Optimizer mode in oracle 10g

Question

what is the difference between setting optimizer mode in oracle 10g
optimizer_mode=choose
optimizer_mode=all_rows

Answer 1

user446367 wrote:
what i have to set for the below parameters . any idea ?
optimizer_index_caching
optimizer_index_cost_adjIn general you would leave them set at the default value in 10g (and probably, in most earlier versions, for most cases as well). Even if you were to change them, asking for specific values on an internet forum is rather asking for trouble, it's not dissimilar to asking "what should I set for the parameter processes?" A reasonable value will be application dependent.
The first parameter reduces the cost of some types of indexed access by assuming that only the specified percentage of index i/o actually results in a physical I/O and therefore only that percentage of the io cost is taken into account. By contrast all tablescan access and the other types of indexed access are assumed to be uncached and therefore need costing.
The second parameter just arbitrarily scales indexed access path costings.
You could argue, and some have, that you could calculate a cache hit ratio for index blocks in much the same way as you can calculate a cache hit ratio generally and then set the first parameter to this value. The standout problems with this approach are
1) It applies to all matching statements and objects not just your problem ones.
2) It doesn't necessarily even apply to your problem statements.
3) It doesn't have anything to do with query duration per se, just efficiency of one particular access path.
4) People tend to choose to round costs down and make the likelihood of two plans getting the same cost and being chosen on a tiebreaker basis higher.
The second parameter just asks you how much you like the idea of indexes generally. If you are going to change it pick a family members age or something - it'll make you smile every time you see it.
Niall Litchfield
http://www.orawin.info/