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EXPLAIN

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 | DESCRIBE | DESC} 
[BASIC | OUTLINE | EXTENDED | EXTENDED_NOADDR | PARTITIONS | FORMAT = {TRADITIONAL| JSON}] 
{SELECT statement | DELETE statement | INSERT statement | REPLACE statement| UPDATE statement}

参数解释

参数

描述

tbl_name

指定表名。

col_name

指定表的列名。

BASIC

指定输出计划的基础信息,如算子 ID、算子名称、所引用的表名。

OUTLINE

指定输出的计划信息包含 outline 信息。

EXTENDED

EXPLAIN 产生附加信息,包括:每个算子的输入列和输出列,访问表的分区信息,当前使用的 filter 信息,如果当前算子使用了索引,显示所使用的索引列及抽取的 query range。

EXTENDED_NOADDR

以简约的方式展示附加信息。

PARTITIONS

显示分区相关信息。

FORMAT = {TRADITIONAL| JSON}

指定 EXPALIN 的输出格式:

  • TRADITIONAL:表格输出格式

  • JSON:KEY:VALUE 输出格式,JSON 显示为JSON 字符串,包括 EXTENDED 和 PARTITIONS 信息。

示例

  • 省略 explain_type
OceanBase(admin@test)>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
OceanBase(admin@test)>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

OceanBase(admin@test)>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
OceanBase(admin@test)>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":5933108,
  "output": [
    "t1.c1",
    "t1.c2",
    "t2.c1",
    "t2.c2"
  ],
  "TABLE SCAN": {
    "ID":0,
    "OPERATOR":"TABLE SCAN",
    "NAME":"TABLE SCAN",
    "EST.ROWS":10000,
    "COST":6218,
    "output": [
      "t2.c2",
      "t2.c1"
    ]
  },
  "TABLE SCAN": {
    "ID":1,
    "OPERATOR":"TABLE SCAN",
    "NAME":"TABLE SCAN",
    "EST.ROWS":100000,
    "COST":68477,
    "output": [
      "t1.c2",
      "t1.c1"
    ]
  }
}

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:

列名

说明

ID

计划执行序列号。

OPERATOR

执行算子。

NAME

算子所引用的表。

EST.ROWS

估计执行到当前算子输出的行数。

COST

执行到当前算子的CPU时间。