Hash Join

step 1 build hash table：
for row_1 in (select * from left_table)
loop
	hash_value = HASH(row_1)
	insert_hash_table(hash_value, row_1)
end loop
step 2 probe hash table:
for row_2 in (select * from right_table)
loop
	hash_value = HASH(row_2)
	row = lookup_hash_table(hash_value, row_2)
	if match join condition(row, row_2)
	then
  	output (row, row_2)
	end if
end loop

Query Plan:

|ID|OPERATOR   |NAME|EST. ROWS|COST  |
--------------------------------------
|0 |HASH JOIN  |    |99000    |194200|
|1 | TABLE SCAN|t1  |100000   |41911 |
|2 | TABLE SCAN|t2  |100000   |41911 |


Outputs & filters:
-------------------------------------
  0 - output([1]), filter(nil),
      equal_conds([t1.c1 = t2.c1]), other_conds(nil)
  1 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0)
  2 - output([t2.c1]), filter(nil),
      access([t2.c1]), partitions(p0)

explain select 1 from t1, t2 where t1.c1 = t2.c1;
Query Plan:

|ID|OPERATOR        |NAME|EST. ROWS|COST   |
--------------------------------------------
|0 |NESTED-LOOP JOIN|    |99000    |4120845|
|1 | TABLE SCAN     |t1  |100000   |41911  |
|2 | TABLE GET      |t2  |1        |40     |


Outputs & filters:
-------------------------------------
  0 - output([1]), filter(nil),
      conds(nil), nl_params_([t1.c1])
  1 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0)
  2 - output([1]), filter(nil),
      access([t2.c1]), partitions(p0)

explain select /*+leading(t1 t2) use_hash(t2)*/ 1 from t1, t2 where t1.c1 = t2.c1;
Query Plan:

|ID|OPERATOR   |NAME|EST. ROWS|COST  |
--------------------------------------
|0 |HASH JOIN  |    |99000    |194200|
|1 | TABLE SCAN|t1  |100000   |41911 |
|2 | TABLE SCAN|t2  |100000   |41911 |


Outputs & filters:
-------------------------------------
  0 - output([1]), filter(nil),
      equal_conds([t1.c1 = t2.c1]), other_conds(nil)
  1 - output([t1.c1]), filter(nil),
      access([t1.c1]), partitions(p0)
  2 - output([t2.c1]), filter(nil),
      access([t2.c1]), partitions(p0)