Re: Query with concatenation in explain plan

From: Shakespeare <>
Date: Sat, 29 Nov 2008 13:57:08 +0100
Message-ID: <49313c25$0$189$>

Jonathan Lewis wrote:

> "Shakespeare" <> wrote in message 
> news:49312a68$0$186$

>> I have a customer who has a bad-performing query, and the explain plan
>> shows a split in the query with a concatenation at the end. Because the
>> query is fairly complex, the two parts of the query take a lot of time,
>> where (to my opinion) the whole part could be done in one run.
>> I stripped the query to the point where the concatenation comes in. In
>> the real query, it is joined with a lot of extra tables, of which some
>> are full table scans (still have to tune that part).
>> This is what it boils down to this (the query is generated by a software
>> package, so I can't change them without changing the code of the
>> package):
>> SELECT mtb.*
>> WHERE mtb.dt_begin between nvl(:b14,mtb.dt_begin) and
>> nvl(:b13,mtb.dt_begin)
>> Plan
>> Cost: 748 Bytes: 275,550 Cardinality: 2,505 6 CONCATENATION 2 FILTER
>> Cost: 727 Bytes: 250,470 Cardinality: 2,277 5 FILTER 4 TABLE
>> Cardinality: 228 3 INDEX RANGE SCAN INDEX APP.mtb_IX_DT_BEGIN
>> Cost: 3 Cardinality: 41
>> Oracle Database 10g Enterprise Edition Release - 64bit
>> PL/SQL Release - Production
>> CORE Production
>> TNS for IBM/AIX RISC System/6000: Version - Production
>> NLSRTL Version - Production
>> Looks like a QBE where a user can put in date limits. If nothing is
>> entered, all rows are returned.
>> I noted that the problem is in the NVL's. A normal 'between' would use
>> the index in one run. But still: a concatenation suggests that rows that
>> are returned in the first clause and rows that are returned in the second
>> are put together in the result set, where I would think only rows that
>> are returned by BOTH clauses should be i the result set.
>> Fron the docs: Concatenation = An operation that accepts multiple sets of
>> rows and returns the union-all of the sets.
>> Am I missing something here?
>> Shakespeare
> Your stripped example shows the optimizer producing a
> run-time option on execution plans.  It's an example of the
> type of thing I've described in the following note:
> At run time, only one of the two parts of the plan will operate
> because the two FILTERs are filtering constants, and are mutually
> exclusive.
> In your case, the "good" part of the plan may be bad because you
> have multiple conditions of the same type (for different columns in
> different tables) and the optimizer can only play this concatenation
> trick once.  The link above has a follow-on link saying more about
> that issue.

thank you for your response. I just concluded I missed that filter part (see my next post...) before reading your response. I'll check out the link. It seems to be bad programming: replacing the column<=column part with column <= "a very large value" seems to perform better. This application has a lot of constructs like this in one query though, and unfortunately, each and everyone generates a concatenation in the plan, which, as you stated, leads to bad performance because of the concatenation trick being played once.

Shakespeare Received on Sat Nov 29 2008 - 06:57:08 CST

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