EXPLAIN
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falseortrue；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)