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Mike Cunningham wrote:
This may be a left over from the old days but we never put a key on any table (Physical file) directly
IIRC (and _only_ IIRC), way back when, there were a couple elements
that combined to bring a small amount of risk to having keys on PFs.
1. Non-keyed LFs were not allowed. (Possibly irrelevant.)
2. Damage to a PF index could make it impossible to read the PF even
in arrival sequence.
Back on the S/38 and in early OS/400, damage to files was far more
common than it is today. Power requirements were higher, UPSs were
less common, stability could be affected more easily than in later
releases, etc.
Recovery of data in a PF could often be done by reading the PF in
pure arrival sequence as long as the PF itself had no index/key. If
a LF key suffered damage, it wasn't much of a problem -- delete it
and recreate it; the damage to the LF index was physically separate
from the PF object that held the data.
But there were circumstances where damage to a PF /index/ would
interfere with reading the physical records even when trying to read
without reference to the index.
Because you couldn't create a LF that presented records in a
reasonably pure arrival-sequence fashion, the PF became the _only_
possible way to access record by record. It's _possible_ that a
non-keyed LF today might allow useful arrival-sequence access if
damage occurs to a PF index. (I don't have a clue whether it
actually would; I'm just dredging up old memories, probably jumbled.)
Regardless, the rarity of damage to [indexes plus data spaces] today
seems to have made the early issue(s) with PF keys almost entirely moot.
Anybody know more details from way back then? Do reasonable reasons
exist today? (Disregarding tables that don't need keys in the first
place, etc.)
Tom Liotta
midrange.com
