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On 13 Feb 2013 07:53, Stone, Joel wrote:
<<SNIP>> Iseries users normally do a SELECT and the rows appear in
arrival sequence.
There is an expression about what "assuming" does. Many of those who
had assumed that "arrival sequence" could be expected, were in for a
shock when they learned they should never have been dependent on that
false assumption. They had been FD&H for perhaps years while all
/seemed/ to work as expected, when suddenly, sometimes, their data was
no longer in arrival sequence. They are shocked, so they call IBM, I
had to tell them... Tough luck, the rules are clear, that without an
explicit order being requested, the database can return the data in *any
order* so there is no defect [as long as the _unordered set_ is
complete\accurate].
But for some reason when UNION is introduced, the arrival sequence
is lost.
Shocker! The arrival sequence was never there as a guarantee before
SQL UNION, nor after.
Using UNION ALL versus UNION DISTINCT to try to influence the effect,
then perhaps seeing an apparent arrival sequence, that would still be a
dependence on a false assumption. Again, the database is under
absolutely *no obligation to return data in arrival sequence* when no
ORDER BY is specified. The optimizer will choose what is felt to be the
fastest method of retrieval, and that may or may not reflect arrival
order. More often than not, arrival order is quickest for a read-only
of all data in a file.member. However if the database can perform
parallel retrieval or determines something about the file or environment
[amount of memory, CPU, etc.] for which an alternate access path is
desirable for quicker access, then the database will easily return the
data in an order that may not reflect arrival sequence.
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