why oracle optimizer not eliminate this case? - sql

Question

i am doubting about this case, but not clear why.
consider the following sql :
create table t1(tid int not null, t1 int not null);
create table t2(t2 int not null, tname varchar(30) null);
create unique index i_t2 on t2(t2);
create or replace view v_1 as
select t1.tid,t1.t1,max(t2.tname) as tname
from t1 left join t2
on t1.t1 = t2.t2
group by t1.tid,t1.t1;
then check the execution plan for select count(1) from v_1, the t2 is eliminated by the optimizer:
SQL> select count(1) from v_1;
Execution Plan
----------------------------------------------------------
Plan hash value: 3243658773
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 3 (34)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | | |
| 2 | VIEW | VM_NWVW_0 | 1 | | 3 (34)| 00:00:01 |
| 3 | HASH GROUP BY | | 1 | 26 | 3 (34)| 00:00:01 |
| 4 | TABLE ACCESS FULL| T1 | 1 | 26 | 2 (0)| 00:00:01 |
----------------------------------------------------------------------------------
but if the index i_t2 is dropped or recreated without unique attribute,
the table t2 is not eliminated in execution plan:
SQL> drop index i_t2;
Index dropped.
SQL> select count(1) from v_1;
Execution Plan
----------------------------------------------------------
Plan hash value: 2710188186
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 5 (20)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | | | |
| 2 | VIEW | VM_NWVW_0 | 1 | | 5 (20)| 00:00:01 |
| 3 | HASH GROUP BY | | 1 | 39 | 5 (20)| 00:00:01 |
|* 4 | HASH JOIN OUTER | | 1 | 39 | 4 (0)| 00:00:01 |
| 5 | TABLE ACCESS FULL| T1 | 1 | 26 | 2 (0)| 00:00:01 |
| 6 | TABLE ACCESS FULL| T2 | 1 | 13 | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
it seems even if the index is removed,
the result of select count(1) from v_1 also equal to
select count(1) from (select tid,t1 from t1 group by tid,t1)
why the optimizer does not eliminate t2 in the second case?
is there any principle or actual data example discribing this?
thanks :)

Answer 1

This is an optimization called join elimination. Because t2.t2 us unique, the optimizer knows that every row retrieved from t1 can only ever retrieve one row from t2. Since there is nothing projected from t2, there is no need to perform the join.
If you do
select tid, t1 from v_1;
you will see that we do not perform the join. However, if we project from t2, then the join is needed.