Re: [HACKERS] Optimizer speed and GEQO (was: nested loops in joins)

> The main thing that jumps out from the profiles is that on these complex
> searches, the optimizer spends practically *all* of its time down inside
> better_path, scanning the list of already known access paths to see if a
> proposed new path is a duplicate of one already known.  (Most of the time
> it is not, as shown by the small number of times path_is_cheaper gets
> called from better_path.)  This scanning is O(N^2) in the number of paths
> considered, whereas it seems that all the other work is no worse than O(N)
> in the number of paths.  The bottom of the duplicate check is equal(),
> which as you can see is getting called a horrendous number of times.
> 
> It would be worth our while to try to eliminate this mostly-unsuccessful
> comparison operation.
> 
> I wonder whether it would work to simply always add the new path to the
> list, and rely on the later "prune" step to get rid of duplicates?
> The prune code is evidently far more efficient at getting rid of useless
> entries than better_path is, because it's nowhere near the top of the
> profile even though it's processing nearly as many paths.


You are definately on to something here.  I have added comments and more
to the optimizer README as I continue exploring options.

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