描述

该语句用于解释 SQL 语句的执行计划，可以是 ​SELECT​、​DELETE​、​INSERT​、​REPLACE​ 或 ​UPDATE​ 语句。

格式

获取表或列的信息：
{EXPLAIN | DESCRIBE | DESC} tbl_name [col_name | wild]

获取SQL计划信息：
{EXPLAIN} 
[BASIC | OUTLINE | EXTENDED | EXTENDED_NOADDR | PARTITIONS | FORMAT = {TRADITIONAL| JSON}] 
{SELECT statement | DELETE statement | INSERT statement | UPDATE statement | MERGE statement}

参数解释

示例

• 省略 ​explain_type​

obclient>CREATE TABLE t1(c1 INT,c2 INT);
Query OK, 0 rows affected (0.04 sec)

obclient>CREATE TABLE t2(c1 INT,c2 INT);
Query OK, 0 rows affected (0.07 sec)

obclient>EXPLAIN SELECT * FROM t1,t2 WHERE t1.c2=t2.c2 AND t2.c1 > 4\G
*************************** 1. row ***************************
Query Plan: =======================================
|ID|OPERATOR   |NAME|EST. ROWS|COST   |
---------------------------------------
|0 |HASH JOIN  |    |9801000  |5933109|
|1 | TABLE SCAN|t2  |10000    |6219   |
|2 | TABLE SCAN|t1  |100000   |68478  |
=======================================

Outputs & filters: 
-------------------------------------
  0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil), 
      equal_conds([t1.c2 = t2.c2]), other_conds(nil)
  1 - output([t2.c2], [t2.c1]), filter(nil), 
      access([t2.c2], [t2.c1]), partitions(p0)
  2 - output([t1.c2], [t1.c1]), filter(nil), 
      access([t1.c2], [t1.c1]), partitions(p0)

• EXTENDED

obclient>EXPLAIN EXTENDED_NOADDR SELECT * FROM t1,t2 
        WHERE t1.c2=t2.c2 AND t2.c1 > 4\G
*************************** 1. row ***************************
Query Plan: =======================================
|ID|OPERATOR   |NAME|EST. ROWS|COST   |
---------------------------------------
|0 |HASH JOIN  |    |9801000  |5933109|
|1 | TABLE SCAN|t2  |10000    |6219   |
|2 | TABLE SCAN|t1  |100000   |68478  |
=======================================

Outputs & filters: 
-------------------------------------
  0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil), 
      equal_conds([t1.c2 = t2.c2]), other_conds(nil)
  1 - output([t2.c2], [t2.c1]), filter(nil), 
      access([t2.c2], [t2.c1]), partitions(p0), 
      is_index_back=false, 
      range_key([t2.c1]), range(4 ; MAX), 
      range_cond([t2.c1 > 4])
  2 - output([t1.c2], [t1.c1]), filter(nil), 
      access([t1.c2], [t1.c1]), partitions(p0), 
      is_index_back=false, 
      range_key([t1.__pk_increment], [t1.__pk_cluster_id], [t1.__pk_partition_id]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true

• ​TRADITIONAL​格式

obclient>EXPLAIN FORMAT=TRADITIONAL SELECT * FROM t1,t2 
        WHERE t1.c2=t2.c2 AND t2.c1 > 4\G
*************************** 1. row ***************************
Query Plan: =======================================
|ID|OPERATOR   |NAME|EST. ROWS|COST   |
---------------------------------------
|0 |HASH JOIN  |    |9801000  |5933109|
|1 | TABLE SCAN|t2  |10000    |6219   |
|2 | TABLE SCAN|t1  |100000   |68478  |
=======================================

Outputs & filters: 
-------------------------------------
  0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil), 
      equal_conds([t1.c2 = t2.c2]), other_conds(nil)
  1 - output([t2.c2], [t2.c1]), filter(nil), 
      access([t2.c2], [t2.c1]), partitions(p0)
  2 - output([t1.c2], [t1.c1]), filter(nil), 
      access([t1.c2], [t1.c1]), partitions(p0)

• JSON 格式

obclient>EXPLAIN FORMAT=JSON SELECT * FROM t1,t2 
       WHERE t1.c2=t2.c2 AND t2.c1 > 4\G
*************************** 1. row ***************************
Query Plan: {
  "ID":2,
  "OPERATOR":"JOIN",
  "NAME":"JOIN",
  "EST.ROWS":9800999,
  "COST":6840524,
  "output": [
    "T1.C1",
    "T1.C2",
    "T2.C1",
    "T2.C2"
  ],
  "CHILD_1": {
    "ID":0,
    "OPERATOR":"TABLE SCAN",
    "NAME":"TABLE SCAN",
    "EST.ROWS":10000,
    "COST":80810,
    "output": [
      "T2.C2",
      "T2.C1"
    ]
  },
  "CHILD_2": {
    "ID":1,
    "OPERATOR":"TABLE SCAN",
    "NAME":"TABLE SCAN",
    "EST.ROWS":100000,
    "COST":64065,
    "output": [
      "T1.C2",
      "T1.C1"
    ]
  }
}
1 row in set (0.01 sec)

EXPLAIN 的每个输出行提供一个表的相关信息，并且每个行包括下面的列：