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From "A B (JIRA)" <derby-...@db.apache.org>
Subject [jira] Updated: (DERBY-1073) Reset optimizer timeout for subqueries on a per-round basis to allow consideration of plans that use pushed predicates.
Date Thu, 16 Mar 2006 00:31:58 GMT
     [ http://issues.apache.org/jira/browse/DERBY-1073?page=all ]

A B updated DERBY-1073:
-----------------------

    Attachment: d1073_timeoutFix_v1.patch

As described in the derby-dev thread here:

http://article.gmane.org/gmane.comp.apache.db.derby.devel/16723

the original patch for DERBY-1073 was incomplete.  For details see the above thread.  In short,
the 'default' behavior for DERBY-1073 (i.e. the case where there are no pushed predicates)
combined with the changes for DERBY-1007 can result in significantly greater compilation time
for some queries--as evidenced by the 4 to 7 time slow-down in the regression tests for nist/dml132.sql.

The resolution to this problem is to take the changes for DERBY-1073 one step further by recognizing
when a subquery's optimizer has timed out and, for each round thereafter, allowing the optimizer
to continue until it finds it's first complete (and valid) plan for that round.  Before d1073_timeoutFix_v1.patch
the optimizer for a subquery would return immediately (i.e. without doing any optimization)
for every round after the initial timeout, which led to incorrect (and exceedingly high) cost
estimates.

More simply put, the changes in d1073_timeoutFix_v1.patch delay subquery timeout until the
optimizer for the subquery has found a legitimate cost to return.

In order to have a legitimate cost, the optimizer must find at least one complete join order.
 Instead of just using the first possible join order, the changes in d1073_timeoutFix_v1.patch
try to guess at what the best join order will be by "jumping" to the join order that was most
recently deemed "best" for the subquery prior to timeout.  It then stops optimizing (times
out) and returns the cost of that join order.

Note that with d1073_timeoutFix_v1.patch, the nist/dml132.sql test will still run more slowly
than it did prior to DERBY-1007 and DERBY-1073.  But a) the slow-down will be far less than
the 4 to 7-time slow-down seen in the current trunk, and b) the slow-down is actually the
result of more correct behavior.  The reason this slow-down is more correct is because it
comes from the fact that optimizer timeout value is now correct (roughly 35ish seconds) whereas
prior to DERBY-1007 and DERBY-1073 the timeout value, which was only 14-ish seconds, was incorrect
because the subquery that timed out was returning an illegitimate cost estimate that in turn
led to an incorrect timeout value.

I ran derbyall with these changes against sane jars on Red Hat with IBM 1.4.2 and saw no new
failures.

I would appreciate any review comments, if anyone has the time...Thanks.

> Reset optimizer timeout for subqueries on a per-round basis to allow consideration of
plans that use pushed predicates.
> -----------------------------------------------------------------------------------------------------------------------
>
>          Key: DERBY-1073
>          URL: http://issues.apache.org/jira/browse/DERBY-1073
>      Project: Derby
>         Type: Sub-task
>   Components: Performance
>     Versions: 10.2.0.0
>     Reporter: A B
>     Assignee: A B
>     Priority: Minor
>  Attachments: d1073_timeoutFix_v1.patch, d1073_v1.patch, d1073_v2.patch
>
> I wanted to file this as subtask to DERBY-805, but since DERBY-805 is itself a subtask
to DERBY-649, I was not able to do so.  So I'm creating this issue as a(nother) subtaks for
DERBY-649.
> [ Based on derby-dev thread found here: http://article.gmane.org/gmane.comp.apache.db.derby.devel/16007
]
> If we have a query such as:
> select <...> from
>    (select t1.i, t2.j from t1, t2 where <...>) X1,
>     T3
> where <...>
> then we would have one "outer" query and one "subquery".  The outer query would be "select
<...> from X1, T3", the subquery would be "select t1.i, t2.j from t1, t2".
> In this case the Derby optimizer will create two instances of OptimizerImpl: one for
the outer query (call it OI_OQ) and one for the subquery (call it OI_SQ).  Each OptimizerImpl
has its own timeout "clock" that it initializes at creation time--but never resets.  If timeout
occurs, the OptimizerImpl will stop searching for "the" best plan and will just take the best
plan found so far.
> That said, for every permutation of the outer query a call will be made to optimize the
subquery.  To simplify things, let's assume there are only two permutations of the outer query:
one with join order {X1, T3} and another with join order {T3, X1}.
> Now let's say we're looking at the first permutation {X1, T3}.  OI_OQ will make a call
to optimize the subquery represented by OI_SQ.  Let's further say that the subquery tries
some permutation {T1, T2} and then times out.  It then returns the plan information for {T1,
T2} to the outer query.  The outer query, which has *not* yet timed out, then decides to try
its second permutation {T3, X1}.  So it again makes a call to optimize the subquery.  In this
case, the subquery--which has already timed out--will *immediately* return without trying
to optimize anything.  The outer query will then make a decision about its second permutation
based on the un-optimized subquery's plan results.
> This hasn't really been an issue to date because the "best plan" chosen by the subquery
is typically independent of the outer query's current permutation--with the exception of "outerCost",
which is passed in from the outer query and is factored into the subquery's cost estimates.
 Because of this relative independence, the plan chosen by the subquery would rarely (if ever?)
change with different permutations of the outer query, so if the subquery timed out once there
was no point in trying to re-optimize it again later.
> With DERBY-805, though, Derby has acquired the ability to push predicates from outer
queries down into subqueries--which means that the outer join order can have a very significant
impact on the plan chosen by the subquery.  But because the timeout mechanism is never reset,
we could end up skipping the second optimization phase of the subquery, which means we never
get a chance to see how much the outer predicates can help, and thus we could end up skipping
over some plans that have the potential to give us significant performance improvement.
> So resolution of this issue would involve resetting the timeout state for subqueries
to allow the Derby optimizer to consider plans that rely on pushed predicates.

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