create table t1(id) as
select 1e3 + rownum from dual connect by rownum <= 5;
create table t2(id, constraint pk_t2 primary key (id)) as
select rownum from
(select * from dual connect by rownum <= 1e3),
(select * from dual connect by rownum <= 1e4);

alter session set statistics_level = all;
select * from t1 join t2 on t1.id = t2.id;
select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
select /*+ use_hash(t1 t2) */ * from t1 join t2 on t1.id = t2.id;
select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

-----------------------------------------------------------------------------------------------
| Id  | Operation          | Name  | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |      1 |        |      5 |00:00:00.01 |      16 |      2 |
|   1 |  NESTED LOOPS      |       |      1 |      5 |      5 |00:00:00.01 |      16 |      2 |
|   2 |   TABLE ACCESS FULL| T1    |      1 |      5 |      5 |00:00:00.01 |       4 |      0 |
|*  3 |   INDEX UNIQUE SCAN| PK_T2 |      5 |      1 |      5 |00:00:00.01 |      12 |      2 |
-----------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      5 |00:00:04.32 |   15237 |       |       |          |
|*  1 |  HASH JOIN         |      |      1 |      5 |      5 |00:00:04.32 |   15237 |  2293K|  2293K|  970K (0)|
|   2 |   TABLE ACCESS FULL| T1   |      1 |      5 |      5 |00:00:00.01 |       3 |       |       |          |
|   3 |   TABLE ACCESS FULL| T2   |      1 |   8986K|     10M|00:00:01.17 |   15234 |       |       |          |
----------------------------------------------------------------------------------------------------------------