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开发指南—DAL语句—EXPLAIN

2022-06-17 55
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简介： 该语句用于解释SQL语句的执行计划，包括SELECT、DELETE、INSERT、REPLACE或UPDATE语句。

语法

获取SQL计划信息：


EXPLAIN
{LOGICALVIEW | LOGIC | SIMPLE | DETAIL | EXECUTE | PHYSICAL | OPTIMIZER | SHARDING
 | COST | ANALYZE | BASELINE | JSON_PLAN | ADVISOR} 
 {SELECT statement | DELETE statement | INSERT statement | REPLACE statement| UPDATE statement}

示例

  • explain语句：展示基本的SQL执行计划，该执行计划是算子组成，主要体现SQL在CN上的整个执行过程。
mysql> explain select count(*) from lineitem group by L_ORDERKEY;

+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | LOGICAL EXECUTIONPLAN |
 +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | Project(count()="count()") |
 | HashAgg(group="L_ORDERKEY", count()="SUM(count())") |
 | Gather(concurrent=true) |
 | LogicalView(tables="[000000-000003].lineitem_[00-15]", shardCount=16, sql="SELECT `L_ORDERKEY`, COUNT() AS `count()` FROM `lineitem` AS `lineitem` GROUP BY `L_ORDERKEY`") |
 | HitCache:false | |
 | TemplateId: 5819c807 |
 +------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
  • 其中，HitCache标记该查询是否命中PlanCache，取值为false or true；TemplateId表示对该计划的标识，具有全局唯一性。
  • explain logicalview语句：展示LogicalView所表示的下推SQL在DN上的执行计划。
mysql> explain LOGICALVIEW select  mysql> explain select logialview count(*) from lineitem group by L_ORDERKEY;
+----------------------------------------------------------+
| LOGICAL EXECUTIONPLAN                                    |
+----------------------------------------------------------+
| Project(count()="count()")                             |
|   HashAgg(group="L_ORDERKEY", count()="SUM(count())")  |
|     Gather(concurrent=true)                              |
|       LogicalView                                        |
|         MysqlAgg(group="L_ORDERKEY", count(*)="COUNT()") |
|           MysqlTableScan(name=[ads, lineitem])           |
| HitCache:true                                            |
| Source:PLAN_CACHE                                        |
| TemplateId: 5819c807
  • explain execute语句：表示下推SQL在mysql的执行情况，这个语句和mysql的explain语句同义。通过该语句可以查看下推SQL在DN上有没有使用索引，有没有做全表扫描。
mysql> explain EXECUTE  select  count(*) from lineitem group by L_ORDERKEY;
+----+-------------+----------+------------+-------+---------------+---------+---------+-----+------+----------+----------------------------------------------+
| id | select_type | table    | partitions | type  | possible_keys | key     | key_len | ref | rows | filtered | Extra                                        |
+----+-------------+----------+------------+-------+---------------+---------+---------+-----+------+----------+----------------------------------------------+
| 1  | SIMPLE      | lineitem | NULL       | index | PRIMARY       | PRIMARY | 8       | NULL | 1    | 100      | Using index; Using temporary; Using filesort |
+----+-------------+----------+------------+-------+---------------+---------+---------+-----+------+----------+----------------------------------------------+
1 row in set (0.24 sec)
  • explain sharding语句：展示当前查询在DN上扫描的物理分片情况。
mysql> explain sharding  select  count(*) from lineitem group by L_ORDERKEY;
+---------------+----------------------------------+-------------+-----------+-----------+
| LOGICAL_TABLE | SHARDING                         | SHARD_COUNT | BROADCAST | CONDITION |
+---------------+----------------------------------+-------------+-----------+-----------+
| lineitem      | [000000-000003].lineitem_[00-15] | 16          | false     |           |
+---------------+----------------------------------+-------------+-----------+-----------+
1 row in set (0.04 sec)
  • explain cost语句：相对于explain语句，除了展示执行计划以外，还会显示各个算子基于统计信息估算的代价，以及这条查询被优化器识别的WORKLOAD。
mysql> explain COST  select  count(*) from lineitem group by L_ORDERKEY;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| LOGICAL EXECUTIONPLAN                                                                                                                                                                                                                                                                                            |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Project(count()="count()"): rowcount = 2508.0, cumulative cost = value = 2.4867663E7, cpu = 112574.0, memory = 88984.0, io = 201.0, net = 4.75, id = 182                                                                                                                                                       |
|   HashAgg(group="L_ORDERKEY", count()="SUM(count())"): rowcount = 2508.0, cumulative cost = value = 2.4867662E7, cpu = 112573.0, memory = 88984.0, io = 201.0, net = 4.75, id = 180                                                                                                                            |
|     Gather(concurrent=true): rowcount = 2508.0, cumulative cost = value = 2.4860069E7, cpu = 105039.0, memory = 29796.0, io = 201.0, net = 4.75, id = 178                                                                                                                                                        |
|       LogicalView(tables="[000000-000003].lineitem_[00-15]", shardCount=16, sql="SELECT `L_ORDERKEY`, COUNT() AS `count()` FROM `lineitem` AS `lineitem` GROUP BY `L_ORDERKEY`"): rowcount = 2508.0, cumulative cost = value = 2.4860068E7, cpu = 105038.0, memory = 29796.0, io = 201.0, net = 4.75, id = 109 |
| HitCache:true                                                                                                                                                                                                                                                                                                    |
| Source:PLAN_CACHE                                                                                                                                                                                                                                                                                                |
| WorkloadType: TP                                                                                                                                                                                                                                                                                                 |
| TemplateId: 5819c807
  • explain analyze语句：相对于explain cost语句，除了显示各个算子基于统计信息估算的代价以外，该语句可以收集真实运行过程中算子输出的rowCount等信息。
mysql> explain ANALYZE  select  count(*) from lineitem group by L_ORDERKEY;
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| LOGICAL EXECUTIONPLAN                                                                                                                                                                                                                                                                                                                                                                                    |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Project(count()="count()"): rowcount = 2508.0, cumulative cost = value = 2.4867663E7, cpu = 112574.0, memory = 88984.0, io = 201.0, net = 4.75, actual time = 0.001 + 0.000, actual rowcount = 2506, actual memory = 0, instances = 1, id = 182                                                                                                                                                        |
|   HashAgg(group="L_ORDERKEY", count()="SUM(count())"): rowcount = 2508.0, cumulative cost = value = 2.4867662E7, cpu = 112573.0, memory = 88984.0, io = 201.0, net = 4.75, actual time = 0.000 + 0.000, actual rowcount = 2506, actual memory = 0, instances = 1, id = 180                                                                                                                             |
|     Gather(concurrent=true): rowcount = 2508.0, cumulative cost = value = 2.4860069E7, cpu = 105039.0, memory = 29796.0, io = 201.0, net = 4.75, actual time = 0.000 + 0.000, actual rowcount = 0, actual memory = 0, instances = 0, id = 178                                                                                                                                                            |
|       LogicalView(tables="[000000-000003].lineitem_[00-15]", shardCount=16, sql="SELECT `L_ORDERKEY`, COUNT() AS `count()` FROM `lineitem` AS `lineitem` GROUP BY `L_ORDERKEY`"): rowcount = 2508.0, cumulative cost = value = 2.4860068E7, cpu = 105038.0, memory = 29796.0, io = 201.0, net = 4.75, actual time = 0.030 + 0.025, actual rowcount = 10000, actual memory = 0, instances = 0, id = 109 |
| HitCache:true                                                                                                                                                                                                                                                                                                                                                                                            |
| Source:PLAN_CACHE                                                                                                                                                                                                                                                                                                                                                                                        |
| TemplateId: 5819c807                                                                                                                                                                                                                                                                                                                                                                                     |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
7 rows in set (1.08 sec)
  • explain physical语句：展示查询在运行过程中执行模式、各个执行片段（Fragment）的依赖关系和并行度。该查询被识别为单机单线程计划模式（TP_LOCAL），执行计划被分为三个片段Fragment-0、Fragment-1和Fragment-2，先做预聚合再做最终的聚合计算，每个片段的执行度可以不同。
mysql> explain physical   select  count(*) from lineitem group by L_ORDERKEY;
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| PLAN                                                                                                                                                                           |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ExecutorMode: TP_LOCAL                                                                                                                                                         |
| Fragment 0 dependency: [] parallelism: 4                                                                                                                                       |
| Gather(concurrent=true)                                                                                                                                                        |
|   LogicalView(tables="[000000-000003].lineitem_[00-15]", shardCount=16, sql="SELECT `L_ORDERKEY`, COUNT() AS `count()` FROM `lineitem` AS `lineitem` GROUP BY `L_ORDERKEY`") |
| Fragment 1 dependency: [] parallelism: 8                                                                                                                                       |
| LocalBuffer                                                                                                                                                                    |
|   RemoteSource(sourceFragmentIds=[0], type=RecordType(INTEGER L_ORDERKEY, BIGINT count(*)))                                                                                    |
| Fragment 2 dependency: [0, 1] parallelism: 8                                                                                                                                   |
| Project(count()="count()")                                                                                                                                                   |
|   HashAgg(group="L_ORDERKEY", count()="SUM(count())")                                                                                                                        |
|     RemoteSource(sourceFragmentIds=[1], type=RecordType(INTEGER L_ORDERKEY, BIGINT count(*)))                                                                                  |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
11 rows in set (0.10 sec)
  • explain advisor语句：主要是会基于统计信息，分析当前查询的执行计划，给用户推荐可以加速查询的全局二级索引。
mysql> explain advisor   select  count(*) from lineitem group by L_ORDERKEY \G;
 1. row **
IMPROVE_VALUE: 4.4%
  IMPROVE_CPU: 340.8%
  IMPROVE_MEM: 0.0%
   IMPROVE_IO: 1910.0%
  IMPROVE_NET: 0.0%
 BEFORE_VALUE: 2.48676627E7
   BEFORE_CPU: 112573.7
   BEFORE_MEM: 88983.8
    BEFORE_IO: 201
   BEFORE_NET: 4.7
  AFTER_VALUE: 2.38256249E7
    AFTER_CPU: 25536
    AFTER_MEM: 88983.8
     AFTER_IO: 10
    AFTER_NET: 4.7
 ADVISE_INDEX: ALTER TABLE `ads`.`lineitem` ADD GLOBAL INDEX `__advise_index_gsi_lineitem_L_ORDERKEY`(`L_ORDERKEY`) DBPARTITION BY HASH(`L_ORDERKEY`) TBPARTITION BY HASH(`L_ORDERKEY`) TBPARTITIONS 4;
     NEW_PLAN:
Project(count()="count()")
  HashAgg(group="L_ORDERKEY", count()="SUM(count())")
    Gather(concurrent=true)
      IndexScan(tables="[000000-000003].lineitem__what_if_gsi_L_ORDERKEY_[00-15]", shardCount=16, sql="SELECT `L_ORDERKEY`, COUNT() AS `count()` FROM `lineitem__what_if_gsi_L_ORDERKEY` AS `lineitem__what_if_gsi_L_ORDERKEY` GROUP BY `L_ORDERKEY`")
         INFO: GLOBAL_INDEX
1 row in set (0.13 sec)
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