rob@XE:11g2> set autotrace traceonly explain

rob@XE:11g2> SELECT COUNT(*) FROM

 2     (SELECT given_name FROM top_french_names

 3      UNION

 4      SELECT given_name FROM top_usa_names

 5     );

Elapsed: 00:00:00.08

Execution Plan

==========================================================

Plan hash value: 1581671293

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| Id  | Operation             | Name             | Rows  | Bytes | Cost (%CPU)|Time     |

------------------------------------------------------------------------------------------

|   0 | SELECT STATEMENT      |                  |     1 |       |     8  (25)|00:00:01 |

|   1 |  SORT AGGREGATE       |                  |     1 |       |            |         |

|   2 |   VIEW                |                  |   400 |       |     8  (25)|00:00:01 |

|   3 |    SORT UNIQUE        |                  |   400 |  2800 |     8  (63)|00:00:01 |

|   4 |     UNION-ALL         |                  |       |       |            |         |

|   5 |      TABLE ACCESS FULL| TOP_FRENCH_NAMES |   200 |  1400 |     3   (0)|00:00:01 |

|   6 |      TABLE ACCESS FULL| TOP_USA_NAMES    |   200 |  1400 |     3   (0)|00:00:01 |

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rob@XE:11g2> SELECT COUNT(*) FROM

 2     (SELECT given_name FROM top_french_names

 3      UNION ALL

 4      SELECT given_name FROM top_usa_names

 5     );

Elapsed: 00:00:00.04

Execution Plan

==========================================================

Plan hash value: 2045709238

---------------------------------------------------------------------------------

| Id  | Operation            | Name             | Rows  | Cost (%CPU)| Time|

---------------------------------------------------------------------------------

|   0 | SELECT STATEMENT     |                  |     1 |     6   (0)| 00:00:01|

|   1 |  SORT AGGREGATE      |                  |     1 |            |         |

|   2 |   VIEW               |                  |   400 |     6   (0)| 00:00:01|

|   3 |    UNION-ALL         |                  |       |            |         |

|   4 |     TABLE ACCESS FULL| TOP_FRENCH_NAMES |   200 |     3   (0)| 00:00:01|

|   5 |     TABLE ACCESS FULL| TOP_USA_NAMES    |   200 |     3   (0)| 00:00:01|

---------------------------------------------------------------------------------