Re: Fuzzy cost comparison to eliminate redundant planning

Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Do we know in the optimizer whether we will be needing cheapest startup
> > or not?
> 
> No.  Higher levels might want either.
> 
> > Is the middle one kept because the optimizer has to mix the startup plus
> > some percentage of the total cost for queries using LIMIT?
> 
> Right.  There are potentially some ranges of LIMIT for which it could
> win, I believe.  Maybe with some math you could prove there is no range
> in which the other two don't dominate it, but I suspect the extra logic
> would slow down add_path more than it's worth.

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.

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