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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-1905) Optimizer cost estimates for subqueries are way (way) too high.
Date Fri, 29 Sep 2006 21:19:20 GMT
Optimizer cost estimates for subqueries are way (way) too high.

                 Key: DERBY-1905
                 URL: http://issues.apache.org/jira/browse/DERBY-1905
             Project: Derby
          Issue Type: Bug
          Components: Performance, SQL
    Affects Versions:,,
            Reporter: A B

If I run the attached repro (pulled from DERBY-1866) with derby.optimizer.noTimeout=true (meaning
the optimizer should take all the time it needs to try out all possible plans within the restrictions
of the optimizer overrides), the estimated cost shown in derby.log (if I log the query plan)
is over 600k and the estimated row count is over 520k.

If, as the code in OptimizerImpl seems to expect, the unit for a cost estimate is milliseconds,
then the optimize here is guessing that the query will take over 10 minutes to execute and
will return a half-million rows.  But in truth the *combined* time for compilation AND execution
is just a couple of seconds, and only 15 rows are returned.

That suggests to me a rather serious problem with the optimizer cost estimates for subqueries.

Among other things this can have a major impact on the optimizer's timeout mechanism for very
deeply-nested queries.  The optimizer will continue to try out different combinations of indexes/joinOrders/joinStrategies
until it either exhausts them all or until the number of milliseconds spent optimizing is
greater than the "best cost" estimate so far. In the case of the repro for this issue, the
optimizer quickly exhausts all of the options and so it finishes in a fair amount of time.

But in larger queries where there are far more combinations to try (see esp. queries attached
to DERBY-1205, DERBY-1777), these severly inflated cost estimates get very large very quickly
(sometimes even reaching infinity--see DERBY-1259, DERBY-1260) and so the optimizer just keeps
on optimizing and never times out.  The result is that for queries like those in DERBY-1777,
the optimizer can spend literally hours optimizing a query which then executes in a matter
of seconds.

I'm still investigating this, but preliminary examination suggests that part of the problem
is in some way related to the treatment of "outer costs" by the optimizer--and in particular,
it looks like the optimizer is perhaps too aggressive in multiplying cost estimates by row
counts pulled from "outer costs".  That's just my first guess at the problem, though; there
could of course be far more to it than that...

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