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
Priority: Minor
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.
--
This message is automatically generated by JIRA.
-
If you think it was sent incorrectly contact one of the administrators:
http://issues.apache.org/jira/secure/Administrators.jspa
-
For more information on JIRA, see:
http://www.atlassian.com/software/jira
