SQL Thinking

s2下半年我在内部有一次部门级别的技术分享会,以本文内容分享为主。
其实有很多人问过我相同的问题,遇到需要改写的慢sql,不知道怎么改,改好了以后也不知道等不等价?不等价了也不知道错在哪?这个要怎么破?
其实都是因为绝大多数人没有做过开发,看不懂sql,不会写sql,没有sql思维,下面通过几个案例将sql思维给引出来。


1、row_number() over() 逻辑实现

这种排名的窗口函数(row_number()、rank()、dense_rank())据我所知,除了在oracle上大数据量执行性能表现得不错,其他各种数据库(包括列式存储的ap库),性能都表现都很一般。
所以如果在项目上遇到row_number() over() 性能不理想的情况下,可以尝试进行改写优化。
PS:当然可以通过加并行hint来提升效率,但是我们这篇文章主要是讲如何将数据库提供的逻辑通过另外一种方式来实现,提升自己sql代码思维。

-- student 数据量
obclient [YZJ]> select count(1) from student;
+----------+
| COUNT(1) |
+----------+
|   500001 |
+----------+
1 row in set (0.002 sec)


-- 案例sql
-- S.CLASS : 学科、包含null值
-- S.S_DATE: 入学时间
SELECT * FROM (
    SELECT 
    S.*,
    row_number() OVER(PARTITION BY S.CLASS ORDER BY S.S_DATE DESC) RN
 FROM student S
 ) WHERE RN = 1;

27 rows in set (0.639 sec)

+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                    |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ================================================================                                                                                              |
| |ID|OPERATOR             |NAME           |EST.ROWS|EST.TIME(us)|                                                                                              |
| ----------------------------------------------------------------                                                                                              |
| |0 |SUBPLAN SCAN         |ANONYMOUS_VIEW1|4       |691395      |                                                                                              |
| |1 |└─WINDOW FUNCTION    |               |500001  |682867      |                                                                                              |
| |2 |  └─PARTITION SORT   |               |500001  |249986      |                                                                                              |
| |3 |    └─TABLE FULL SCAN|S              |500001  |46352       |                                                                                              |
| ================================================================                                                                                              |
| Outputs & filters:                                                                                                                                            |
| -------------------------------------                                                                                                                         |
|   0 - output([.ID], [.NAME], [.AGE], [.SEX], [.S_DATE], [.CLASS], [.RN]), filter([.RN = 1]), rowset=256                                                       |
|       access([.RN], [.ID], [.NAME], [.AGE], [.SEX], [.S_DATE], [.CLASS])                                                                                      |
|   1 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS], [T_WIN_FUN_ROW_NUMBER()]), filter(nil), rowset=256                                    |
|       win_expr(T_WIN_FUN_ROW_NUMBER()), partition_by([S.CLASS]), order_by([S.S_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED PRECEDING), lower(UNBOUNDED  |
|       FOLLOWING)                                                                                                                                              |
|   2 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256                                                              |
|       sort_keys([HASH(S.CLASS), ASC], [S.CLASS, ASC], [S.S_DATE, DESC])                                                                                       |
|   3 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256                                                              |
|       access([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), partitions(p0)                                                                       |
|       is_index_back=false, is_global_index=false,                                                                                                             |
|       range_key([S.ID]), range(MIN ; MAX)always true                                                                                                          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------+
21 rows in set (0.004 sec)

1.1、改写方式一

改写窗口函数的逻辑,必须要将表关联2次或者2次+,需要将另外一张关联的表理解成一个“滑动”的窗口。
一般有两种改法:

  • 方法1、相同表 join 两次。
  • 方法2、表量子查询实现,将子查询的那张表理解成一个“滑动”的窗口。
SELECT s.* FROM student s   -- 原表
INNER JOIN 
    (
      
    SELECT 
      class, 
      max(s_date) max_date   
      FROM student
    GROUP BY  class 
    ) s1 -- 滑动的窗口表
    ON decode(s.class,NULL,'aab',s.class) = decode(s1.class,NULL,'aab',s1.class)
       -- decode 处理 s表和s1窗口表的分组列class
        AND s.s_date = s1.max_date  
		-- s表的s_date和窗口表s1的最大的日期去比较

        WHERE exists(
          
            SELECT 1 FROM student s2 where s1.max_date = s2.s_date GROUP BY s2.class having MAX(s2.ID) = S.ID
      /* exists 这段比较抽象,我也是想了一会才实现逻辑。
	     实现 row_number() 唯一排名的数据
      */
        );


27 rows in set (0.038 sec)

+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                            |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ==============================================================================================                                                                        |
| |ID|OPERATOR                          |NAME                            |EST.ROWS|EST.TIME(us)|                                                                        |
| ----------------------------------------------------------------------------------------------                                                                        |
| |0 |SUBPLAN FILTER                    |                                |4       |28745       |                                                                        |
| |1 |├─NESTED-LOOP JOIN                |                                |8       |28555       |                                                                        |
| |2 |│ ├─SUBPLAN SCAN                  |S1                              |26      |27712       |                                                                        |
| |3 |│ │ └─MERGE GROUP BY              |                                |26      |27712       |                                                                        |
| |4 |│ │   └─TABLE FULL SCAN           |STUDENT(IDX_CLASS_SDATE_STUDENT)|500001  |14409       |                                                                        |
| |5 |│ └─DISTRIBUTED TABLE RANGE SCAN  |S(IDX_S_DATE_CALSS_ID)          |1       |32          |                                                                        |
| |6 |└─LIMIT                           |                                |1       |27          |                                                                        |
| |7 |  └─MERGE GROUP BY                |                                |1       |27          |                                                                        |
| |8 |    └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID)         |56      |23          |                                                                        |
| ==============================================================================================                                                                        |
| Outputs & filters:                                                                                                                                                    |
| -------------------------------------                                                                                                                                 |
|   0 - output([S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter([(T_OP_EXISTS, subquery(1))]), rowset=256                                             |
|       exec_params_([S1.MAX_DATE(:0)], [S.ID(:1)]), onetime_exprs_(nil), init_plan_idxs_(nil), use_batch=false                                                         |
|   1 - output([S1.MAX_DATE], [S.ID], [S.NAME], [S.AGE], [S.SEX], [S.S_DATE], [S.CLASS]), filter(nil), rowset=256                                                       |
|       conds(nil), nl_params_([S1.CLASS(:4)], [S1.MAX_DATE(:5)]), use_batch=false                                                                                      |
|   2 - output([S1.CLASS], [S1.MAX_DATE]), filter(nil), rowset=256                                                                                                      |
|       access([S1.CLASS], [S1.MAX_DATE])                                                                                                                               |
|   3 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)]), filter(nil), rowset=256                                                                                   |
|       group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)])                                                                                                   |
|   4 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256                                                                                              |
|       access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0)                                                                                                       |
|       is_index_back=false, is_global_index=false,                                                                                                                     |
|       range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true                                                         |
|   5 - output([S.ID], [S.CLASS], [S.S_DATE], [S.NAME], [S.AGE], [S.SEX]), filter([ora_decode(cast(S.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab',    |
|        VARCHAR2(3 BYTE)), cast(S.CLASS, VARCHAR2(3 BYTE))) = ora_decode(cast(:4, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(:4, |
|        VARCHAR2(3 BYTE)))]), rowset=256                                                                                                                               |
|       access([S.ID], [S.CLASS], [S.S_DATE], [S.NAME], [S.AGE], [S.SEX]), partitions(p0)                                                                               |
|       is_index_back=true, is_global_index=false, filter_before_indexback[true],                                                                                       |
|       range_key([S.S_DATE], [S.CLASS], [S.ID]), range(MIN ; MAX),                                                                                                     |
|       range_cond([S.S_DATE = :5])                                                                                                                                     |
|   6 - output([1]), filter(nil), rowset=256                                                                                                                            |
|       limit(1), offset(nil)                                                                                                                                           |
|   7 - output(nil), filter([T_FUN_MAX(S2.ID) = :1]), rowset=256                                                                                                        |
|       group([S2.CLASS]), agg_func([T_FUN_MAX(S2.ID)])                                                                                                                 |
|   8 - output([S2.ID], [S2.CLASS]), filter(nil), rowset=256                                                                                                            |
|       access([S2.ID], [S2.CLASS]), partitions(p0)                                                                                                                     |
|       is_index_back=false, is_global_index=false,                                                                                                                     |
|       range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true,                                                                       |
|       range_cond([:0 = S2.S_DATE])                                                                                                                                    |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
43 rows in set (0.008 sec)

1.2、改写方式二

SELECT S1.*
FROM student S1
INNER JOIN 
    (
      
    SELECT 
      decode(class,null,'aab',class) class, 
      MAX(S2.S_DATE) MAX_S_DATE,
      MAX(ID) KEEP (DENSE_RANK LAST ORDER BY S_DATE) MAX_ID
    FROM student S2
    GROUP BY  S2.CLASS
    
    /*
      改写逻辑:
        1、decode(class,null,'aab',class) class,  提前处理null值。如果有其他案例谓词过滤条件要转换列名,
           例如:upper(列名) = upper(:1) ,可以提前在先写个select括起来内表处理字段,弄个别名,然后外面再过滤。
      
           select * from (
            select upper(col) col1 from tb 
           ) where col1 = upper(:1);

        2、MAX(S2.S_DATE) MAX_S_DATE 不多说,找最大日期,等价 desc

        3、MAX(ID) KEEP (DENSE_RANK LAST ORDER BY S_DATE) MAX_ID 等价于上个sql 的 exists逻辑,
          利用keep 唯一的特性,取出不同组里面最大的 id + ORDER BY S_DATE LAST 最后的唯一数据。

        相当于 MAX_S_DATE,和 MAX(id) keep 两个列的数据都是一条 ,相当于rn=1。
      
      
    */

      
    ) S2
    ON decode(S1.CLASS,null,'aab',S1.CLASS) = S2.CLASS     /* 处理关联列 */
        AND S1.ID = S2.MAX_ID ; 
       -- AND S1.S_DATE = S2.MAX_S_DATE;   

    /* 其实用S1.ID = S2.MAX_ID 关联就行,
     S1.S_DATE = S2.MAX_S_DATE 这个写不写都行,如果想逻辑更严谨点可以写上。
    */



27 rows in set (0.114 sec)


+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                       |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ==============================================================================                                                                                   |
| |ID|OPERATOR               |NAME                       |EST.ROWS|EST.TIME(us)|                                                                                   |
| ------------------------------------------------------------------------------                                                                                   |
| |0 |NESTED-LOOP JOIN       |                           |2       |44008       |                                                                                   |
| |1 |├─SUBPLAN SCAN         |S2                         |26      |43466       |                                                                                   |
| |2 |│ └─MERGE GROUP BY     |                           |26      |43466       |                                                                                   |
| |3 |│   └─TABLE FULL SCAN  |S2(IDX_CLASS_SDATE_STUDENT)|500001  |22423       |                                                                                   |
| |4 |└─DISTRIBUTED TABLE GET|S1                         |1       |21          |                                                                                   |
| ==============================================================================                                                                                   |
| Outputs & filters:                                                                                                                                               |
| -------------------------------------                                                                                                                            |
|   0 - output([S1.ID], [S1.NAME], [S1.AGE], [S1.SEX], [S1.S_DATE], [S1.CLASS]), filter(nil), rowset=256                                                           |
|       conds(nil), nl_params_([S2.CLASS(:3)], [S2.MAX_S_DATE(:4)], [S2.MAX_ID(:5)]), use_batch=false                                                              |
|   1 - output([S2.CLASS], [S2.MAX_S_DATE], [S2.MAX_ID]), filter(nil), rowset=256                                                                                  |
|       access([S2.CLASS], [S2.MAX_S_DATE], [S2.MAX_ID])                                                                                                           |
|   2 - output([ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(S2.CLASS, VARCHAR2(3 BYTE)))],          |
|       [T_FUN_MAX(S2.S_DATE)], [T_FUN_KEEP_MAX(S2.ID) order_items(S2.S_DATE)]), filter(nil), rowset=256                                                           |
|       group([S2.CLASS]), agg_func([T_FUN_MAX(S2.S_DATE)], [T_FUN_KEEP_MAX(S2.ID) order_items(S2.S_DATE)])                                                        |
|   3 - output([S2.ID], [S2.CLASS], [S2.S_DATE]), filter(nil), rowset=256                                                                                          |
|       access([S2.ID], [S2.CLASS], [S2.S_DATE]), partitions(p0)                                                                                                   |
|       is_index_back=false, is_global_index=false,                                                                                                                |
|       range_key([S2.CLASS], [S2.S_DATE], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true                                                                   |
|   4 - output([S1.ID], [S1.CLASS], [S1.S_DATE], [S1.NAME], [S1.AGE], [S1.SEX]), filter([S1.S_DATE = :4], [ora_decode(cast(S1.CLASS, VARCHAR2(1 BYTE)), cast(NULL, |
|        VARCHAR2(0 )), cast('aab', VARCHAR2(3 BYTE)), cast(S1.CLASS, VARCHAR2(3 BYTE))) = :3]), rowset=256                                                        |
|       access([S1.ID], [S1.CLASS], [S1.S_DATE], [S1.NAME], [S1.AGE], [S1.SEX]), partitions(p0)                                                                    |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false,false],                                                                          |
|       range_key([S1.ID]), range(MIN ; MAX),                                                                                                                      |
|       range_cond([S1.ID = :5])                                                                                                                                   |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
28 rows in set (0.011 sec)

row_number() over() 的逻辑还可以用表量子查询来实现,不过性能会更慢,实现起来很麻烦,就不在这里演示。


2、keep() 函数逻辑实现

某些数据库可能不支持keep函数,或者keep函数的性能并不理想,我们也可以尝试通过另外一种方式来实现keep函数的逻辑。

2.1、keep 形式一

SELECT 
  s1.class,
  max(s1.s_date) MAX_S_DATE,
  MAX(name) KEEP (DENSE_RANK LAST ORDER BY s1.s_date) max_name,
  min(s1.s_date) MIN_S_DATE,
  MIN(name) KEEP (DENSE_RANK FIRST ORDER BY s1.s_date) min_name
 FROM student s1 
GROUP BY
    s1.class;

27 rows in set (0.495 sec)


+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                                                            |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ===================================================                                                                                                                                                   |
| |ID|OPERATOR           |NAME|EST.ROWS|EST.TIME(us)|                                                                                                                                                   |
| ---------------------------------------------------                                                                                                                                                   |
| |0 |MERGE GROUP BY     |    |26      |144304      |                                                                                                                                                   |
| |1 |└─PARTITION SORT   |    |500001  |107784      |                                                                                                                                                   |
| |2 |  └─TABLE FULL SCAN|S1  |500001  |22309       |                                                                                                                                                   |
| ===================================================                                                                                                                                                   |
| Outputs & filters:                                                                                                                                                                                    |
| -------------------------------------                                                                                                                                                                 |
|   0 - output([S1.CLASS], [T_FUN_MAX(S1.S_DATE)], [T_FUN_KEEP_MAX(S1.NAME) order_items(S1.S_DATE)], [T_FUN_MIN(S1.S_DATE)], [T_FUN_KEEP_MIN(S1.NAME) order_items(S1.S_DATE)]), filter(nil), rowset=256 |
|       group([S1.CLASS]), agg_func([T_FUN_MAX(S1.S_DATE)], [T_FUN_KEEP_MAX(S1.NAME) order_items(S1.S_DATE)], [T_FUN_MIN(S1.S_DATE)], [T_FUN_KEEP_MIN(S1.NAME)                                          |
|       order_items(S1.S_DATE)])                                                                                                                                                                        |
|   1 - output([S1.CLASS], [S1.S_DATE], [S1.NAME]), filter(nil), rowset=256                                                                                                                             |
|       sort_keys([HASH(S1.CLASS), ASC], [S1.CLASS, ASC])                                                                                                                                               |
|   2 - output([S1.CLASS], [S1.S_DATE], [S1.NAME]), filter(nil), rowset=256                                                                                                                             |
|       access([S1.CLASS], [S1.S_DATE], [S1.NAME]), partitions(p0)                                                                                                                                      |
|       is_index_back=false, is_global_index=false,                                                                                                                                                     |
|       range_key([S1.ID]), range(MIN ; MAX)always true                                                                                                                                                 |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)

2.2、keep 形式一改写标量子查询

改写逻辑很简单,先分组找出最大、最小的 日期,然后通过日期 + class 外部标量子查询找出最大、最小的name。

SELECT 
  CLASS,
  MAX_S_DATE,
  (SELECT max(NAME) FROM student s2 WHERE 
       decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS) AND s2.S_DATE = s1.MAX_S_DATE) max_name,
  MIN_S_DATE,
  (SELECT min(NAME) FROM student s2 WHERE 
       decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS) AND s2.S_DATE = s1.MIN_S_DATE) min_name
 FROM (
SELECT 
  CLASS,
  max(S_DATE) MAX_S_DATE,
  min(S_DATE) MIN_S_DATE
 FROM student 
GROUP BY
    CLASS 
) s1 ;


27 rows in set (0.040 sec)

+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                                 |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ============================================================================================                                                                               |
| |ID|OPERATOR                        |NAME                            |EST.ROWS|EST.TIME(us)|                                                                               |
| --------------------------------------------------------------------------------------------                                                                               |
| |0 |SUBPLAN FILTER                  |                                |26      |36802       |                                                                               |
| |1 |├─MERGE GROUP BY                |                                |26      |35450       |                                                                               |
| |2 |│ └─TABLE FULL SCAN             |STUDENT(IDX_CLASS_SDATE_STUDENT)|500001  |14409       |                                                                               |
| |3 |├─SCALAR GROUP BY               |                                |1       |26          |                                                                               |
| |4 |│ └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID)         |1       |26          |                                                                               |
| |5 |└─SCALAR GROUP BY               |                                |1       |26          |                                                                               |
| |6 |  └─DISTRIBUTED TABLE RANGE SCAN|S2(IDX_S_DATE_CALSS_ID)         |1       |26          |                                                                               |
| ============================================================================================                                                                               |
| Outputs & filters:                                                                                                                                                         |
| -------------------------------------                                                                                                                                      |
|   0 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [subquery(1)], [T_FUN_MIN(STUDENT.S_DATE)], [subquery(2)]), filter(nil), rowset=256                             |
|       exec_params_([STUDENT.CLASS(:0)], [T_FUN_MAX(STUDENT.S_DATE)(:1)], [STUDENT.CLASS(:2)], [T_FUN_MIN(STUDENT.S_DATE)(:3)]), onetime_exprs_(nil), init_plan_idxs_(nil), |
|        use_batch=false                                                                                                                                                     |
|   1 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]), filter(nil), rowset=256                                                           |
|       group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)])                                                                           |
|   2 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256                                                                                                   |
|       access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0)                                                                                                            |
|       is_index_back=false, is_global_index=false,                                                                                                                          |
|       range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true                                                              |
|   3 - output([T_FUN_MAX(S2.NAME)]), filter(nil), rowset=256                                                                                                                |
|       group(nil), agg_func([T_FUN_MAX(S2.NAME)])                                                                                                                           |
|   4 - output([S2.NAME]), filter([ora_decode(cast(:0, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(:0, VARCHAR2(2 BYTE)))                |
|       = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), rowset=256               |
|       access([S2.ID], [S2.CLASS], [S2.NAME]), partitions(p0)                                                                                                               |
|       is_index_back=true, is_global_index=false, filter_before_indexback[true],                                                                                            |
|       range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true,                                                                            |
|       range_cond([S2.S_DATE = :1])                                                                                                                                         |
|   5 - output([T_FUN_MIN(S2.NAME)]), filter(nil), rowset=256                                                                                                                |
|       group(nil), agg_func([T_FUN_MIN(S2.NAME)])                                                                                                                           |
|   6 - output([S2.NAME]), filter([ora_decode(cast(:2, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(:2, VARCHAR2(2 BYTE)))                |
|       = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), rowset=256               |
|       access([S2.ID], [S2.CLASS], [S2.NAME]), partitions(p0)                                                                                                               |
|       is_index_back=true, is_global_index=false, filter_before_indexback[true],                                                                                            |
|       range_key([S2.S_DATE], [S2.CLASS], [S2.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true,                                                                            |
|       range_cond([S2.S_DATE = :3])                                                                                                                                         |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
38 rows in set (0.008 sec)

2.3、keep 形式一改写left join

改成 left join 也是按照标量的逻辑,都比较简单。

SELECT s1.CLASS,
       s1.MAX_S_DATE,
       max(CASE WHEN s2.S_DATE = s1.MAX_S_DATE THEN s2.NAME END)  max_name,
       s1.MIN_S_DATE,
       min(CASE WHEN s2.S_DATE = s1.MIN_S_DATE THEN s2.NAME END ) min_name
 FROM (

SELECT 
  CLASS,
  max(S_DATE) MAX_S_DATE,
  min(S_DATE) MIN_S_DATE
 FROM student 
GROUP BY
    CLASS 
    )s1 LEFT JOIN  student s2 ON decode(s1.CLASS,null,'bb',s1.CLASS) = decode(s2.CLASS,null,'bb',s2.CLASS)
    GROUP BY 
    s1.CLASS,
       s1.MAX_S_DATE,
       s1.MIN_S_DATE;

27 rows in set (0.213 sec)

+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                         |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ===================================================================================                                                                                |
| |ID|OPERATOR               |NAME                            |EST.ROWS|EST.TIME(us)|                                                                                |
| -----------------------------------------------------------------------------------                                                                                |
| |0 |HASH GROUP BY          |                                |17      |200679      |                                                                                |
| |1 |└─HASH OUTER JOIN      |                                |65001   |193221      |                                                                                |
| |2 |  ├─SUBPLAN SCAN       |S1                              |26      |35450       |                                                                                |
| |3 |  │ └─MERGE GROUP BY   |                                |26      |35450       |                                                                                |
| |4 |  │   └─TABLE FULL SCAN|STUDENT(IDX_CLASS_SDATE_STUDENT)|500001  |14409       |                                                                                |
| |5 |  └─TABLE FULL SCAN    |S2                              |500001  |22309       |                                                                                |
| ===================================================================================                                                                                |
| Outputs & filters:                                                                                                                                                 |
| -------------------------------------                                                                                                                              |
|   0 - output([S1.CLASS], [S1.MAX_S_DATE], [T_FUN_MAX(CASE WHEN S2.S_DATE = S1.MAX_S_DATE THEN S2.NAME ELSE NULL END)], [S1.MIN_S_DATE], [T_FUN_MIN(CASE            |
|       WHEN S2.S_DATE = S1.MIN_S_DATE THEN S2.NAME ELSE NULL END)]), filter(nil), rowset=256                                                                        |
|       group([S1.CLASS]), agg_func([T_FUN_MAX(CASE WHEN S2.S_DATE = S1.MAX_S_DATE THEN S2.NAME ELSE NULL END)], [T_FUN_MIN(CASE WHEN S2.S_DATE = S1.MIN_S_DATE      |
|       THEN S2.NAME ELSE NULL END)])                                                                                                                                |
|   1 - output([S1.CLASS], [S2.S_DATE], [S2.NAME], [S1.MAX_S_DATE], [S1.MIN_S_DATE]), filter(nil), rowset=256                                                        |
|       equal_conds([ora_decode(cast(S1.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S1.CLASS, VARCHAR2(2 BYTE)))          |
|       = ora_decode(cast(S2.CLASS, VARCHAR2(1 BYTE)), cast(NULL, VARCHAR2(0 )), cast('bb', VARCHAR2(2 BYTE)), cast(S2.CLASS, VARCHAR2(2 BYTE)))]), other_conds(nil) |
|   2 - output([S1.CLASS], [S1.MAX_S_DATE], [S1.MIN_S_DATE]), filter(nil), rowset=256                                                                                |
|       access([S1.CLASS], [S1.MAX_S_DATE], [S1.MIN_S_DATE])                                                                                                         |
|   3 - output([STUDENT.CLASS], [T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)]), filter(nil), rowset=256                                                   |
|       group([STUDENT.CLASS]), agg_func([T_FUN_MAX(STUDENT.S_DATE)], [T_FUN_MIN(STUDENT.S_DATE)])                                                                   |
|   4 - output([STUDENT.CLASS], [STUDENT.S_DATE]), filter(nil), rowset=256                                                                                           |
|       access([STUDENT.CLASS], [STUDENT.S_DATE]), partitions(p0)                                                                                                    |
|       is_index_back=false, is_global_index=false,                                                                                                                  |
|       range_key([STUDENT.CLASS], [STUDENT.S_DATE], [STUDENT.ID]), range(MIN,MIN,MIN ; MAX,MAX,MAX)always true                                                      |
|   5 - output([S2.CLASS], [S2.S_DATE], [S2.NAME]), filter(nil), rowset=256                                                                                          |
|       access([S2.CLASS], [S2.S_DATE], [S2.NAME]), partitions(p0)                                                                                                   |
|       is_index_back=false, is_global_index=false,                                                                                                                  |
|       range_key([S2.ID]), range(MIN ; MAX)always true                                                                                                              |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
31 rows in set (0.005 sec)

2.4、keep 形式二

SELECT 
  DEPARTMENT_ID,
  HIRE_DATE,
  MAX(FIRST_NAME) KEEP (DENSE_RANK LAST ORDER BY HIRE_DATE)  over(PARTITION BY DEPARTMENT_ID) max_name,
  MIN(FIRST_NAME) KEEP (DENSE_RANK FIRST ORDER BY HIRE_DATE) over(PARTITION BY DEPARTMENT_ID) min_name
 FROM EMPLOYEES ;

 107 rows in set (0.002 sec)

2.5、keep 形式二改写left join

SELECT 
  e1.DEPARTMENT_ID,
  e1.HIRE_DATE,
  e2.max_name,
  e2.min_name
FROM EMPLOYEES e1 LEFT JOIN (
SELECT DEPARTMENT_ID,
       MAX(CASE WHEN max_rn = 1 THEN FIRST_NAME ELSE NULL END) max_name,
       MIN(CASE WHEN min_rn = 1 THEN FIRST_NAME ELSE NULL END) min_name
     
 FROM (
    SELECT 
      FIRST_NAME,
      HIRE_DATE,
      DEPARTMENT_ID,
      DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) max_rn,
      DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE ) min_rn

   
    FROM  EMPLOYEES
)
GROUP BY DEPARTMENT_ID

) e2 ON   // 最后关联
decode(e1.DEPARTMENT_ID, null, 'aaa', e1.DEPARTMENT_ID) =
                                   decode(e2.DEPARTMENT_ID, null, 'aaa', e2.DEPARTMENT_ID);
                                   
107 rows in set (0.007 sec)

2.6、keep 形式二改写标量子查询


SELECT DEPARTMENT_ID,
         HIRE_DATE,
    (SELECT e2.max_name
    FROM 
        (SELECT DEPARTMENT_ID,
        max(FIRST_NAME) max_name
        FROM 
            (SELECT x.*,
         DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID
            ORDER BY  HIRE_DATE DESC) max_rn
            FROM EMPLOYEES x )
            WHERE max_rn = 1
            GROUP BY  DEPARTMENT_ID ) e2
            WHERE decode(e1.department_id, null, 'aaa', e1.department_id) = decode(e2.department_id, null, 'aaa', e2.department_id) ) max_name, 
            (SELECT e2.min_name
            FROM 
                (SELECT DEPARTMENT_ID,
        min(FIRST_NAME) min_name
                FROM 
                    (SELECT x.*,
         DENSE_RANK() OVER(PARTITION BY DEPARTMENT_ID
                    ORDER BY  HIRE_DATE ) min_rn
                    FROM EMPLOYEES x )
                    WHERE min_rn = 1
                    GROUP BY  DEPARTMENT_ID ) e2
                    WHERE decode(e1.department_id, null, 'aaa', e1.department_id) = decode(e2.department_id, null, 'aaa', e2.department_id) ) min_name
                FROM EMPLOYEES e1;

107 rows in set (0.030 sec)

/* 改SQL 逻辑等价上面join sql, 如果会写join版本改成表量子查询是更简单的操作 */

3、rank() over() 函数逻辑实现

rank() over() 函数会跳号,如果符合over()逻辑的数据有两条的话,那么这两条会并列第一,然后第二条数据会加二(即显示第三)。
这个函数在业务逻辑上比较少使用到,但是我们也可以尝试使用其他逻辑来实现rank() over() 函数的功能。
**** 注意:以下实现的rank() over() 逻辑的代码性能,会比原来rank() over() 的性能更差,这里只做功能实现,不考虑性能问题 ****

SELECT * FROM (
SELECT 
  FIRST_NAME,
  DEPARTMENT_ID,
  HIRE_DATE,
   RANK() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) MAX_RN
 FROM EMPLOYEES )  WHERE DEPARTMENT_ID = 80;

 34 rows in set (0.004 sec)

+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                   |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ========================================================                                                                                                     |
| |ID|OPERATOR           |NAME     |EST.ROWS|EST.TIME(us)|                                                                                                     |
| --------------------------------------------------------                                                                                                     |
| |0 |WINDOW FUNCTION    |         |34      |42          |                                                                                                     |
| |1 |└─SORT             |         |34      |16          |                                                                                                     |
| |2 |  └─TABLE FULL SCAN|EMPLOYEES|34      |10          |                                                                                                     |
| ========================================================                                                                                                     |
| Outputs & filters:                                                                                                                                           |
| -------------------------------------                                                                                                                        |
|   0 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE], [T_WIN_FUN_RANK()]), filter(nil), rowset=256                          |
|       win_expr(T_WIN_FUN_RANK()), partition_by(nil), order_by([EMPLOYEES.HIRE_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED PRECEDING), lower(UNBOUNDED  |
|       FOLLOWING)                                                                                                                                             |
|   1 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256                                              |
|       sort_keys([EMPLOYEES.HIRE_DATE, DESC])                                                                                                                 |
|   2 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter([EMPLOYEES.DEPARTMENT_ID = 80]), rowset=256                   |
|       access([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), partitions(p0)                                                       |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                            |
|       range_key([EMPLOYEES.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                        |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)

3.1、改写方式一:标量子查询逻辑实现

 SELECT 
  e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE,
  (
    SELECT count(HIRE_DATE) + 1 FROM EMPLOYEES e1 WHERE 
      DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND 
      e1.HIRE_DATE > e.HIRE_DATE
  ) MAX_RN
 FROM EMPLOYEES  e  WHERE e.DEPARTMENT_ID = 80;

/*
  重点是在标量子查询内的 e1.HIRE_DATE > e.HIRE_DATE 逻辑,可以将 e1 表理解成一个滑动的窗口。  
*/
 
 
34 rows in set (0.006 sec)

+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| =========================================================                                                                                                           |
| |ID|OPERATOR                 |NAME|EST.ROWS|EST.TIME(us)|                                                                                                           |
| ---------------------------------------------------------                                                                                                           |
| |0 |MERGE GROUP BY           |    |34      |29          |                                                                                                           |
| |1 |└─NESTED-LOOP OUTER JOIN |    |34      |22          |                                                                                                           |
| |2 |  ├─TABLE FULL SCAN      |E   |34      |11          |                                                                                                           |
| |3 |  └─MATERIAL             |    |1       |11          |                                                                                                           |
| |4 |    └─TABLE FULL SCAN    |E1  |1       |11          |                                                                                                           |
| =========================================================                                                                                                           |
| Outputs & filters:                                                                                                                                                  |
| -------------------------------------                                                                                                                               |
|   0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(E1.HIRE_DATE) + 1]), filter(nil), rowset=256                                            |
|       group([E.EMPLOYEE_ID]), agg_func([T_FUN_COUNT(E1.HIRE_DATE)])                                                                                                 |
|   1 - output([E.EMPLOYEE_ID], [E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [E1.HIRE_DATE]), filter(nil), rowset=256                                            |
|       conds([E1.HIRE_DATE > E.HIRE_DATE]), nl_params_(nil), use_batch=false                                                                                         |
|   2 - output([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), filter([E.DEPARTMENT_ID = 80]), rowset=256                                         |
|       access([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), partitions(p0)                                                                     |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                   |
|       range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                       |
|   3 - output([E1.HIRE_DATE]), filter(nil), rowset=256                                                                                                               |
|   4 - output([E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)),             |
|       cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40  |
|       BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256                                                                                        |
|       access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0)                                                                                                    |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                   |
|       range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                      |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
26 rows in set (0.006 sec)

3.2、改写方式二:LEFT JOIN 逻辑实现

会写标量子查询的情况下,改写成LEFT JOIN 是很简单的事,逻辑一样不再赘述。

 SELECT 
  e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE,
  count(e1.HIRE_DATE) + 1 MAX_RN
 FROM EMPLOYEES  e LEFT JOIN EMPLOYEES e1 ON (
  DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND  e1.HIRE_DATE > e.HIRE_DATE)
  WHERE e.DEPARTMENT_ID = 80
  GROUP BY 
    e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE;

  34 rows in set (0.012 sec)


  +----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                           |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ==========================================================                                                                                                           |
| |ID|OPERATOR                 |NAME |EST.ROWS|EST.TIME(us)|                                                                                                           |
| ----------------------------------------------------------                                                                                                           |
| |0 |HASH GROUP BY            |     |34      |49          |                                                                                                           |
| |1 |└─NESTED-LOOP OUTER JOIN |     |34      |33          |                                                                                                           |
| |2 |  ├─SUBPLAN SCAN         |VIEW1|34      |22          |                                                                                                           |
| |3 |  │ └─HASH GROUP BY      |     |34      |22          |                                                                                                           |
| |4 |  │   └─TABLE FULL SCAN  |E    |34      |10          |                                                                                                           |
| |5 |  └─MATERIAL             |     |1       |11          |                                                                                                           |
| |6 |    └─SUBPLAN SCAN       |VIEW2|1       |11          |                                                                                                           |
| |7 |      └─HASH GROUP BY    |     |1       |11          |                                                                                                           |
| |8 |        └─TABLE FULL SCAN|E1   |1       |11          |                                                                                                           |
| ==========================================================                                                                                                           |
| Outputs & filters:                                                                                                                                                   |
| -------------------------------------                                                                                                                                |
|   0 - output([VIEW1.E.FIRST_NAME], [VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [T_FUN_COUNT_SUM(VIEW1.T_FUN_COUNT(*) * CASE WHEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE)   |
|       IS NOT NULL THEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) ELSE 0 END) + 1]), filter(nil), rowset=256                                                                    |
|       group([VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME]), agg_func([T_FUN_COUNT_SUM(VIEW1.T_FUN_COUNT(*) * CASE WHEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) IS NOT            |
|       NULL THEN VIEW2.T_FUN_COUNT(E1.HIRE_DATE) ELSE 0 END)])                                                                                                        |
|   1 - output([VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.E.DEPARTMENT_ID], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)], [VIEW1.T_FUN_COUNT(*)]), filter(nil), rowset=256 |
|       conds([VIEW2.E1.HIRE_DATE > VIEW1.E.HIRE_DATE]), nl_params_(nil), use_batch=false                                                                              |
|   2 - output([VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.T_FUN_COUNT(*)]), filter(nil), rowset=256                                    |
|       access([VIEW1.E.DEPARTMENT_ID], [VIEW1.E.HIRE_DATE], [VIEW1.E.FIRST_NAME], [VIEW1.T_FUN_COUNT(*)])                                                             |
|   3 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME], [T_FUN_COUNT(*)]), filter(nil), rowset=256                                                            |
|       group([E.HIRE_DATE], [E.FIRST_NAME]), agg_func([T_FUN_COUNT(*)])                                                                                               |
|   4 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), filter([E.DEPARTMENT_ID = 80]), rowset=256                                                           |
|       access([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), partitions(p0)                                                                                       |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                    |
|       range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                        |
|   5 - output([VIEW2.T_FUN_COUNT(E1.HIRE_DATE)], [VIEW2.E1.HIRE_DATE]), filter(nil), rowset=256                                                                       |
|   6 - output([VIEW2.E1.HIRE_DATE], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)]), filter(nil), rowset=256                                                                       |
|       access([VIEW2.E1.HIRE_DATE], [VIEW2.T_FUN_COUNT(E1.HIRE_DATE)])                                                                                                |
|   7 - output([E1.HIRE_DATE], [T_FUN_COUNT(E1.HIRE_DATE)]), filter(nil), rowset=256                                                                                   |
|       group([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), agg_func([T_FUN_COUNT(E1.HIRE_DATE)])                                                                               |
|   8 - output([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080',              |
|       VARCHAR2(40 BYTE)), cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )),        |
|       cast('8080', VARCHAR2(40 BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256                                                                |
|       access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0)                                                                                                     |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                    |
|       range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                       |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------+
40 rows in set (0.011 sec)

4、 dense_rank() over() 函数逻辑实现

dense_rank() over() 函数不会跳号,如果符合over()逻辑的数据有两条的话,那么这两条会并列第一,然后第二条数据会递增加一(即显示第二)。
这个函数在业务逻辑上也比较少用到,但是比rank()用得多,但是我们也可以尝试使用其他逻辑来实现dense_rank() over() 函数的功能。
**** 注意:以下实现的dense_rank() over() 逻辑的代码性能,会比原来dense_rank() over() 的性能更差,这里只做功能实现,不考虑性能问题 ****

SELECT * FROM (
SELECT 
  FIRST_NAME,
  DEPARTMENT_ID,
  HIRE_DATE,
   dense_rank() OVER(PARTITION BY DEPARTMENT_ID ORDER BY HIRE_DATE DESC) MAX_RN
 FROM EMPLOYEES )  WHERE DEPARTMENT_ID = 80;

 34 rows in set (0.001 sec)

 +--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                   |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ========================================================                                                                                                     |
| |ID|OPERATOR           |NAME     |EST.ROWS|EST.TIME(us)|                                                                                                     |
| --------------------------------------------------------                                                                                                     |
| |0 |WINDOW FUNCTION    |         |107     |115         |                                                                                                     |
| |1 |└─PARTITION SORT   |         |107     |33          |                                                                                                     |
| |2 |  └─TABLE FULL SCAN|EMPLOYEES|107     |9           |                                                                                                     |
| ========================================================                                                                                                     |
| Outputs & filters:                                                                                                                                           |
| -------------------------------------                                                                                                                        |
|   0 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE], [T_WIN_FUN_DENSE_RANK()]), filter(nil), rowset=256                    |
|       win_expr(T_WIN_FUN_DENSE_RANK()), partition_by([EMPLOYEES.DEPARTMENT_ID]), order_by([EMPLOYEES.HIRE_DATE, DESC]), window_type(RANGE), upper(UNBOUNDED  |
|       PRECEDING), lower(UNBOUNDED FOLLOWING)                                                                                                                 |
|   1 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256                                              |
|       sort_keys([HASH(EMPLOYEES.DEPARTMENT_ID), ASC], [EMPLOYEES.DEPARTMENT_ID, ASC], [EMPLOYEES.HIRE_DATE, DESC])                                           |
|   2 - output([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), filter(nil), rowset=256                                              |
|       access([EMPLOYEES.FIRST_NAME], [EMPLOYEES.DEPARTMENT_ID], [EMPLOYEES.HIRE_DATE]), partitions(p0)                                                       |
|       is_index_back=false, is_global_index=false,                                                                                                            |
|       range_key([EMPLOYEES.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                        |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------+
18 rows in set (0.004 sec)

4.1、改写方式一:标量子查询逻辑实现

 SELECT 
  e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE,
  (SELECT COUNT(DISTINCT HIRE_DATE) +1 FROM EMPLOYEES e1 WHERE DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND e1.HIRE_DATE > e.HIRE_DATE ) MAX_RN
 FROM EMPLOYEES e WHERE DEPARTMENT_ID = 80 ;

 /* 
 COUNT(DISTINCT HIRE_DATE)  逻辑是求   e1.HIRE_DATE > e.HIRE_DATE 符合这个逻辑的 e1表的 HIRE_DATE 数据,等价 dense_rank。
 */

34 rows in set (0.006 sec)

+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| =========================================================                                                                                                           |
| |ID|OPERATOR                 |NAME|EST.ROWS|EST.TIME(us)|                                                                                                           |
| ---------------------------------------------------------                                                                                                           |
| |0 |MERGE GROUP BY           |    |34      |29          |                                                                                                           |
| |1 |└─NESTED-LOOP OUTER JOIN |    |34      |22          |                                                                                                           |
| |2 |  ├─TABLE FULL SCAN      |E   |34      |11          |                                                                                                           |
| |3 |  └─MATERIAL             |    |1       |11          |                                                                                                           |
| |4 |    └─TABLE FULL SCAN    |E1  |1       |11          |                                                                                                           |
| =========================================================                                                                                                           |
| Outputs & filters:                                                                                                                                                  |
| -------------------------------------                                                                                                                               |
|   0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(distinct E1.HIRE_DATE) + 1]), filter(nil), rowset=256                                   |
|       group([E.EMPLOYEE_ID]), agg_func([T_FUN_COUNT(distinct E1.HIRE_DATE)])                                                                                        |
|   1 - output([E.EMPLOYEE_ID], [E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [E1.HIRE_DATE]), filter(nil), rowset=256                                            |
|       conds([E1.HIRE_DATE > E.HIRE_DATE]), nl_params_(nil), use_batch=false                                                                                         |
|   2 - output([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), filter([E.DEPARTMENT_ID = 80]), rowset=256                                         |
|       access([E.EMPLOYEE_ID], [E.DEPARTMENT_ID], [E.FIRST_NAME], [E.HIRE_DATE]), partitions(p0)                                                                     |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                   |
|       range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                       |
|   3 - output([E1.HIRE_DATE]), filter(nil), rowset=256                                                                                                               |
|   4 - output([E1.HIRE_DATE]), filter([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)),             |
|       cast(E1.DEPARTMENT_ID, VARCHAR2(40 BYTE))) = ora_decode(cast(cast(80, NUMBER(4, 0)), VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40  |
|       BYTE)), cast(cast(80, NUMBER(4, 0)), VARCHAR2(40 BYTE)))]), rowset=256                                                                                        |
|       access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0)                                                                                                    |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                   |
|       range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                      |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
26 rows in set (0.006 sec)

4.2、改写方式二:LEFT JOIN 逻辑实现

会写标量子查询的情况下,改写成LEFT JOIN 是很简单的事,逻辑一样不再赘述。

 SELECT 
  e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE,
  count(DISTINCT e1.HIRE_DATE) + 1 MAX_RN
 FROM EMPLOYEES  e LEFT JOIN EMPLOYEES e1 ON (
  DECODE(e1.DEPARTMENT_ID,NULL,'8080',e1.DEPARTMENT_ID) = DECODE(e.DEPARTMENT_ID,NULL,'8080',e.DEPARTMENT_ID) AND  e1.HIRE_DATE > e.HIRE_DATE)
  WHERE e.DEPARTMENT_ID = 80
  GROUP BY 
  e.FIRST_NAME,
  e.DEPARTMENT_ID,
  e.HIRE_DATE;

  34 rows in set (0.006 sec)

  +---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Query Plan                                                                                                                                                          |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| =====================================================                                                                                                               |
| |ID|OPERATOR             |NAME|EST.ROWS|EST.TIME(us)|                                                                                                               |
| -----------------------------------------------------                                                                                                               |
| |0 |MERGE GROUP BY       |    |34      |76          |                                                                                                               |
| |1 |└─PARTITION SORT     |    |34      |70          |                                                                                                               |
| |2 |  └─HASH OUTER JOIN  |    |34      |62          |                                                                                                               |
| |3 |    ├─TABLE FULL SCAN|E   |34      |10          |                                                                                                               |
| |4 |    └─TABLE FULL SCAN|E1  |107     |9           |                                                                                                               |
| =====================================================                                                                                                               |
| Outputs & filters:                                                                                                                                                  |
| -------------------------------------                                                                                                                               |
|   0 - output([E.FIRST_NAME], [E.DEPARTMENT_ID], [E.HIRE_DATE], [T_FUN_COUNT(distinct E1.HIRE_DATE) + 1]), filter(nil), rowset=256                                   |
|       group([E.FIRST_NAME], [E.HIRE_DATE]), agg_func([T_FUN_COUNT(distinct E1.HIRE_DATE)])                                                                          |
|   1 - output([E.FIRST_NAME], [E.HIRE_DATE], [E.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256                                                             |
|       sort_keys([HASH(E.FIRST_NAME, E.HIRE_DATE), ASC], [E.FIRST_NAME, ASC], [E.HIRE_DATE, ASC])                                                                    |
|   2 - output([E.FIRST_NAME], [E.HIRE_DATE], [E.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256                                                             |
|       equal_conds([ora_decode(cast(E1.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), cast(E1.DEPARTMENT_ID,         |
|        VARCHAR2(40 BYTE))) = ora_decode(cast(E.DEPARTMENT_ID, VARCHAR2(1048576 )), cast(NULL, VARCHAR2(0 )), cast('8080', VARCHAR2(40 BYTE)), cast(E.DEPARTMENT_ID, |
|        VARCHAR2(40 BYTE)))]), other_conds([E1.HIRE_DATE > E.HIRE_DATE])                                                                                             |
|   3 - output([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), filter([E.DEPARTMENT_ID = 80]), rowset=256                                                          |
|       access([E.DEPARTMENT_ID], [E.HIRE_DATE], [E.FIRST_NAME]), partitions(p0)                                                                                      |
|       is_index_back=false, is_global_index=false, filter_before_indexback[false],                                                                                   |
|       range_key([E.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                       |
|   4 - output([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), filter(nil), rowset=256                                                                                           |
|       access([E1.DEPARTMENT_ID], [E1.HIRE_DATE]), partitions(p0)                                                                                                    |
|       is_index_back=false, is_global_index=false,                                                                                                                   |
|       range_key([E1.EMPLOYEE_ID]), range(MIN ; MAX)always true                                                                                                      |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
27 rows in set (0.005 sec)

5、 总结

SQL语句虽然很简单,但是要培养出SQL思维可不是一件容易的事,还是得多看,多写,多思考。
这个是要通过不同的开发需求日积月累形成的思维逻辑,而不是一蹴而就。

posted @ 2024-07-15 23:31  小至尖尖  阅读(524)  评论(1编辑  收藏  举报