RE: 54 is less than 4 for CBO ?

Milen

>But how is is possible to ignore the fact that cost = 54 (index acces) is
>less than cost = 4 ?
>Or there are some peculiarities hard-coded in the logic of CBO that are
>roughly saying : "If you have FIRST_ROWS as hint or optimizer_mode then use
>the index at (almost) any cost".

The old first row optimizer (that you activated with the hint FIRST_ROWS) works in this way. Therefore I usually say it isn't a true cost-based optimizer. Is more or less a hybrid between rule-based and cost-based.

If you want a better first row optimizer you should use the new one (which is available as of 9.0). It can be activated with FIRST_ROWS_n, where n is the number of rows for which the query optimizer will compute the costs. With it you will get full table scans as well if they make sense from a costing point of view...

HTH
Chris

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