APPROX_COUNT_DISTINCT test

CREATE TABLE tt AS SELECT rownum id, t.* FROM dba_objects t WHERE rownum < 100000;

INSERT INTO tt AS SELECT * FROM tt;

INSERT INTO tt AS SELECT * FROM tt;

commit;

SET autot ON EXP stat
SQL> SELECT COUNT(DISTINCT id) FROM tt;

COUNT(DISTINCTID)
-----------------
           999999

Elapsed: 00:00:02.15

Execution Plan
----------------------------------------------------------
Plan hash VALUE: 1056889062

-----------------------------------------------------------------------------------------
| Id  | Operation            | Name     | ROWS  | Bytes |TempSpc| Cost (%CPU)| TIME     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |          |     1 |    13 |       |  6834   (1)| 00:00:01 |
|   1 |  SORT AGGREGATE      |          |     1 |    13 |       |            |          |
|   2 |   VIEW               | VW_DAG_0 |   999K|    12M|       |  6834   (1)| 00:00:01 |
|   3 |    HASH GROUP BY     |          |   999K|  4882K|    11M|  6834   (1)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| TT       |   999K|  4882K|       |  3769   (1)| 00:00:01 |
-----------------------------------------------------------------------------------------


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      95708  consistent gets
      54725  physical reads
          0  redo SIZE
        553  bytes sent via SQL*Net TO client
        552  bytes received via SQL*Net FROM client
          2  SQL*Net roundtrips TO/FROM client
          0  sorts (memory)
          0  sorts (disk)
          1  ROWS processed


SQL> SELECT APPROX_COUNT_DISTINCT(id) FROM tt;

APPROX_COUNT_DISTINCT(ID)
-------------------------
                   971092

Elapsed: 00:00:00.71

Execution Plan
----------------------------------------------------------
Plan hash VALUE: 3133740314

------------------------------------------------------------------------------
| Id  | Operation             | Name | ROWS  | Bytes | Cost (%CPU)| TIME     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |     1 |     5 |  3769   (1)| 00:00:01 |
|   1 |  SORT AGGREGATE APPROX|      |     1 |     5 |            |          |
|   2 |   TABLE ACCESS FULL   | TT   |   999K|  4882K|  3769   (1)| 00:00:01 |
------------------------------------------------------------------------------


Statistics
----------------------------------------------------------
          5  recursive calls
          0  db block gets
      95710  consistent gets
      54725  physical reads
          0  redo SIZE
        561  bytes sent via SQL*Net TO client
        552  bytes received via SQL*Net FROM client
          2  SQL*Net roundtrips TO/FROM client
          0  sorts (memory)
          0  sorts (disk)
          1  ROWS processed