Question

哪个更有效2，特别是在使用带有varchar类型数据的Listagg的情况下？

SELECT department_id AS "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) AS "Employees"
  FROM employees
  GROUP BY department_id
  ORDER BY department_id;

OR

SELECT department_id AS "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) 
       OVER (PARTITION BY department_id) AS "Employees"
  FROM employees
  ORDER BY department_id;

对我来说，除非我只选择1/2列，否则我将始终使用分区，因为我不必在'GROUP BY'子句中包含每一列。

Answer 1

这两个查询提供不同的输出。使用GROUP BY将返回每组一行，而使用OVER ( PARTITION BY .. )将返回所有行，并为分区中的每一行复制LISTAGG结果。

使用哪种解决方案更适合您所需的输出 - 但它们不相同。

SQL Fiddle

Oracle 11g R2架构设置：

CREATE TABLE employees ( department_id, last_name, hire_date ) AS
SELECT 1, 'AAA', DATE '2018-01-01' FROM DUAL UNION ALL
SELECT 1, 'BBB', DATE '2018-01-02' FROM DUAL UNION ALL
SELECT 1, 'CCC', DATE '2018-01-03' FROM DUAL UNION ALL
SELECT 2, 'DDD', DATE '2018-01-01' FROM DUAL UNION ALL
SELECT 2, 'EEE', DATE '2018-01-02' FROM DUAL;

查询1 ：

SELECT department_id AS "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) AS "Employees"
  FROM employees
  GROUP BY department_id
  ORDER BY department_id

<强> Results ：

| Dept. |     Employees |
|-------|---------------|
|     1 | AAA; BBB; CCC |
|     2 |      DDD; EEE |

查询2 ：

SELECT department_id AS "Dept.",
       LISTAGG(last_name, '; ') WITHIN GROUP (ORDER BY hire_date) 
       OVER (PARTITION BY department_id) AS "Employees"
  FROM employees
  ORDER BY department_id

<强> Results ：

| Dept. |     Employees |
|-------|---------------|
|     1 | AAA; BBB; CCC |
|     1 | AAA; BBB; CCC |
|     1 | AAA; BBB; CCC |
|     2 |      DDD; EEE |
|     2 |      DDD; EEE |

Answer 2

两个查询都执行相同数量的工作，因为每个查询都必须执行EMPLOYEES的完整扫描，对LAST_NAME中DEPARTMENT_ID的值进行排序。

使用Oracle“HR”演示模式（其中EMPLOYEES有107行）和SQL * Plus set autotrace工具，我们可以看到执行7次一致获取和1次排序。

SQL> set autotrace traceonly explain statistics

SQL> select department_id as "Dept."
  2       , listagg(last_name, '; ') within group(order by hire_date) as "Employees"
  3  from   employees
  4  group  by department_id
  5  order  by department_id;

12 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 2107619104

--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |    11 |   209 |     4  (25)| 00:00:01 |
|   1 |  SORT GROUP BY     |           |    11 |   209 |     4  (25)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMPLOYEES |   107 |  2033 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
       1605  bytes sent via SQL*Net to client
        608  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
         12  rows processed

SQL> select department_id as "Dept."
  2       , listagg(last_name, '; ') within group(order by hire_date) over(partition by department_id) as "Employees"
  3  from   employees
  4  order  by department_id;

107 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1919783947

--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |   107 |  2033 |     3   (0)| 00:00:01 |
|   1 |  WINDOW SORT       |           |   107 |  2033 |     3   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMPLOYEES |   107 |  2033 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          7  consistent gets
          0  physical reads
          0  redo size
       3703  bytes sent via SQL*Net to client
        685  bytes received via SQL*Net from client
          9  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
        107  rows processed

第二个查询有更多网络活动，因为它会返回更多行。

Listagg分区而不是分组依据

2 个答案: