit_trans (thd=0x7fff2c014430, all=false, ignore_global_read_lock=false)
    at /root/mysql5.7.14/percona-server-5.7.14-7/sql/handler.cc:1818

实际上innodb comit才是需要真正做的，这里包含一些事情要做，比如事物状态的改变，资源的释放。

最后select也会增加Handler_commit，增加为1。