Is an anti-join more efficient than a left outer join?

Question

A comment on this answer notes that anti-joins may have been optimized to be more efficient that outer joins in Oracle. I\'d be interested to see what explanations/evidence

Answer 1

If Oracle can transform left join + where is null into ANTI join then it's exactly the same.

create table ttt1 as select mod(rownum,10) id from dual connect by level <= 50000;
insert into ttt1 select 10 from dual;
create table ttt2 as select mod(rownum,10) id from dual connect by level <= 50000;

select ttt1.id
  from ttt1
  left join ttt2
    on ttt1.id = ttt2.id
 where ttt2.id is null;

select * from ttt1 where id not in (select id from ttt2);

If you have a look at

Final query after transformations:******* UNPARSED QUERY IS *******

in trace for event 10053 then you'll find two exactly the same queries (you can see "=" in predicate in the trace file because there is no special sign for ANTI join)

SELECT "TTT1"."ID" "ID" FROM "TTT2" "TTT2","TTT1" "TTT1" WHERE "TTT1"."ID"="TTT2"."ID"

And they have exactly the same plans

-----------------------------------
| Id  | Operation          | Name |
-----------------------------------
|   0 | SELECT STATEMENT   |      |
|   1 |  HASH JOIN ANTI    |      |
|   2 |   TABLE ACCESS FULL| TTT1 |
|   3 |   TABLE ACCESS FULL| TTT2 |
-----------------------------------

If you, however, put a hint to disable transformations then plan will be

select --+ no_query_transformation
       ttt1.id
  from ttt1, ttt2
 where ttt1.id = ttt2.id(+) and ttt2.id is null;

------------------------------------
| Id  | Operation           | Name |
------------------------------------
|   0 | SELECT STATEMENT    |      |
|   1 |  FILTER             |      |
|   2 |   HASH JOIN OUTER   |      |
|   3 |    TABLE ACCESS FULL| TTT1 |
|   4 |    TABLE ACCESS FULL| TTT2 |
------------------------------------

and performance will degrade significantly.

If you will use ANSI join syntax with deisabled transformation it will be even worse.

select --+ no_query_transformation
       ttt1.id
  from ttt1
  left join ttt2
    on ttt1.id = ttt2.id
 where ttt2.id is null;
select * from table(dbms_xplan.display_cursor(format => 'BASIC'));

--------------------------------------------------
| Id  | Operation              | Name            |
--------------------------------------------------
|   0 | SELECT STATEMENT       |                 |
|   1 |  VIEW                  |                 |
|   2 |   FILTER               |                 |
|   3 |    MERGE JOIN OUTER    |                 |
|   4 |     TABLE ACCESS FULL  | TTT1            |
|   5 |     BUFFER SORT        |                 |
|   6 |      VIEW              | VW_LAT_2131DCCF |
|   7 |       TABLE ACCESS FULL| TTT2            |
--------------------------------------------------

So, in a nutshell, if Oracle can apply transformation to ANTI join then performance is exactly the same otherwise it can be worse. You can also use hint "--+ rule" to disable CBO transformations and see what happenes.

PS. On a separate note, SEMI join may be in some cases much better than inner join + distinct even with enabled CBO transformations.