On 11/10/2010 5:43 PM, Kevin Grittner wrote:
The only half-sane answer I've thought of is to apply a different
cost to full-table or full-index scans based on the ratio with
effective cache size.
The "effective_cache_size" is, in my humble opinion, a wrong method. It would be much easier to have a parameter, let's call it "optimizer_index_caching", which would give the assumption of the percentage of an index that is cached. In other words, if "optimizer_index_caching" was set to 80, the optimizer would assume that 80% of any index is cached and would apply different cost estimate. It's not exact but it's simple and modifiable. It would also be a great tool in the hands of the DBA which has to manage OLTP database or DW database and would be able to create a definitive bias toward one type of the execution plan.
I have to confess that the idea about such parameter is not entirely mine:
http://tinyurl.com/33gu4f6--
Mladen Gogala
Sr. Oracle DBA
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