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  • Subject: RE: Query Optimizer
  • From: pytel@xxxxxxxxxx
  • Date: Thu, 4 Nov 1999 18:29:01 -0600

Another possible difference between LF and temporary access path:
LF (without omits or includes) will have an entry for every record in PF,
while temporary access path will have entries only for records, selected by
WHERE predicate.
Thus optimizer creates an index and selects records in one pass.

This combined process may indeed be faster than using access path from LF -
it's a matter of arithmetic.

This process can be influenced by ALWCPYDTA parameter during SQL
preprocessor.
Another factor is OPTIMIZE FOR <n> ROWS clause.
These "tuning knobs" can bias optimizer decision this way or another.

Best regards
    Alexei Pytel


Scott Mildenberger <Smildenber@Washcorp.com> on 11/04/99 01:36:07 PM

Please respond to MIDRANGE-L@midrange.com

To:   "'MIDRANGE-L@midrange.com'" <MIDRANGE-L@midrange.com>
cc:
Subject:  RE: Query Optimizer




I'm not sure why the optimizer chose what it did, one guess might be that
maybe the access path maintenance wasn't set to *immed so it wasn't
current.
But I do know one thing, if you create a logical file that can reuse an
existing access path it is built almost immediately.  If the temporary
access path that the optimizer built exactly matched the existing LF then
it
would have been built in a very short time.  I'm assuming since you asked
the question that this temporary index took some time to build.  If that is
the case then there must have been something different between the
temporary
index and the existing LF.  Not sure this helps but you may want to look at
all the attributes of the existing LF.

Scott Mildenberger

> -----Original Message-----
> From:   Pete Hall [SMTP:pbhall@execpc.com]
> Sent:   Wednesday, November 03, 1999 6:12 PM
> To:     MIDRANGE-L@midrange.com
> Subject:     Query Optimizer
>
> Does anyone have a clue as to why the query optimizer would chose to
build
>
> a temporary access path with a key that exactly matches the second of two
> logical files (no includes or omits)? The explanation says that "The cost
> to use this access path, as determined by the optimizer, was higher than
> the cost associated with the chosen access method" (which was to build a
> temporary index). Does the fact that this is a logical file (DDS) as
> opposed to and SQL index have any bearing on the matter? This is at V4R3,
> current on database PTFs.
> Pete Hall
> pbhall@execpc.com
> http://www.execpc.com/~pbhall
>
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