196 lines
7.2 KiB
Markdown
196 lines
7.2 KiB
Markdown
EXPLAIN
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============================
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描述
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-----------
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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。提供正则过滤功能，通过session变量explain_regex对输出行/JSON进行过滤。
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格式
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-----------
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```javascript
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获取表或列的信息：
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{EXPLAIN | DESCRIBE | DESC} tbl_name [col_name | wild]
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获取SQL计划信息：
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{EXPLAIN | DESCRIBE | DESC}
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[BASIC | OUTLINE | EXTENDED | EXTENDED_NOADDR | PARTITIONS | FORMAT = {TRADITIONAL| JSON}]
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{SELECT statement | DELETE statement | INSERT statement | REPLACE statement| UPDATE statement}
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```
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参数解释
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-------------
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| **参数** | **描述** |
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|-------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| tbl_name | 指定表名。 |
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| col_name | 指定表的列名。 |
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| 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。 |
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| OUTLINE | 指定输出的计划信息包含outline信息。 |
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| 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。 |
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| EXTENDED_NOADDR | 以简约的方式展示附加信息。 |
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| PARTITIONS | 显示分区相关信息。 |
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| 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。 |
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示例
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-----------
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* **省略explain_type**
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```javascript
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OceanBase(admin@test)>explain select * from t1,t2 where t1.c2=t2.c2 and t2.c1 > 4\G
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*************************** 1. row ***************************
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Query Plan: =======================================
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|ID|OPERATOR |NAME|EST. ROWS|COST |
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---------------------------------------
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|0 |HASH JOIN | |9801000 |5933109|
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|1 | TABLE SCAN|t2 |10000 |6219 |
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|2 | TABLE SCAN|t1 |100000 |68478 |
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=======================================
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Outputs & filters:
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-------------------------------------
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0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil),
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equal_conds([t1.c2 = t2.c2]), other_conds(nil)
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1 - output([t2.c2], [t2.c1]), filter(nil),
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access([t2.c2], [t2.c1]), partitions(p0)
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2 - output([t1.c2], [t1.c1]), filter(nil),
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access([t1.c2], [t1.c1]), partitions(p0)
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```
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* **EXTENDED**
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```javascript
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OceanBase(admin@test)>explain extended_noaddr select * from t1,t2 where t1.c2=t2.c2 and t2.c1 > 4\G
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*************************** 1. row ***************************
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Query Plan: =======================================
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|ID|OPERATOR |NAME|EST. ROWS|COST |
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---------------------------------------
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|0 |HASH JOIN | |9801000 |5933109|
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|1 | TABLE SCAN|t2 |10000 |6219 |
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|2 | TABLE SCAN|t1 |100000 |68478 |
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=======================================
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Outputs & filters:
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-------------------------------------
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0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil),
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equal_conds([t1.c2 = t2.c2]), other_conds(nil)
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1 - output([t2.c2], [t2.c1]), filter(nil),
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access([t2.c2], [t2.c1]), partitions(p0),
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is_index_back=false,
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range_key([t2.c1]), range(4 ; MAX),
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range_cond([t2.c1 > 4])
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2 - output([t1.c2], [t1.c1]), filter(nil),
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access([t1.c2], [t1.c1]), partitions(p0),
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is_index_back=false,
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range_key([t1.__pk_increment], [t1.__pk_cluster_id], [t1.__pk_partition_id]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true
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```
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* **TRADITIONAL** **格式**
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```javascript
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OceanBase(admin@test)>explain format=TRADITIONAL select * from t1,t2 where t1.c2=t2.c2 and t2.c1 > 4\G
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*************************** 1. row ***************************
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Query Plan: =======================================
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|ID|OPERATOR |NAME|EST. ROWS|COST |
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---------------------------------------
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|0 |HASH JOIN | |9801000 |5933109|
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|1 | TABLE SCAN|t2 |10000 |6219 |
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|2 | TABLE SCAN|t1 |100000 |68478 |
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=======================================
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Outputs & filters:
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-------------------------------------
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0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil),
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equal_conds([t1.c2 = t2.c2]), other_conds(nil)
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1 - output([t2.c2], [t2.c1]), filter(nil),
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access([t2.c2], [t2.c1]), partitions(p0)
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2 - output([t1.c2], [t1.c1]), filter(nil),
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access([t1.c2], [t1.c1]), partitions(p0)
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```
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* **JSON格式**
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```javascript
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OceanBase(admin@test)>explain format=JSON select * from t1,t2 where t1.c2=t2.c2 and t2.c1 > 4\G
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*************************** 1. row ***************************
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Query Plan: {
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"ID":2,
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"OPERATOR":"JOIN",
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"NAME":"JOIN",
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"EST.ROWS":9800999,
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"COST":5933108,
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"output": [
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"t1.c1",
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"t1.c2",
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"t2.c1",
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"t2.c2"
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],
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"TABLE SCAN": {
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"ID":0,
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"OPERATOR":"TABLE SCAN",
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"NAME":"TABLE SCAN",
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"EST.ROWS":10000,
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"COST":6218,
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"output": [
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"t2.c2",
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"t2.c1"
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]
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},
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"TABLE SCAN": {
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"ID":1,
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"OPERATOR":"TABLE SCAN",
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"NAME":"TABLE SCAN",
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"EST.ROWS":100000,
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"COST":68477,
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"output": [
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"t1.c2",
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"t1.c1"
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]
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}
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}
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```
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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：
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| **列名** | **说明** |
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|----------|-----------------|
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| ID | 计划执行序列号。 |
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| OPERATOR | 执行算子。 |
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| NAME | 算子所引用的表。 |
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| EST.ROWS | 估计执行到当前算子输出的行数。 |
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| COST | 执行到当前算子的CPU时间。 |
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