SQL ORACLE, difference between LEFT JOIN to a table and LEFT JOIN to a SUB QUERY

Question

I'm trying to optimize some SQL code from former coworker and something comes up. I want to know if LEFT JOIN to a whole table or LEFT JOIN to a part of table with only needed columns (create by sub query) were better?

Is there any way to test the performance between these two cases? Example:

SELECT A.*, B.COL1, B.COL2, B.COL3
FROM TABLE_A A
LEFT JOIN TABLE_B ON A.ID = B.ID;

SELECT A.*, C.*
FROM TABLE_A 
LEFT JOIN 
    (SELECT B.ID, B.COL1, B.COL2, B.COL3 FROM TABLE_B) C ON C.ID = A.ID

Answer 1

In this instance there's no difference. You can verify this by getting the execution plans for the two queries. There are many ways to do this.

A good technique to get a plan you can share on Q&A sites like this is:

• Before running your query, set serverouput off
• Run your queries with the gather_plan_statistics hint
• Get their executions plans by calling dbms_xplan.display_cursor

Do this and you'll see:

create table table_a (
  id int
);

create table table_b (
  id int,
  col1 int,
  col2 int,
  col3 int
);

insert into table_a values ( 1 );
insert into table_a values ( 2 );

insert into table_b values ( 1, 1, 1, 1 );
insert into table_b values ( 3, 3, 3, 3 );

commit;

set serveroutput off

select /*+ gather_plan_statistics */
       a.*, b.col1, b.col2, b.col3
from   table_a a
left join table_b b
on     a.id = b.id;

select * 
from   table(dbms_xplan.display_cursor(null, null, 'IOSTATS LAST'));

PLAN_TABLE_OUTPUT                                                                          
SQL_ID  64516xvpa898t, child number 1                                                      
-------------------------------------                                                      
select /*+ gather_plan_statistics */        a.*, b.col1, b.col2, b.col3                    
from   table_a a left join table_b b on     a.id = b.id                                    

Plan hash value: 1267695137                                                                

----------------------------------------------------------------------------------------   
| Id  | Operation          | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |   
----------------------------------------------------------------------------------------   
|   0 | SELECT STATEMENT   |         |      1 |        |      2 |00:00:00.01 |      14 |   
|*  1 |  HASH JOIN OUTER   |         |      1 |      2 |      2 |00:00:00.01 |      14 |   
|   2 |   TABLE ACCESS FULL| TABLE_A |      1 |      2 |      2 |00:00:00.01 |       7 |   
|   3 |   TABLE ACCESS FULL| TABLE_B |      1 |      2 |      2 |00:00:00.01 |       7 |   
----------------------------------------------------------------------------------------   

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

   1 - access("A"."ID"="B"."ID")                                                           

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

select /*+ gather_plan_statistics */
       a.*, c.*
from   table_a a
left join (
  select b.id, b.col1, b.col2, b.col3 
  from   table_b b
) c 
on c.id = a.id;

select * 
from   table(dbms_xplan.display_cursor(null, null, 'IOSTATS LAST'));

PLAN_TABLE_OUTPUT                                                                          
SQL_ID  b0abq59kzw8df, child number 0                                                      
-------------------------------------                                                      
select /*+ gather_plan_statistics */        a.*, c.* from   table_a a                      
left join (   select b.id, b.col1, b.col2, b.col3    from   table_b b )                    
c  on c.id = a.id                                                                          

Plan hash value: 1267695137                                                                

----------------------------------------------------------------------------------------   
| Id  | Operation          | Name    | Starts | E-Rows | A-Rows |   A-Time   | Buffers |   
----------------------------------------------------------------------------------------   
|   0 | SELECT STATEMENT   |         |      1 |        |      2 |00:00:00.01 |      14 |   
|*  1 |  HASH JOIN OUTER   |         |      1 |      2 |      2 |00:00:00.01 |      14 |   
|   2 |   TABLE ACCESS FULL| TABLE_A |      1 |      2 |      2 |00:00:00.01 |       7 |   
|   3 |   TABLE ACCESS FULL| TABLE_B |      1 |      2 |      2 |00:00:00.01 |       7 |   
----------------------------------------------------------------------------------------   

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

   1 - access("B"."ID"="A"."ID")                                                           

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

Note that:

• The "Plan hash value" is the same for both queries (1267695137)
• The values in the Starts, A-rows, & Buffers columns of the plans are the same

=> The queries used the same plan & did the same amount of work.