Re: strange cost in explain plan

The statement: "the cost of the parent should be equal or larger than the child" is not strictly true.

Your basic plan is for

    select {} from PATH_RELATIONSHIP_TEMP x;

You then have a scalar subquery in the select list. Oracle therefore shows you two independent plans in the plan table. One for your driving query - which as a cost of two, and one for (each) scalar subquery, which has a cost of 2083 EACH TIME IT EXECUTES.

But Oracle does not know how many times the scalar subquery will run, and does not make any allowance for multiple executions in the total cost of the query.

Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

• Original Message -----
  > Date: Fri, 12 Oct 2007 10:10:51 +0800
  > From: "qihua wu" <staywithpin_at_gmail.com>
  > Subject: strange cost in explain plan
  >
  > Hi, everyone,
  > For the plan, the cost of the parent should be equal or larger than the
  > child. But in the following plan, the total cost (Id=0) is 2, but it's child
  > "HASH UNIQUE "(ID=1) has a cost of 2083 which is much larger than 2, how
  > could it be like that?
  >
  > Plan hash value: 3028553145
  >
  > ----------------------------------------------------------------------------------------------------------
  >
  > | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
  > ----------------------------------------------------------------------------------------------------------
  >
  > | 0 | SELECT STATEMENT | | 1 | 482 | 2 (0)| 00:00:01 |
  > | 1 | HASH UNIQUE | | 1 | 48 | 2083 (1)| 00:00:25 |
  > |* 2 | COUNT STOPKEY | | | | | |
  > | 3 | TABLE ACCESS BY INDEX ROWID | ATTRIBUTE_VALUE | 1 | 24 | 3 (0)|
  > 00:00:01 |
  > | 4 | NESTED LOOPS | | 11 | 528 | 2082 (1)| 00:00:25 |
  > |* 5 | TABLE ACCESS BY INDEX ROWID| ATTRIBUTE_VALUE | 11 | 264 | 2049 (1)|
  > 00:00:25 |
  > |* 6 | INDEX RANGE SCAN | IDX_ATTRIBUTE_VALUE_2 | 19155 | | 59 (0)| 00:00:01
  > |
  > |* 7 | INDEX RANGE SCAN | IDX_ATTRIBUTE_VALUE_1 | 1 | | 2 (0)| 00:00:01 |
  > | 8 | TABLE ACCESS FULL | PATH_RELATIONSHIP_TEMP | 1 | 482 | 2 (0)| 00:00:01
  > |
  > ----------------------------------------------------------------------------------------------------------
  >
  >
  > Predicate Information (identified by operation id):
  > ---------------------------------------------------
  >
  > 2 - filter(ROWNUM=1)
  > 5 - filter("A"."ATTR_STR"=:B1)
  >
  >
  > Select (select distinct b.attr_str
  > from ATTRIBUTE_VALUE a,
  > ATTRIBUTE_VALUE b
  > where a.locale_lang_id = :a
  > and a.attr_id = :b
  > and a.attr_str = x.product_line_name
  > and a.node_id = b.node_id
  > and b.locale_lang_id = :c
  > and b.attr_id = :d
  > and a.change_lifecycle_id = 0
  > and b.change_lifecycle_id = 0
  > and rownum = 1)
  > from PATH_RELATIONSHIP_TEMP x;
  >
  >
  

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