RE: Costs in explain plan

From: Toepke, Kevin M <ktoepke_at_trilegiant.com>
Date: Wed, 12 Dec 2001 06:27:41 -0800
Message-ID: <F001.003DAAD4.20011212050027@fatcity.com>

Also, even if 2 queries have an identical cost _AND_ explain plan that does not mean they'll execute in the same amount of time! (At least with Oracle 8.1.6, 8.1.6 & 8.1.7 on Sun Solaris)

Yes, I have tested this and No I can't explain it.

The specific situation was somthing like this: Original:

        SELECT id
        FROM   credit_card
        WHERE  credit_card_number = :ccn;
Modified:
        SELECT /*+ FIRST_ROWS */
        FROM   credit_card
        WHERE  credit_card_number = :ccn;

The hinted query ran about 10-15% faster (over thousands of executions) than the unhinted version. The credit_card table was hash partitioned on credit_card_number.

Caver

-----Original Message-----
Sent: Wednesday, December 12, 2001 4:55 AM To: Multiple recipients of list ORACLE-L

For the SAME query, the costs can be compared - but remember that the cost is basically just a mathematical algorithm that pops out a number at the end. It is somewhat analagous to the number of IO's expected to satisfy the query, but of course

1. Oracle can get it wrong occasionally
2. its only as good as your optimizer stats
3. By default, Oracle tends to undercost full table scans (since it treats a single block IO the same as a multiblock IO)

hth
connor

• Arn Klammer <Arn.Klammer_at_austrac.gov.au> wrote: > Hi,
  >
  > Can you compare the costs that are displayed in
  > explain plan output? For example, I have a query
  > that contains a merge join. The cost reported in
  > the explain plan is 134. By including a hint to use
  > a nested loop instead, the cost becomes 1160. Does
  > this mean the merge join plan is more efficient,
  > despite generally being considered a less efficient
  > join method than a NL?
  >
  > Thanx
  >
  > -a
  >
  >
  >
  >
  

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Received on Wed Dec 12 2001 - 08:27:41 CST