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
a) Oracle can get it wrong occasionally
b) its only as good as your optimizer stats
c) 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 <[EMAIL PROTECTED]> 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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