Re: Implicit Access Path Sharing

Adding to what Larry said,

    If LF1 is created with keys A,B followed by LF2 which is created with key A.
LF2 would share the access path of LF1.  This has the effect of saving storage,
but more importantly improving performance.  Now the important item with the
above example of LF1 & LF2, if you create LF2 before you create LF1, then the
files would NOT be implicitly shared by OS/400.  You need to create the LF which
has the longer key first, and then create the subset key for OS/400 to 
implicitly
share an access path.  Using select/Omits as Larry said, would negate any
sharing.

    Pete Massiello

Larry Bolhuis wrote:

> Mark,
>
>    When LFs are created with CRTLF (as opposed to via SQL) access
> paths are shared when the key fields of the file being created match
> those in an existing access path. For example an LF being created for
> key fields 'A' and 'B' would share an existing access path for LF with
> keys 'A' 'B'(obvious case) but it would also share with LF that has
> keys 'A' 'B' 'C'.  It would NOT share an access path with LF keyed 'D'
> 'A' 'B'.  Additionally other attributes must match.  These include
> sequence (ascending/descending), Select/Omit and Dynamic Select.  In
> some cases certain attributes may be overriden.  I have seen Dynamic
> Select ignored when the existing access path does not use it.
>
>    I don't believe the format name has any effect. For example it you
> have a PF with fields 'A' through 'F'.  LF 1 has keys 'A' and 'B' and
> includes fields 'A' 'B' 'C' and 'D'.  LF 2 with keys 'A' and 'B' but
> selecting fields 'A' 'B' 'E' and 'F' would still share the access
> path.  Remember that the access paths select and sequence ROWS.
> Column selection is independant of the access path.  We RPG types tend
> to equate access paths with LF and this is not the case.
>
>  - Larry
>
> Mark Lazarus wrote:
> >
> >  Can anyone explain when Implicit Access Path Sharing is not used?  Would
> > it be used when creating the identical *LF, but the fields are specified in
> > the DDS?  How about if the format name is different?  According to my
> > observations the latter would not share the AP.  Why not?
> >
> --
> Larry Bolhuis         | What do you want to reload today?
> Arbor Solutions, Inc  |
> (616) 451-2500        | Two rules to success in life:
> lbolhui@ibm.net       | 1. Never tell people everything you know.
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--
Pete Massiello
OS Solutions International
Phone: (203)-744-7854  Ext 11.
http://www.os-solutions.com
mailto:pmassiello@os-solutions.com


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