Tag Archives: explain plan

high cost

What’s wrong with this query ?


select
(t6.column_value).getstringval() t6
from
table(xmlsequence(extract(xmltype(‘<x/>’),’/x’))) t1,
table(xmlsequence(t1.column_value))t2,
table(xmlsequence(t2.column_value))t3,
table(xmlsequence(t3.column_value))t4,
table(xmlsequence(t4.column_value))t5,
table(xmlsequence(t5.column_value))t6;
T6
————————
<x/>

Elapsed: 00:00:00.01

Well, let’s check the plan :


--------------------------------------------------------------------
| Id  | Operation          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT   |    18E|    15E|    18E  (0)|999:59:59 |
|   1 |  NESTED LOOPS      |    18E|    15E|    18E  (0)|999:59:59 |
|   2 |   NESTED LOOPS     |    18E|    15E|    99P  (3)|999:59:59 |
|   3 |    NESTED LOOPS    |  4451T|    31P|    12T  (3)|999:59:59 |
|   4 |     NESTED LOOPS   |   544G|  3045G|  1490M  (3)|999:59:59 |
|   5 |      NESTED LOOPS  |    66M|   254M|   182K  (3)| 00:36:31 |
|   6 |       COLLECTION I |       |       |            |          |
|   7 |       COLLECTION I |       |       |            |          |
|   8 |      COLLECTION IT |       |       |            |          |
|   9 |     COLLECTION ITE |       |       |            |          |
|  10 |    COLLECTION ITER |       |       |            |          |
|  11 |   COLLECTION ITERA |       |       |            |          |
--------------------------------------------------------------------

It is returning 18 quadrillions of rows, 15 exabytes, the cost is 1.8E19 and the time is about one month :mrgreen: