Re: Changing isolation level?

"Howard J. Rogers" <hjr_at_dizwell.com> wrote: >
> Time to don the Devil's Advocate costume, I think.

I'll be the Devil's Other Advocate. (God doesn't have any lawyers).

> I can think of lots of cases where it would be rather unfortuate to see
> an old piece of data just prior to it being changed. Rather than see the
> old data, I could well imagine that it would be more helpful to see
> nothing until the situation stabilises.

I thought I could think of lots of them too, but when I looked into them more they didn't hold. Of course, maybe that just means I'm better at shooting things down than dreaming them up.

> If I have a customer demanding to know whether I have a turbo-widget in
> stock, I can select from my products table to check. It tells me I have
> one such item left. Mrs. Miggens therefore, on the strength of that,
> undertakes the arduous two-hour journey from remote rural New South Wales
> into the City Centre to pick up said turbo-widget. Unbeknownst to her,
> however, the information I gave her was wrong, because at the time I did
> my query, Fred in the next cubicle was selling the same product to Miss
> Marple. He hadn't committed the transaction, which is why I saw a stock
> level of 1. But it would have been nice not to give the wrong info out to
> Mrs. Miggens.

Given the two hour drive in which someone could have purchased that widget out from under her, that 3 minute open transaction in which you had (soon to be) stale data is insignificant.

> I suppose I could have badgered her into placing an order,
> because then my attempt to update the stock level myself would have
> failed. But Mrs. Miggens ran into a bit of credit rating trouble a while
> back, and doesn't have a credit card. The most she can do on the phone is
> to enquire.

If you could mark the item as "held for intended buyer" it would solve both the 3 minute window problem, and the 2 hour window problem. Given that, it seems pointless to try to come up with solution that only solves the minor problem and not the major one.

> There are two choices there, I suppose: see nothing at all (the blocking
> idea). Or see a stock level of 0 (the dirty reads issue beloved of, er,
> certain other products). Oracle's long-time answer has been: you'll see
> 1, and only 1, and nothing else, ever... which some might consider to be
> a lack of choice in answering technology.

I agree there. There are lots of times I would like to be able to do a dirty read (mostly in debugging situations, not production). It's unfortunate that Oracle "protects" me from making that choice.

Cheers,

Xho

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