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Re: how to retrieve records and make sure they arrive in RRN sequence



fixed

Are records ever deleted? If records are deleted and REUSEDLT(*YES)
is applied, rrn may not reflect arrival sequence.

John McKee

On Wed, Feb 13, 2013 at 1:48 PM, Stone, Joel <Joel.Stone@xxxxxxxxxx> wrote:
OK I understand that you are fore-warning me.

And I understand that there should be a seq# or date/time stamp on each record.

But, given that this file has been around for 30 years, and the only key is the MEMO#, and the file hasn't been reorg'd;

Is UNION ALL any different than READE on the MEMO # as the only key?

I mean would SQL know to do anything differently than read by arrival seq?

I don't know much about the inner workings of SQL on DB2/400, but it seems unlikely that the SQL engine would start in the middle of a file and read backwards & forwards. Also unlikely that it would scramble the sequence from arrival.

Yes this could occur in RPG READE also (without a second key to guarantee proper ordering of the records). But it doesn't fail - rather it works well.

Would SQL retrieval be any more likely to scramble the sequence than RPG READE or Cobol or any other access method?

And I hear loud and clear that it is not a good idea. But will it really fail given the circumstances?



-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of CRPence
Sent: Wednesday, February 13, 2013 1:26 PM
To: midrange-l@xxxxxxxxxxxx
Subject: Re: how to retrieve records and make sure they arrive in RRN sequence

On 13 Feb 2013 09:56, Stone, Joel wrote:
Wow - that worked!! Union ALL is the ticket. So simple and elegant too.

There is a huge difference between "that worked" and "that specific
request to perform that SQL without an ORDER BY produced the desired
effect that time, but at the whim of the optimizer for that invocation,
so that same collation can not be expected to occur on every invocation
of that same query." Dependence on a false assumption is not a good
proposition for software programming.

<<SNIP>>

UNION has an added feature of sorting the data - I didn't know that.

The UNION is a synonym for UNION DISTINCT. UNION DISTINCT does not
"sort" the data, even though its DISTINCT processing may produce a set
that might appear to have some apparent order. Appearances can be
deceiving. There is no obligation of the database to return DISTINCT
results in any particular order, when no ORDER BY is specified.

UNION ALL leaves the data alone - like a CPYF. It simply concatenates
a file onto the end of another, which is what I am looking for.

The result of a UNION ALL is not defined as a concatenation of one
file onto the end of another. The UNION ALL returns an unordered set of
rows, as a union of data from the tables named as a party to the union.
The presumption of an arrival sequence concatenation is a fallacy, bound
to be learned later, to have been based on a false assumption; though
having been warned, that mistake can be avoided.

Also FWiW the CPYF provides arrival sequence for copying all data
from a file.mbr only when using FROMRCD(1) versus the default of
FROMRCD(*START) which uses a keyed access path if one exists.

--
Regards, Chuck
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