If you want both the max and the min, then things are going to be a bit more work. You are either going to want to do two separate selects or join two selects or use subselects. If there aren't enough prices per stock, the sequential scan might be fastest since you only need to go through the table once and don't have to hit the index blocks.
It is still odd that you didn't get a big speed up for just the min though.
I found I'm suffering from an effect detailed in a previous thread titled
Does "correlation" mislead the optimizer on large tables?
Ken
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