标量子查询 子查询执行次数计算公式

select a.username,(select count(*) from all_objects b where b.owner=a.username) cnt from all_users a;

create table a as select * from all_users;

create table b as select * from all_objects;


select a.username,(select count(*) from b where b.owner=a.username) cnt from a;


SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	2q0bhbzak85z9, child number 0
-------------------------------------
select a.username,(select count(*) from b where b.owner=a.username) cnt
from a

Plan hash value: 3049668959

----------------------------------------------------------------------------------------------
| Id  | Operation	   | Name | Starts | E-Rows | A-Rows |	 A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	  |	 1 |	    |	  36 |00:00:00.01 |	  6 |	   0 |
|   1 |  SORT AGGREGATE    |	  |	36 |	  1 |	  36 |00:00:01.09 |   36756 |  36648 |
|*  2 |   TABLE ACCESS FULL| B	  |	36 |	780 |  43773 |00:00:01.02 |   36756 |  36648 |
|   3 |  TABLE ACCESS FULL | A	  |	 1 |	 36 |	  36 |00:00:00.01 |	  6 |	   0 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("B"."OWNER"=:B1)

Note
-----
   - dynamic sampling used for this statement (level=2)


25 rows selected.

标量子查询会执行传入的值 DISTINCT那么多次,这里传入的值是a.username 就是distinct(a.username)次


SQL> 
select count(distinct a.username) from a;SQL> 

COUNT(DISTINCTA.USERNAME)
-------------------------
		       36


改写成左链接:



  select a.username,nvl(b.cnt,0) from a left join (select count(*) cnt ,b.owner from b group by b.owner ) b
  on b.owner = a.username
  
  select a.username,count(b.owner) from  a left join  b on b.owner=a.username group by a.username