On 04/10/2011 07:27 PM, Tomas Vondra wrote:
But if you need more details, then I quess the best approach to get it is to read the sources (search for the cost estimation etc.).
There's a small fully worked out example of this in my book too, where I duplicate the optimizer's EXPLAIN cost computations for a simple query. The main subtle thing most people don't appreciate fully is how much the optimizer takes into account two things: the selectivity of operators, and the expected size of the data being moved around. For example, a lot of the confusion around "why didn't it use my index?" comes from not noticing the size of the index involved, and therefore how expensive its page cost is considered to be.
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