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From "A B (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2130) Optimizer performance slowdown from 10.1 to 10.2
Date Sun, 03 Dec 2006 00:18:21 GMT
    [ http://issues.apache.org/jira/browse/DERBY-2130?page=comments#action_12455141 ] 
            
A B commented on DERBY-2130:
----------------------------

Everything written in Bryan's preceding comment sounds correct to me, so I won't dwell.  As
for the specific questions:

> I've seen almost no discussion of the first point (that the cheapest actual plan
> should have the cheapest estimated cost); does this mean that we are pretty
> confident about this aspect of cost estimation at this point? That is, the cost
> estimating may be off, but it seems to be off for all queries equally?

This has been my general assumption about the code, yes--at least, after DERBY-1007 was resolved.
 It seems to me that at some point in the last year I found a scenario where the optimizer's
"best" cost estimate did not (appear to) correspond to the best query plan, but I don't remember
the details and it may have ended up being correct after all.  In any event, in all of the
discussion that I've had/written, my general assumption has been that "Yes", the *relative*
accuracy of the cost estimates is correct--i.e. that better plans have lower cost estimates.
 Note, though, that this is just an assumption of mine which I have not bothered trying to
debunk; if you find info to the contrary, please say so!

> DERBY-1907 is certainly relevant here; are there other issues logged like this?

None come to mind, no.  But I admit that's an answer based strictly on memory; I didn't actually
do any searching...

> I guess I'm wondering (out loud) whether it is worth investigating a simple tuning 
> of the cost estimation algorithm. If the optimizer was *much* faster at generating
> and estimating possible plans, wouldn't that be a big benefit? 

Yes, definitely!

> Also, how confident are we that permutation jumping (as described in
> http://wiki.apache.org/db-derby/JoinOrderPermutations) is working properly? 

I had to laugh out loud when I read this question.  It sounds to me like the kind of question
someone asks when they've found a somewhat serious bug but don't want to rock the boat ;)

So far as I know, the jumping code is working properly.  But if you told me there was a problem
with the code, I think I'd assume you were right.  Is that the case?

> Optimizer performance slowdown from 10.1 to 10.2
> ------------------------------------------------
>
>                 Key: DERBY-2130
>                 URL: http://issues.apache.org/jira/browse/DERBY-2130
>             Project: Derby
>          Issue Type: Bug
>          Components: Performance, SQL
>    Affects Versions: 10.2.1.6, 10.3.0.0, 10.1.3.1
>            Reporter: Bryan Pendleton
>         Attachments: repro.sql
>
>
> Attached is 'repro.sql', an IJ script which demonstrates what I
> believe to be a serious performance issue in the Optimizer.
> I have run this script in a number of configurations:
>  - 10.1.2.1: the script runs successfully. The 'prepare' statement
>    takes about 90 seconds, on a fairly powerful Windows machine
>  - 10.1.3.1: the script produces a NPE. I believe this is DERBY-1777
>  - 10.2.1.8/trunk: the script runs successfully. The 'prepare' statement
>    often takes about 220 seconds, on the same Windows machine
>    Intermittently, on 10.2 and on the trunk, the prepare statement takes
>    15+ minutes. I cannot reliably reproduce this; I run the same script
>    several times in a row and I cannot predict whether it will take 220
>    seconds or whether it will take 15+ minutes.
> I am quite motivated to work on this problem, as this is blocking me from
> using Derby for a project that I'm quite keen on, but I need some
> suggestions and ideas about how to attack it. From my perspective
> there are 3 primary topics:
> 1) Why did optimizer performance for this query degrade so significantly
> from 10.1.2.1 to 10.2? The optimizer seems to be at least 2.5 times slower,
> for this particular query at least, in 10.2. Sometimes it is 10x slower.
> 2) What is the source of the non-determinism? Why does the optimizer
> often take 4 minutes to optimize this query on the trunk, but sometimes
> take 15+ minutes? I don't believe that I'm changing anything from
> run to run.
> 3) Can we improve the optimizer performance even beyond what it was
> for 10.1.2? I realize that this is an ugly query, but I was hoping to
> see an optimization time of 5-10 seconds, not 90 seconds (and certainly
> not 220 seconds).
> I have attempted to start answering some of these questions, with
> limited success. Here is some of what I think I've discovered so far:
>  - the optimizer changes in 10.2 seem to have given the optimizer many
>    more choices of possible query plans to consider. I think this means
>    that, if the optimizer does not time out, it will spend substantially
>    more time optimizing because there are more choices to evaluate. Does
>    this by itself mean that the optimizer will take 2.5 times longer in
>    10.2 than it did in 10.1?
>  - something about this query seems to make the costing mechanism go
>    haywire, and produce extreme costs. While stepping through the
>    optimization of this query in the debugger I have seen it compute
>    costs like 1e63 and 1e200. This might be very closely related to
>    DERBY-1905, although I don't think I'm doing any subqueries here.
>    But maybe I'm misunderstanding the term "subquery" in DERBY-1905.
>    At any rate, due to the enormous estimated costs, timeout does not
>    occur.
>  - the WHERE clause in this query is converted during compilation to 
>    an equivalent IN clause, I believe, which then causes me to run into
>    a number of the problems described in DERBY-47 and DERBY-713.
>    Specifically, rather than constructing a plan which involves 4
>    index probes for the 4 WHERE clause values, the optimizer decides
>    that an index scan must be performed and that it will have to process
>    the entire index (because the query uses parameter markers, not
>    literal values). So perhaps solving DERBY-47 would help me
>  - the optimizer in fact comes up with a "decent" query plan quite quickly.
>    I have experimented with placing a hard limit into the optimizer
>    timeout code, so that I can force optimization to stop after an
>    arbitrary fixed period of time. Then I have been able to set that
>    value to as low as 1 second, and the optimizer has produced plans
>    that then execute in a few milliseconds. Of course, I have only tried
>    this with a trivial amount of data in my database, so it's possible
>    that the plan produced by the optimizer after just a second of
>    optimizing is in fact poor, and I'm just not noticing it because my
>    data sizes are so small.
> At this point, what would be really helpful to me would be some suggestions
> about some general approaches or techniques to try to start breaking down
> and analyzing this problem.

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