Demand Indexes and deleting records.

[Brian K. White]

On 12/14/20 1:26 PM, Scott Walker via Filepro-list wrote,
> In my mind that would make Demand indexes totally unusable on any system with more than one user, and maybe not even then.

Agreed. I never used them. Even a single-user system is not a 
single-process system.

I think the feature is only good for setting up non-trivial searches 
based on things you don't have automatic indexes for already, and where 
it's not important if it's 100% correct. Actually, because of what I'll 
point out later, it needs to be ok if it's wildly not even remotely 
close, let alone 100%.

No definitive reports for accounting, no actionable business logic like 
what you described or like automatic inventory replenishment or really 
anything important, but maybe useful to get a rough answer about 
something. I never found any such situation that wasn't solved by just 
using the most appropriate auto index and filtering the rest of the way 
in processing, but maybe they exist.

Main reason I never used them is actually even simpler than this, which 
was just that it's effectively a temp file in a shared, non-temp, 
non-unique path and filename. If the generated index file was 
/tmp/foo_${REALLY_UNIQUE}, then ok, but it's the same 
/path/to/foo/index.4 for all other processes on the system. (Well, all 
other processes using the same PFDATA+PFDIR) Utterly un-usable outside 
of ms-dos or earlier.

By unusable, I don't mean it's impossible to try to rig up some sort of 
locking scheme, I mean it's wrong to even try. You can't ever actually 
count on something like that. "all my programs know to check for X 
before touching, and I'm the only developer..." Right. Ok. Is the rule 
enforced by the kernel or by the fp binaries? I didn't think so, and so, no.

All that said, maybe it could be hacked to work. The unique temp file 
aspect I mean.  Maybe you could generate a random unique temp qualifier 
and use that just for the demand index? I never tried and I don't know 
off the top of my head if it's possible or maybe possible but just 
impractical.

-- 
bkw