you tell the database WHAT you want and it is its job and prerogative to decide HOW
to do it. So the HOW may change at any time, but the results should not. If
they do then it is a bug.
We should also clarify what "explain is unreliable" means.
Through "explain plan for ..." the optimizer tells you "Given the current
circumstances and my current knowledge, this is how I plan to process your sql".
If the circumstances change - e.g. add or drop an index-, or the optimizer's
knowledge changes - analyzing tables/indexes changes statistics, the access
plan may turn out different. That is where the "unreliability" comes in. As the
CBO evolves, it is becoming increasingly difficult to ensure that the session
where and when you do the explain does have the exact same parameters as the
session where the sql was executed, or will be executed. If all conditions are
the same, the result of the parse will reliably be the same. It is just that
it becomes increasingly uncertain that you can (re)create the runtime conditions for the
explain. Especially once the optimizer takes past execution statistics into
account.
At 11:30 AM 3/5/2003 -0800, you wrote:
Wolfgang ...
Now that explain plan is unreliable and will be even more in 10i, I'll always keep a silver dollar in my pocket. The coin toss is still right (in almost) 50% of the time.
Jeez ... what's next ... in 11i, SELECT statement *may* return data and in some cases would actually perform inserts into some other tables, because that's what you *wanted to do* anyways ... right?
I want my dBase IV back ... Raj
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