[Info-vax] Indexed file read question

[Hein RMS van den Heuvel]

On Thursday, November 19, 2020 at 10:47:07 AM UTC-5, Stephen Hoffman wrote:
> On 2020-11-19 04:50:35 +0000, Hein RMS van den Heuvel said: 
> 
> > ...with duplicates the first (by time) inserted with that first key... 
> 
> Which I've incorrectly referred to as "indeterminate" here, though I've 
> approximately never been seeking duplicate records returned in 
> insertion-time order. 

You have... Remember VMSmail? The FOLDER name? That's an alternate key with duplicates!
So a DIR/FOL=xxx returns messages in the order you'd expect.
Least surprise engineering.

Business applications might have a 'status' flag say 'A' for Action, 'P' for Print and ' ' <space> for 'done' with a NULL key value of space.
The application can subsequently ask for the first A row to be processed or P row to be printed.
It can be useful, but it is not  very robust.

To build on my earlier example, see what CONVERT does to the file:

$ conv/key=1 tmp.idx tt:/fdl=nl:
noot A
 aap A
vuur B
mies B
$ conv tmp.idx new.idx
$ conv/key=1 new.idx tt:/fdl=nl:
 aap A
noot A
mies B
vuur B

Mind you for VMSmail, this caused no issue because the PK was a timestamp and thus convert delivered messages in folders by time order. 
Many, if not most, business application essentially hand out PK's in always increasing order again making the right thing happen in general, but not guaranteed. I'm sure this has caused head-scratching or perhaps support calls over time.

Fun?
Hein