Re: In what circumstances might optimizer not choose the lowest cost?

From: Lothar Flatz <l.flatz_at_bluewin.ch>
Date: Tue, 20 Jan 2015 14:49:32 +0100
Message-ID: <54BE5CEC.5090207_at_bluewin.ch>



Hi Kim,

first of all to finally answer your question I think we would need a 10053 trace.
However, maybe we can get away with a bit simpler approach. I would try to add to your query "case dataset when 'DAT' then upperalphanum(eksterntvarenr) end" is not NULL. You can well expect the optimizer does not understand your function. Therefore it does not know that you deliberately are skipping nulls. It could be that the optimizer thinks the index access is not save (in terms of missing some rows) and you are helping him with your Index FBI hint.
Actually the optimizer should never do an unsafe transformation. Not even hinted. But maybe Jonathan Lewis would know about exceptions.

One more additional suggestion: If you can, avoid upperalphanum(:bind1) , but rather send a pre calculated bind variable. The estimate of the optimizer should be better.
Did you try cardinality feedback?

regards

Lothar

(BTW: a salted banana is a throw away of a table row. A banana is a throw away from the index entry) ;-)

Am 20.01.2015 um 14:20 schrieb Kim Berg Hansen:
> Hi, fellows.
>
> I had a weird little case today, which I'll try to describe simply:
>
>
> Two tables - Tab1 with 4 million rows, Tab2 with 3/4 million rows.
> Tab1 has a function based index on an expression: "case dataset when
> 'DAT' then upperalphanum(eksterntvarenr) end" - upperalphanum is a
> function returning uppercase of input stripped of any whitespace and
> non-alphanumeric characters. The FBI contains about two hundred
> thousand of the 4 million rows of Tab1, for the rest the expression is
> NULL.
>
> Query is a simple join between the two tables joining on a two-column key.
> There is a predicate on Tab1 on the FBI expression:
> "case dataset when 'DAT' then upperalphanum(eksterntvarenr) end
> BETWEEN upperalphanum(:bind1) and upperalphanum(:bind2)"
> And a filter predicate on two columns of Tab2.
> The access I want (and normally get) is index range scan of the FBI
> index on Tab1 and nested loop/index access of Tab2.
> (The whole purpose of the FBI is to have a small fast index for this
> and other similar queries.)
>
>
> I have three versions of the query for testing:
> Q1: Hinted to use FBI index access on Tab1.
> Q2: Hinted to use an alternative normal index on Tab1 containing the
> columns of the FBI expression, where the expression then will be
> evaluated for all rows.
> Q3: Unhinted (my normal query.)
> Apart from hints, the three queries are identical.
>
> Normally they get plans costed like this:
> Q1 hinted to FBI gets total Cost=26276.
> Q2 hinted to normal index gets total Cost=40473.
> So normal index has a higher cost than FBI.
> Q3 unhinted picks the lower cost access plan and uses FBI with total
> Cost=26676.
>
> Then I added a check constraint "check( dataset='DAT' )" on Tab2 on
> one of the two key columns used for the join.
> This changed the access plans for the queries - suddenly appeared
> (optimizer generated) a filter predicate dataset='DAT' on Tab1, as the
> optimizer know nows via the check constraint on Tab2 and the join
> between Tab1 and Tab2, that accessing any Tab1 rows with dataset NOT
> equal to 'DAT' would be folly, because they would be certain to be
> "thrown away" when we join to Tab1 on dataset column. ("Salted
> banana", as NoCoug Journal recently called it ;-)
>
> When that filter predicate was added, my three test queries got new
> costs, of course:
> Q1 hinted to FBI gets total Cost=24374.
> Q2 hinted to normal index gets total Cost=35493.
> So even with the new filter predicate reducing estimated cardinality
> (and cost) slightly, normal index is still higher cost than FBI.
>
> BUT... Q3 unhinted picks the HIGHER cost access plan and uses normal
> index with total Cost=35493 ??
>
>
> I can understand that my check constraint has a sideeffect of adding a
> filter predicate.
> I have also tested dropping the constraint again and instead added the
> same filter predicate manually to the queries - it gives the same
> result (so it is not specifically because there's a check constraint.)
>
> What I canNOT understand is, that with the extra filter predicate in
> place, the optimizer picks the HIGHER costed of the two access plans?
>
>
> So my question really is:
>
> Are there known circumstances where the optimizer does NOT choose the
> lowest cost, even though same query with a hint CAN produce a plan
> with a lower cost?
>
> Or is this "buggy" behaviour? (My version is 11.2.0.3.0 EE.)
>
>
> Thanks in advance for any hints I can research ;-)
>
>
> Regards
>
>
> Kim Berg Hansen
>
> http://dspsd.blogspot.com
> kibeha_at_gmail.com <mailto:kibeha_at_gmail.com>
> _at_kibeha
>

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Received on Tue Jan 20 2015 - 14:49:32 CET

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