Re: [HACKERS] Optimizer speed and GEQO (was: nested loops in joins) - Mailing list pgsql-hackers

> It took a certain amount of patience to run these cases with profiling
> turned on :-(, but here are the results:

Interesting numbers.

> 
>             7T+12I        7T+7I        7T
> 
> Runtime, sec        1800        457        59
> equal() calls        585 mil        157 mil        21.7 mil

> A fairly decent approximation is that the runtime varies as the
> square of the number of create_nestloop_path calls.  That number
> in turn seems to vary as the factorial of the number of tables,
> with a weaker but still strong term involving the number of indexes.
> I understand the square and the factorial terms, but the form of
> the dependency on the index count isn't real clear.

This is exactly what I would expect.  See optimizer/README.  The
factorial makes a lot of sense.  It evaluates all joins join, removes
the cheapest from the list, and goes to find the next best one.
8 compares7 compares6 compares...

Here is a good illustration of what you are seeing:
> 6!        720> 8!        40320

That's why I reduced GEQO from 8 to 6.

> 
> It might be worthwhile to try a GEQO threshold based on the number of
> tables plus half the number of indexes on those tables.  I have no idea
> where to find the number of indexes, so I'll just offer the idea for
> someone else to try.

Sounds like a good idea.  I can easily get that information.  The
optimizer does that lower in the code.  Perhaps we can just move the
GEQO test to after the index stuff is created.  I will be able to look
at it after the TEMP stuff.

> 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.

I think I optimized prune.c white a bit around 6.1.  I broke it during
the optimization, and Vadim fixed it for me.

One of my TODO items has been to 100% understand the optimizer.  Haven't
had time to do that yet.  Been on my list for a while.


> 
> Anybody here know much about how the optimizer works?  I'm hesitant to
> hack on it myself.

Let me help you.

--  Bruce Momjian                        |  http://www.op.net/~candle maillist@candle.pha.pa.us            |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026