Re: Fuzzy cost comparison to eliminate redundant planning work

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.
        regards, tom lane