HOw does the optimizer evaluate db_file_multiblock_read_count ?

Does anybody know if the cost based optimizer evaluates db_file_multiblock_read_count's face value or what it can actually do ?

The background of my question is, we all know CBO's behaviour may change depending on your multiblock_read_count value. If it is high, it "thinks" full table scan is cheap, and therefore may favor using more.

But we also know that what you set on multiblock_read_count is not necessarily what you get, because there are lots of other bottlenecks, such as maxphys, your file system cache.. etc. For 8K block, you can set it to 128, but may only get actual I/O size of 32 blocks, or in some cases, only 1, depending on the file system you use.

So the question is, when CBO evaluates this, does it use the face value (which is whatever you set it to), or does it use what you can actually get to determine the cost of a full table scan ?

Thanks for sharing your insight in advance. Received on Thu Oct 31 2002 - 18:15:49 CST