基于规则的查询改写 
==============================

基于规则的查询改写方式主要包括子查询相关改写、外联接消除、简化条件改写和非 SPJ（Select Project Join）的改写等。

子查询相关改写 
----------------

优化器对于子查询一般使用嵌套执行的方式，也就是父查询每生成一行数据后，都需要执行一次子查询。使用这种方式需要多次执行子查询，执行效率很低。对于子查询的优化方式，一般会将其改写为联接操作，可大大提高执行效率，主要优点如下：

* 可避免子查询多次执行。

  

* 优化器可根据统计信息选择更优的联接顺序和联接方法。

  

* 子查询的联接条件、过滤条件改写为父查询的条件后，优化器可以进行进一步优化，例如条件下压等。

  




子查询改写的方式主要包括视图合并、子查询展开和将 `ANY/ALL` 使用 `MAX/MIN` 改写等。

#### **视图合并** 

视图合并是将代表一个视图的子查询合并到包含该视图的查询中，视图合并后，有助于优化器增加联接顺序的选择、访问路径的选择以及进一步做其他改写操作，从而选择更优的执行计划。

OceanBase 数据库支持对 SPJ 视图进行合并。如下示例为将查询 Q1 改写为 Q2：

```javascript
obclient>CREATE TABLE t1 (c1 INT, c2 INT);
Query OK, 0 rows affected (0.00 sec)

obclient>CREATE TABLE t2 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.00 sec)

obclient>CREATE TABLE t3 (c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.00 sec)

Q1: 
obclient>SELECT t1.c1, v.c1 
                FROM t1, (SELECT t2.c1, t3.c2 
                FROM t2, t3 
                WHERE t2.c1 = t3.c1) v 
                WHERE t1.c2 = v.c2;
<==>
Q2: 
obclient>SELECT t1.c1, t2.c1 
                FROM t1, t2, t3 
                WHERE t2.c1 = t3.c1 AND t1.c2 = t3.c2;
```



如果 Q1 不进行改写，其联接顺序有以下几种：

* `t1`, `v(t2,t3)`

  

* `t1`, `v(t3,t2)`

  

* `v(t2,t3)`, `t1`

  

* `v(t3,t2)`, `t1`

  




进行视图合并改写后，可选择的联接顺序有：

* `t1`, `t2`, `t3`

  

* `t1`, `t3`, `t2`

  

* `t2`, `t1`, `t3`

  

* `t2`, `t3`, `t1`

  

* `t3`, `t1`, `t2`

  

* `t3`, `t2`, `t1`

  




由此可以得出，视图合并增加了联接顺序的选择性。对于复杂查询，视图合并后，对路径的选择和可改写的空间均会增大，从而使优化器生成更优的计划。

#### **子查询展开** 

子查询展开是将 `WHERE` 条件中子查询提升到父查询中，并作为联接条件与父查询并列进行展开。转换后子查询将不存在，外层父查询中会变成多表联接。

这样改写的好处是优化器在进行路径选择、联接方法和联接排序时都会考虑到子查询中的表，从而可以获得更优的执行计划。涉及的子查询表达式一般有 `NOT IN`、`IN`、`NOT EXIST`、`EXIST`、`ANY` 和 `ALL`。

子查询展开的方式如下：

* 改写条件使生成的联接语句能够返回与原始语句相同的行。

  

* 展开为半联接（Semi Join/Anti Join）

  如下例所示，`t2.c2` 不具有唯一性，改写为 Semi Join，该语句改写后的执行计划如下：

  ```javascript
  obclient>CREATE TABLE t1 (c1 INT, c2 INT);
  Query OK, 0 rows affected (0.17 sec)
  
  obclient>CREATE TABLE t2 (c1 INT PRIMARY KEY, c2 INT);
  Query OK, 0 rows affected (0.01 sec)
  
  obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 IN (SELECT t2.c2 FROM t2)\G;
  *************************** 1. row ***************************
  Query Plan: 
  =======================================
  |ID|OPERATOR      |NAME|EST. ROWS|COST|
  ---------------------------------------
  |0 |HASH SEMI JOIN|    |495      |3931|
  |1 | TABLE SCAN   |t1  |1000     |499 |
  |2 | TABLE SCAN   |t2  |1000     |433 |
  =======================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.c1], [t1.c2]), filter(nil),
        equal_conds([t1.c1 = t2.c2]), other_conds(nil)
    1 - output([t1.c1], [t1.c2]), filter(nil),
        access([t1.c1], [t1.c2]), partitions(p0)
    2 - output([t2.c2]), filter(nil),
        access([t2.c2]), partitions(p0)
  ```

  

  将查询前面的操作符改为 `NOT IN` 后，可改写为 Anti Join，具体计划如下例所示：

  ```javascript
  obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 NOT IN (SELECT t2.c2 FROM t2)\G;
  *************************** 1. row ***************************
  Query Plan:
  ================================================
  |ID|OPERATOR             |NAME|EST. ROWS|COST  |
  ------------------------------------------------
  |0 |NESTED-LOOP ANTI JOIN|    |0        |520245|
  |1 | TABLE SCAN          |t1  |1000     |499   |
  |2 | TABLE SCAN          |t2  |22       |517   |
  ================================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.c1], [t1.c2]), filter(nil),
        conds(nil), nl_params_([t1.c1], [(T_OP_IS, t1.c1, NULL, 0)])
    1 - output([t1.c1], [t1.c2], [(T_OP_IS, t1.c1, NULL, 0)]), filter(nil),
        access([t1.c1], [t1.c2]), partitions(p0)
    2 - output([t2.c2]), filter([(T_OP_OR, ? = t2.c2, ?, (T_OP_IS, t2.c2, NULL, 0))]),
        access([t2.c2]), partitions(p0)
  ```

  




<!-- -->

* 子查询展开为内联接

  在上述示例的查询 Q1 中，如果将 `t2.c2` 改为 `t2.c1`，由于 `t2.c1` 为主键，子查询输出具有唯一性，此时可以直接转换为内联接，如下所示：

  ```javascript
  Q1: 
  obclient>SELECT * FROM t1 WHERE t1.c1 IN  (SELECT t2.c1 FROM t2)\G;
  <==>
  Q2: 
  obclient>SELECT t1.* FROM t1, t2 WHERE t1.c1 = t2.c1;
  ```

  

  Q1 改写后的计划如下所示：

  ```javascript
  obclient>EXPLAIN SELECT * FROM t1 WHERE t1.c1 IN (SELECT t2.c1 FROM t2)\G;
  *************************** 1. row ***************************
  Query Plan:
   ====================================
  |ID|OPERATOR   |NAME|EST. ROWS|COST|
  ------------------------------------
  |0 |HASH JOIN  |    |1980     |3725|
  |1 | TABLE SCAN|t2  |1000     |411 |
  |2 | TABLE SCAN|t1  |1000     |499 |
  ====================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t1.c1], [t1.c2]), filter(nil),
        equal_conds([t1.c1 = t2.c1]), other_conds(nil)
    1 - output([t2.c1]), filter(nil),
        access([t2.c1]), partitions(p0)
    2 - output([t1.c1], [t1.c2]), filter(nil),
        access([t1.c1], [t1.c2]), partitions(p0)
  ```

  

  对于 `NOT IN`、`IN`、`NOT EXIST`、`EXIST`、`ANY` 和 `ALL` 子查询表达式都可以对应做类似的改写操作。
  




#### **ANY/ALL 使用 MAX/MIN 改写** 

对于 `ANY/ALL` 的子查询，如果子查询中没有 `GROUP BY` 子句、聚集函数和 `HAVING` 条件时，以下表达式可以使用聚集函数 `MIN/MAX` 进行等价转换，其中 `col_item` 为单独列且有非 `NULL` 属性：

```sql
val > ALL(SELECT col_item ...)  <==> val > ALL(SELECT MAX(col_item) ...);
val >= ALL(SELECT col_item ...) <==> val >= ALL(SELECT MAX(col_item) ...);
val < ALL(SELECT col_item ...)  <==> val < ALL(SELECT MIN(col_item) ...);
val <= ALL(SELECT col_item ...) <==> val <= ALL(SELECT MIN(col_item) ...);
val > ANY(SELECT col_item ...)  <==> val > ANY(SELECT MIN(col_item) ...);
val >= ANY(SELECT col_item ...) <==> val >= ANY(SELECT MIN(col_item) ...);
val < ANY(SELECT col_item ...)  <==> val < ANY(SELECT MAX(col_item) ...);
val <= ANY(SELECT col_item ...) <==> val <= ANY(SELECT MAX(col_item) ...);
```



将子查询更改为含有 `MAX/MIN` 的子查询后，再结合使用 `MAX/MIN` 进行改写，可减少改写前对内表的多次扫描，如下例所示：

```javascript
obclient>SELECT c1 FROM t1 WHERE c1 > ANY(SELECT c1 FROM t2);
<==>
obclient>SELECT c1 FROM t1 WHERE c1 > ANY(SELECT MIN(c1) FROM t2);
```



结合 `MAX/MIN` 进行改写后，可利用 `t2.c1` 的主键序将 `LIMIT 1` 直接下压到 `TABLE SCAN`，将 `MIN` 值输出，执行计划如下所示：

```javascript
obclient>EXPLAIN SELECT c1 FROM t1 WHERE c1 > ANY(SELECT c1 FROM t2)\G;
*************************** 1. row ***************************
Query Plan:
 ===================================================
|ID|OPERATOR        |NAME          |EST. ROWS|COST|
---------------------------------------------------
|0 |SUBPLAN FILTER  |              |1        |73  |
|1 | TABLE SCAN     |t1            |1        |37  |
|2 | SCALAR GROUP BY|              |1        |37  |
|3 |  SUBPLAN SCAN  |subquery_table|1        |37  |
|4 |   TABLE SCAN   |t2            |1        |36  |
===================================================

Outputs & filters:
-------------------------------------
  0 - output([t1.c1]), filter([t1.c1 > ANY(subquery(1))]),
      exec_params_(nil), onetime_exprs_(nil), init_plan_idxs_([1])
  1 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0)
  2 - output([T_FUN_MIN(subquery_table.c1)]), filter(nil),
      group(nil), agg_func([T_FUN_MIN(subquery_table.c1)])
  3 - output([subquery_table.c1]), filter(nil),
      access([subquery_table.c1])
  4 - output([t2.c1]), filter(nil),
      access([t2.c1]), partitions(p0),
      limit(1), offset(nil)
```





外联接消除 
--------------

外联接操作可分为左外联接、右外联接和全外联接。在联接过程中，由于外联接左右顺序不能变换，优化器对联接顺序的选择会受到限制。外联接消除是指将外联接转换成内联接，从而可以提供更多可选择的联接路径，供优化器使用。

如果进行外联接消除，需要存在"空值拒绝条件"，即在 `WHERE` 条件中存在，当内表生成的值为 `NULL` 时，输出为 `FALSE` 的条件。

如下例所示：

```javascript
obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT JOIN t2 ON t1.c2 = t2.c2;
```



这是一个外联接，在其输出行中 `t2.c2` 可能为 `NULL`。如果加上一个条件 ` t2.c2 > 5`，则通过该条件过滤后，`t2.c1` 输出不可能为 `NULL`， 从而可以将外联接转换为内联接。

```javascript
obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT JOIN t2 ON t1.c2 = t2.c2 WHERE t2.c2 > 5;
<==>
obclient>SELECT t1.c1, t2.c2 FROM t1 LEFT INNER JOIN t2 ON t1.c2 = t2.c2 
            WHERE t2.c2 > 5;
```





简化条件改写 
---------------

#### HAVING 条件消除 

如果查询中没有聚集操作和 `GROUP BY`，则 `HAVING` 可以合并到 `WHERE` 条件中，并将 `HAVING` 条件删除， 从而可以将 `HAVING` 条件在 `WHERE` 条件中统一管理，并做进一步优化。

```javascript
obclient>SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 HAVING t1.c2 > 1;
<==>
obclient>SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 AND t1.c2 > 1;
```



改写后计划如下例所示, `t1.c2 > 1` 条件被下压到了 `TABLE SCAN` 层。

```javascript
obclient>EXPLAIN SELECT * FROM t1, t2 WHERE t1.c1 = t2.c1 HAVING t1.c2 > 1\G;
*************************** 1. row ***************************
Query Plan: 
=========================================
|ID|OPERATOR        |NAME|EST. ROWS|COST|
-----------------------------------------
|0 |NESTED-LOOP JOIN|    |1        |59  |
|1 | TABLE SCAN     |t1  |1        |37  |
|2 | TABLE GET      |t2  |1        |36  |
=========================================

Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2], [t2.c1], [t2.c2]), filter(nil),
      conds(nil), nl_params_([t1.c1])
  1 - output([t1.c1], [t1.c2]), filter([t1.c2 > 1]),
      access([t1.c1], [t1.c2]), partitions(p0)
  2 - output([t2.c1], [t2.c2]), filter(nil),
      access([t2.c1], [t2.c2]), partitions(p0)
```



#### **等价关系推导** 

等价关系推导是指利用比较操作符的传递性，推倒出新的条件表达式，从而减少需要处理的行数或者选择到更有效的索引。

OceanBase 数据库可对等值联接进行推导，例如，一张表中有 `a` 列和 `b` 列，通过 `a ＝ b AND a > 1` 可以推导出 `a ＝ b AND a > 1 AND b > 1`， 如果 `b` 列上有索引，且 `b > 1` 在该索引选择率很低，则可以大大提升访问 `b` 列所在表的性能。

如下例所示，条件 `t1.c1 = t2.c2 AND t1.c1 > 2`，等价推导后为 `t1.c1 = t2.c2 AND t1.c1 > 2 AND t2.c2 > 2`，从计划中可以看到 `t2.c2` 已下压到 `TABLE SCAN`，并且使用了 `t2.c2` 对应的索引。

```sql
obclient>CREATE TABLE t1(c1 INT PRIMARY KEY, c2 INT);
Query OK, 0 rows affected (0.15 sec)

obclient>CREATE TABLE t2(c1 INT PRIMARY KEY, c2 INT, c3 INT, KEY IDX_c2(c2));
Query OK, 0 rows affected (0.10 sec)
/*此命令需运行于 MySQL 模式下*/

obclient>EXPLAIN EXTENDED_NOADDR SELECT t1.c1, t2.c2 FROM t1, t2 
              WHERE t1.c1 = t2.c2 AND t1.c1 > 2\G;
*************************** 1. row ***************************
Query Plan: 
==========================================
|ID|OPERATOR   |NAME      |EST. ROWS|COST|
------------------------------------------
|0 |MERGE JOIN |          |5        |78  |
|1 | TABLE SCAN|t2(IDX_c2)|5        |37  |
|2 | TABLE SCAN|t1        |3        |37  |
==========================================

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



#### **恒真/假消除** 

对于如下恒真恒假条件可以进行消除：

* `false and expr` ＝ 恒 False

  

* t`rue or expr` = 恒 True

  




如下例所示，对于 `WHERE 0 > 1 AND c1 ＝ 3`，由于 `0 > 1` 使得 `AND` 恒假， 所以该 SQL 不用执行，可直接返回，从而加快查询的执行。

```javascript
obclient>EXPLAIN EXTENDED_NOADDR SELECT * FROM t1 WHERE 0 > 1 AND c1 = 3\G;
*************************** 1. row ***************************
Query Plan: 
===================================
|ID|OPERATOR  |NAME|EST. ROWS|COST|
-----------------------------------
|0 |TABLE SCAN|t1  |0        |38  |
===================================

Outputs & filters:
-------------------------------------
  0 - output([t1.c1], [t1.c2]), filter([0], [t1.c1 = 3]), startup_filter([0]),
      access([t1.c1], [t1.c2]), partitions(p0),
      is_index_back=false, filter_before_indexback[false,false],
      range_key([t1.__pk_increment], [t1.__pk_cluster_id], [t1.__pk_partition_id]),
      range(MAX,MAX,MAX ; MIN,MIN,MIN)always false
```





非 SPJ 的改写 
------------------

#### **冗余排序消除** 

冗余排序消除是指删除 Order Item 中不需要的项，减少排序开销。以下三种情况可进行排序消除：

* `ORDER BY` 表达式列表中有重复列，可进行去重后排序。

  ```javascript
  obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c1, c2, c3 ;
  <==>
  obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c2, c3;
  ```

  




<!-- -->

* `ORDER BY` 列中存在 `WHERE` 中有单值条件的列，该列排序可删除。

  ```javascript
  obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c2, c3;
  <==>
  obclient>SELECT * FROM t1 WHERE c2 = 5 ORDER BY c1, c3;
  ```

  




<!-- -->

* 如果本层查询有 `ORDER BY` 但是没有 `LIMIT`，且本层查询位于父查询的集合操作中，则 `ORDER BY` 可消除。因为对两个有序的集合做 `UNION` 操作，其结果是乱序的。但是如果 `ORDER BY` 中有 `LIMIT`，则语义是取最大/最小的 N 个，此时不能消除 `ORDER BY`，否则有语义错误。

  ```javascript
  obclient>(SELECT c1,c2 FROM t1 ORDER BY c1) UNION (SELECT c3,c4 FROM t2 ORDER BY c3)；
  <==>
  obclient>(SELECT c1,c2 FROM t1) UNION (SELECT c3,c4 FROM t2)；
  ```

  




#### **LIMIT 下压** 

`LIMIT` 下压改写是指将 `LIMIT` 下降到子查询中，OceanBase 数据库现在支持在不改变语义的情况下，将 `LIMIT` 下压到视图（示例 1）或 `UNION` 对应的子查询（示例 2）中。

示例 1：将 `LIMIT` 下压到视图。

```sql
obclient>SELECT * FROM (SELECT * FROM t1 ORDER BY c1) a LIMIT 1; 
<==>
obclient>SELECT * FROM (SELECT * FROM t1 ORDER BY c1 LIMIT 1) a LIMIT 1;
```



示例 2：将 `LIMIT` 下压到 `UNION` 对应的子查询中。

```sql
obclient>(SELECT c1,c2 FROM t1) UNION ALL (SELECT c3,c4 FROM t2) LIMIT 5;
<==>
obclient>(SELECT c1,c2 FROM t1 LIMIT 5) UNION ALL (SELECT c3,c4 FROM t2 limit 5) LIMIT 5;
```



#### **DISTINCT 消除** 

* 如果 Select Item 中只包含常量，则可以消除 `DISTINCT`，并加上 `LIMIT 1`。

  ```sql
  obclient>SELECT DISTINCT 1,2 FROM t1 ;
  <==> 
  obclient>SELECT DISTINCT 1,2 FROM t1 LIMIT 1;
  
  obclient>CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT);
  Query OK, 0 rows affected (0.17 sec)
  
  obclient>EXPLAIN EXTENDED_NOADDR SELECT DISTINCT 1,2 FROM t1\G;
  *************************** 1. row ***************************
  Query Plan: 
  ===================================
  |ID|OPERATOR  |NAME|EST. ROWS|COST|
  -----------------------------------
  |0 |TABLE SCAN|t1  |1        |36  |
  ===================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([1], [2]), filter(nil),
        access([t1.c1]), partitions(p0),
        limit(1), offset(nil),
        is_index_back=false,
        range_key([t1.c1]), range(MIN ; MAX)always true
  ```

  




<!-- -->

* 如果 Select Item 中包含确保唯一性约束的列，则 `DISTINCT` 能够被消除。如下示例中 `(c1, c2)` 为主键，可确保 `c1`、`c2` 和 `c3` 唯一性， 从而 `DISTINCT` 可消除。

  ```javascript
  obclient>CREATE TABLE t2(c1 INT, c2 INT, c3 INT, PRIMARY KEY(c1, c2));
  Query OK, 0 rows affected (0.17 sec)
  
  obclient>SELECT DISTINCT c1, c2, c3 FROM t2;
  <==>
  obclient>SELECT c1, c2 c3 FROM t2;
  
  obclient>EXPLAIN SELECT DISTINCT c1, c2, c3 FROM t2\G;
  *************************** 1. row ***************************
  Query Plan: 
  ===================================
  |ID|OPERATOR  |NAME|EST. ROWS|COST|
  -----------------------------------
  |0 |TABLE SCAN|t2  |1000     |455 |
  ===================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([t2.c1], [t2.c2], [t2.c3]), filter(nil),
        access([t2.c1], [t2.c2], [t2.c3]), partitions(p0)
  ```

  




#### **MIN/MAX 改写** 

* 当 `MIN/MAX` 函数中的参数为索引前缀列，且不含 `GROUP BY` 时，可将该 `scalar aggregate` 转换为走索引扫描 1 行的情况，如下例所示：

  ```javascript
  obclient>CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 INT, c3 INT, KEY IDX_c2_c3(c2,c3));
  Query OK, 0 rows affected (0.17 sec)
  
  obclient>SELECT MIN(c2) FROM t1;
  <==>
  obclient>SELECT MIN(c2) FROM (SELECT c2 FROM t2 ORDER BY c2 LIMIT 1) AS t;
  
  obclient>EXPLAIN SELECT MIN(c2) FROM t1\G;
  *************************** 1. row ***************************
  Query Plan: 
  ==================================================
  |ID|OPERATOR       |NAME          |EST. ROWS|COST|
  --------------------------------------------------
  |0 |SCALAR GROUP BY|              |1        |37  |
  |1 | SUBPLAN SCAN  |subquery_table|1        |37  |
  |2 |  TABLE SCAN   |t1(idx_c2_c3) |1        |36  |
  ==================================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([T_FUN_MIN(subquery_table.c2)]), filter(nil),
        group(nil), agg_func([T_FUN_MIN(subquery_table.c2)])
    1 - output([subquery_table.c2]), filter(nil),
        access([subquery_table.c2])
    2 - output([t1.c2]), filter([(T_OP_IS_NOT, t1.c2, NULL, 0)]),
        access([t1.c2]), partitions(p0),
        limit(1), offset(nil)
  ```

  




<!-- -->

* 如果 `SELECT MIN/MAX` 的参数为常量，而且包含 `GROUP BY`，可以将 `MIN/MAX` 改为常量，从而减少 `MIN/MAX` 的计算开销。

  ```javascript
  obclient>SELECT MAX(1) FROM t1 GROUP BY c1;
  <==>
  obclient>SELECT 1 FROM t1 GROUP BY c1;
  
  obclient>EXPLAIN EXTENDED_NOADDR SELECT MAX(1) FROM t1 GROUP BY c1\G;
  *************************** 1. row ***************************
  Query Plan: 
  ===================================
  |ID|OPERATOR  |NAME|EST. ROWS|COST|
  -----------------------------------
  |0 |TABLE SCAN|t1  |1000     |411 |
  ===================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([1]), filter(nil),
        access([t1.c1]), partitions(p0),
        is_index_back=false,
        range_key([t1.c1]), range(MIN ; MAX)always true
  ```

  




<!-- -->

* 如果 `SELECT MIN/MAX` 的参数为常量，而且不含 `GROUP BY`，可以按照如下示例进行改写，从而走索引只需扫描 1 行。

  ```javascript
  obclient>SELECT MAX(1) FROM t1;
  <==> 
  obclient>SELECT MAX(t.a) FROM (SELECT 1 AS a FROM t1 LIMIT 1) t;
  
  obclient>EXPLAIN EXTENDED_NOADDR SELECT MAX(1) FROM t1\G;
  *************************** 1. row ***************************
  Query Plan: 
  ==================================================
  |ID|OPERATOR       |NAME          |EST. ROWS|COST|
  --------------------------------------------------
  |0 |SCALAR GROUP BY|              |1        |37  |
  |1 | SUBPLAN SCAN  |subquery_table|1        |37  |
  |2 |  TABLE SCAN   |t1            |1        |36  |
  ==================================================
  
  Outputs & filters:
  -------------------------------------
    0 - output([T_FUN_MAX(subquery_table.subquery_col_alias)]), filter(nil),
        group(nil), agg_func([T_FUN_MAX(subquery_table.subquery_col_alias)])
    1 - output([subquery_table.subquery_col_alias]), filter(nil),
        access([subquery_table.subquery_col_alias])
    2 - output([1]), filter(nil),
        access([t1.c1]), partitions(p0),
        limit(1), offset(nil),
        is_index_back=false,
        range_key([t1.c1]), range(MIN ; MAX)always true
  ```

  



