Bruce Momjian <[EMAIL PROTECTED]> writes: > Tom Lane wrote: >> Right. There are potentially some ranges of LIMIT for which it could >> win, I believe.
> What if we take the total cost and divide it by the number of rows returned --- > then we have a per-row cost for each plan. Then we subtract the two, and > that difference compared to the difference in startup costs tell us how > many rows could potentially use this plan. You're missing the point. We are comparing plans where one has a higher start cost and a lower total cost than the other. For example (pardon the crummy ASCII art): A A A A B A B A B A B AB BA B A B A B A A A A A where the X-axis is the percentage of tuples we expect to fetch, and the Y-axis is the estimated cost. We have to keep both plans since upper levels might want to fetch different percentages depending on what plan is being considered up there; so either A or B might be the thing to use. Now if we consider *three* plans: A A A A B A B A B A B AB C BA C B CA C B A B A A A A A Here, plan B loses everywhere: to A at lower percentages and to C at higher ones. But I don't see how to eliminate B on the basis of one-at-a-time comparisons. It seems like getting rid of B would require turning add_path into an O(N^3) instead of O(N^2) algorithm ... which would almost certainly eat more cycles than it'd save. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match