查看并行查询

Recent Pages

查看并行查询

最后更新时间：2023-01-06 11:35:30
TDSQL-C MySQL 版支持查看并行查询的执行计划，以及查看线程中哪些线程在执行并行查询计划。您可清晰了解到并行查询是如何在数据库中稳定生效，也可在并行查询执行过程中遇到问题时，帮助快速定位问题。
本文为您介绍查看并行查询的两种常用方法。
方法一：使用 EXPLAIN 语句
示例 SQL 语句：
SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty
FROM lineitem
WHERE l_shipdate <= '1998-09-02'
GROUP BY l_returnflag, l_linestatus
ORDER BY l_returnflag, l_linestatus;
本示例为 TPC-H Q1 的简化形式，是典型的报表运算。
执行计划打印语句（EXPLAIN）：
EXPLAIN  SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty
FROM lineitem
WHERE l_shipdate <= '1998-09-02'
GROUP BY l_returnflag, l_linestatus
ORDER BY l_returnflag, l_linestatus;
查询结果:
MySQL [tpch100g]> explain SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty FROM lineitem WHERE l_shipdate <= '1998-09-02' GROUP BY l_returnflag, l_linestatus ORDER BY l_returnflag, l_linestatus;
+----+-------------+-------------+------------+------+---------------+------+---------+------+-----------+----------+-----------------------------------------------------------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows      | filtered | Extra                                                     |
+----+-------------+-------------+------------+------+---------------+------+---------+------+-----------+----------+-----------------------------------------------------------+
|  1 | SIMPLE      | lineitem    | NULL       | ALL  | i_l_shipdate  | NULL | NULL    | NULL | 593184480 |    50.00 | Parallel scan (4 workers); Using where; Using temporary   |
|  1 | SIMPLE      | <sender1>   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |         0 |     0.00 | Send to (<receiver1>)                                     |
|  1 | SIMPLE      | <receiver1> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |         0 |     0.00 | Receive from (<sender1>); Using temporary; Using filesort |
+----+-------------+-------------+------------+------+---------------+------+---------+------+-----------+----------+-----------------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)
树状执行计划打印语句（EXPLAIN format=tree）:
EXPLAIN format=tree query  SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty
FROM lineitem
WHERE l_shipdate <= '1998-09-02'
GROUP BY l_returnflag, l_linestatus
ORDER BY l_returnflag, l_linestatus;
查询结果:
MySQL [tpch100g]> explain format=tree SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty FROM lineitem WHERE l_shipdate <= '1998-09-02' GROUP BY l_returnflag, l_linestatus ORDER BY l_returnflag, l_linestatus\\G
*************************** 1. row ***************************
EXPLAIN: -> Sort: lineitem.L_RETURNFLAG, lineitem.L_LINESTATUS
    -> Table scan on <temporary>
        -> Final Aggregate using temporary table
            -> PX Receiver (slice: 0; workers: 1)
                -> PX Sender (slice: 1; workers: 4)
                    -> Table scan on <temporary>
                        -> Aggregate using temporary table
                            -> Filter: (lineitem.L_SHIPDATE <= DATE'1998-09-02')  (cost=65449341.10 rows=296592240)
                                -> Parallel table scan on lineitem  (cost=65449341.10 rows=593184480)
﻿
1 row in set (0.00 sec)
由上述结果可以看出：
并行查询计划将语句分布给了4个工作线程进行运算。
将聚合运算拆分为了上下段，用户线程和并行线程分别执行。
对 lineitem 表采用了并行扫描算子。
实例中树状执行计划打印（EXPLAIN format=tree query）相较于传统执行计划打印（EXPLAIN）效果更好。
方法二：线程列表查看
show processlist 命令的输出结果显示了有哪些线程在运行，不仅可以查看当前所有的连接数，还可以查看当前的连接状态帮助识别出有问题的查询语句等。
基于 show processlist 命令，TDSQL-C MySQL 版自研了 show parallel processlist 语句，帮助您过滤线程中非并行查询的线程，使用该命令行后，将只展示与并行查询有关的线程。
示例 SQL 语句：
SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty
FROM lineitem
WHERE l_shipdate <= '1998-09-02'
GROUP BY l_returnflag, l_linestatus
ORDER BY l_returnflag, l_linestatus;
本示例为 TPC-H Q1 的简化形式，是典型的报表运算。
show processlist 查询结果：
mysql> show processlist;
+--------+-------------+-----------------+-----------+---------+-------+------------+------------------------------------------------------------------------------------------------------+
| Id     | User        | Host            | db        | Command | Time  | State      | Info                                                                                                 |
+--------+-------------+-----------------+-----------+---------+-------+------------+------------------------------------------------------------------------------------------------------+
|      7 | tencentroot | 127.0.0.1:49238 | NULL      | Sleep   |     0 |            | NULL                                                                                                 |
|     11 | tencentroot | 127.0.0.1:49262 | NULL      | Sleep   |     0 |            | NULL                                                                                                 |
|     13 | tencentroot | 127.0.0.1:49288 | NULL      | Sleep   |     1 |            | NULL                                                                                                 |
| 237062 | tencentroot | localhost       | tpch100g  | Query   |    24 | Scheduling | SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty FROM lineitem WHERE l_shipdate <= '199 |
| 237107 | tencentroot | localhost       | NULL      | Query   |     0 | init       | show processlist                                                                                     |
+--------+-------------+-----------------+-----------+---------+-------+------------+------------------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)
show parallel processlist 查询结果：
mysql> show parallel processlist;
+--------+-------------+-----------+----------+---------+------+-------------+------------------------------------------------------------------------------------------------------+
| Id     | User        | Host      | db       | Command | Time | State       | Info                                                                                                 |
+--------+-------------+-----------+----------+---------+------+-------------+------------------------------------------------------------------------------------------------------+
| 237062 | tencentroot | localhost | tpch100g | Query   |   18 | Scheduling  | SELECT l_returnflag, l_linestatus, sum(l_quantity) as sum_qty FROM lineitem WHERE l_shipdate <= '199 |
| 237110 |             |           |          | Task    |   18 | Task runing | connection 237062, worker 0, task 1                                                                  |
| 237111 |             |           |          | Task    |   18 | Task runing | connection 237062, worker 1, task 1                                                                  |
| 237112 |             |           |          | Task    |   18 | Task runing | connection 237062, worker 2, task 1                                                                  |
| 237113 |             |           |          | Task    |   18 | Task runing | connection 237062, worker 3, task 1                                                                  |
+--------+-------------+-----------+----------+---------+------+-------------+------------------------------------------------------------------------------------------------------+
5 rows in set (0.00 sec)
由上述结果可以看出：
上述查询由并行计划分布给四个 work 线程进行执行：user 仅有一行有显示，表明 ID 237062 为用户线程，将 SQL 语句执行计划下推至下面四个 work 线程中进行，通过 info 列可看到，这四个工作线程均在执行 task1。
每个线程均可查询出来，精准进行定位。
show parallel processlist 相较于 show processlist 可以精准查询到所有进行并行查询的线程，不被其余线程影响。
相关文档
开启或关闭并行查询
hint 语句控制
并行查询指标
联系我们
联系我们，为您的业务提供专属服务。
技术支持
如果你想寻求进一步的帮助，通过工单与我们进行联络。我们提供7x24的工单服务。
7x24 电话支持
tencent cloud

Recent Pages

查看并行查询

方法一：使用 EXPLAIN 语句

方法二：线程列表查看

相关文档

本页内容是否解决了您的问题？

本页内容是否解决了您的问题？

tencent cloud

注册

登录

Recent Pages

查看并行查询

方法一：使用 EXPLAIN 语句

方法二：线程列表查看

相关文档

本页内容是否解决了您的问题？

本页内容是否解决了您的问题？
