SQL> show table t1;
X                               INTEGER Nullable
SQL> show table t2;
X                               INTEGER Nullable

SQL> delete from t1; insert into t1 select row_number()over()*3 from rdb$types,rdb$types rows 1000; commit;
SQL> delete from t2; insert into t2 select row_number()over()*4 from rdb$types,rdb$types rows 1000; commit;
SQL> select count(*) from t1 full join t2 on t1.x=t2.x where t1.x is null or t2.x is null;               
                COUNT
=====================
                 1500

SQL> set stat on; set explain on;
SQL> select count(*) from t1 full join t2 on t1.x=t2.x where t1.x is null or t2.x is null;

Select Expression
    -> Aggregate
        -> Filter
            -> Full Outer Join
                ->  Nested Loop Join (outer)
                    -> Table "T2" Full Scan
                    -> Filter
                        -> Table "T1" Full Scan
                ->  Nested Loop Join (anti)
                    -> Table "T1" Full Scan
                    -> Filter
                        -> Table "T2" Full Scan

                COUNT
=====================
                 1500

Current memory = 2453051016
Delta memory = 0
Max memory = 2468754464
 Elapsed time= 4.40 sec 
Cpu = 0.00 sec
Buffers = 524288
Reads = 0
Writes = 0
 Fetches = 3753454 


SQL> select count(*) from (select x from (select x from t1 union all select x from t2) group by x having count(*)=1);

Select Expression
    -> Aggregate
        -> Filter
            -> Aggregate
                -> Sort
                    -> Union
                        -> Table "T1" Full Scan
                        -> Table "T2" Full Scan

                COUNT
=====================
                 1500

Current memory = 2453072064
Delta memory = 21048
Max memory = 2468754464
 Elapsed time= 0.01 sec 
Cpu = 0.00 sec
Buffers = 524288
Reads = 0
Writes = 0
 Fetches = 4062 

set feed off
create table t1 as select level*3 x from dual connect by level<=1000;
create table t2 as select level*4 x from dual connect by level<=1000;
set autot on
select count(*) from t1 full join t2 on t1.x=t2.x where t1.x is null or t2.x is null;

  COUNT(*)
----------
      1500

План выполнения
----------------------------------------------------------
Plan hash value: 612863728

-----------------------------------------------------------------------------------
| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |          |     1 |    26 |     5  (20)| 00:00:01 |
|   1 |  SORT AGGREGATE        |          |     1 |    26 |            |          |
|*  2 |   VIEW                 | VW_FOJ_0 |  1000 | 26000 |     5  (20)| 00:00:01 |
|*  3 |     HASH JOIN FULL OUTER |          |  1000 | 26000 |     5  (20)| 00:00:01 |
|   4 |     TABLE ACCESS FULL  | T1       |  1000 | 13000 |     2   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL  | T2       |  1000 | 13000 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("T1"."X" IS NULL OR "T2"."X" IS NULL)
   3 - access("T1"."X"="T2"."X")

Note
-----
   - dynamic sampling used for this statement (level=2)


Статистика
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
        339  bytes sent via SQL*Net to client
        333  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

"Aggregate"
"  CTE r"
"    ->  Result"
"          ->   Append "
<...>