Left to right parsing

Question

hi all,
I wnat to join two tables t1,t2. t1 contains 100000 records and t2
contains 100 records.
which query performs better?
1. select t1.col1,t1.col2,t2.col1 from t1 inner join t2 on t1.col1=t2.col1
2.1. select t1.col1,t1.col2,t2.col1 from t2 inner join t1 on t2.col1=t1.col1
Pls show me a link to the details
thanks

Answer 1

In the simplest situation Oracle knows about extent number in tables and about
row structure. Using these information it can choose the leading table which is
t2 in your case.
SQL> create table t1 (col1 int, col2 int);
Table created.
SQL> create table t2 (col1 int, col2 int);
Table created.
SQL> insert into t1 select rownum, rownum from dual connect by level <= 100000;
100000 rows created.
SQL> insert into t2 select rownum, rownum from dual connect by level <= 100;
100 rows created.
SQL> commit;
Commit complete.
SQL> explain plan for select t1.col1,t1.col2,t2.col1 from t1 inner join t2 on t1.col1=t2.col1;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)|
|   0 | SELECT STATEMENT   |      |   100 |  3900 |    60   (4)|
|*  1 |  HASH JOIN         |      |   100 |  3900 |    60   (4)|
|   2 |   TABLE ACCESS FULL| T2   |   100 |  1300 |     3   (0)|
|   3 |   TABLE ACCESS FULL| T1   | 85524 |  2171K|    56   (2)|
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - access("T1"."COL1"="T2"."COL1")
18 rows selected.
SQL> explain plan for select t1.col1,t1.col2,t2.col1 from t2 inner join t1 on t2.col1=t1.col1;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)|
|   0 | SELECT STATEMENT   |      |   100 |  3900 |    60   (4)|
|*  1 |  HASH JOIN         |      |   100 |  3900 |    60   (4)|
|   2 |   TABLE ACCESS FULL| T2   |   100 |  1300 |     3   (0)|
|   3 |   TABLE ACCESS FULL| T1   | 85524 |  2171K|    56   (2)|
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
   1 - access("T2"."COL1"="T1"."COL1")
18 rows selected.In the real world this depends on the statistic, indexes, etc..
See Performance Tuning Guide
Rgds.