AW: Original design approach to Oracle REDO Logs

> Would that break some existing applications?

I think so.

This prospective behavioural change, or breaking change if you prefer, could be circumvented by a developer, just as it was possible for the developer to change the codebase to swallow the (silly) NO_DATA_NEEDED error and work around the disappearance of (OK I know it was unsupported but I still used it) WM_CONCAT function.

Mike
http://www.strychnine.co.uk

> Am 18/06/2021 um 18:25 schrieb Frank Gordon <frankagordon_at_gmail.com>:
>
> In this new scheme, where if the value is already the value that exists don't do the update, how should AFTER UPDATE FOR EACH ROW TRIGGERS work?
> Ignore the update if the value is being set to "itself"? Would that break some existing applications?
>
> Frank
>
>
>
> On Fri, Jun 18, 2021 at 4:02 PM Jonathan Lewis <jlewisoracle_at_gmail.com <mailto:jlewisoracle_at_gmail.com>> wrote:
>
> Mark,
>
> Don't forget the transformation would also have to cater for "column1 is null", because that should be updated too (unless the update were to set it to null).
>
> But the problem is a "notable change in behaviour" - the no-change rows would not be locked.
> I can't think of an example why this might be bad news in a well-designed and coded sysatem, but imagine:
>
> user A: update rows set ownership = 'USERA' where ...';
> user B: update rows set ownership = 'USERB' where ...;
> user A: update where ownership='USERA';
> user B: update where ownership='USERB'
>
> Assuming the ownership updates have some rows in common, and some of those rows are already set to ownership = 'USERA'.
>
> Current implementation:
> ===================
> All userA rows are locked at step 1
> User B waits at step 2 for User A to commit, then probably does a "write consistent restart"
>
>
> Transformed/Block image/Non-locking implementation
> =========================================
> User B at step 2 overwrites some rows which should be owned by userA
> User A at step 3 waits for user B to commit, then probably does a "write consistent restart".
>
> It shouldn't be too hard to produce refine this framework to produce an example where the final values in the rows that both users were interested in change because of the absence of locking.
>
> Regards
> Jonathan Lewis
>
>
>
>
> On Fri, 18 Jun 2021 at 15:15, Clay Jackson <dmarc-noreply_at_freelists.org <mailto:dmarc-noreply_at_freelists.org>> wrote:
> MWF wrote:
>
> “At the query level I wonder if permuting the query when it can be certainly iso-functional as
>
> update someTable
>
> set someColumn1 = 1.234
>
> becomes
>
> update someTable
>
> set someColumn1 = 1.234
>
> where someColumn1 != 1.234”
>
>
>
> I almost can’t believe I’m suggesting this; but is there a nugget of an “enhancement” request in there? I could see cases where if the optimizer was going to do an index lookup (i.e. there was a unique index on that column), OR, a full table scan (i.e. NO other path) a query rewrite might give some dramatic results (I think any paths other than “direct index” or full table scan would probably not be deterministic enough to “take a chance”).
>
>
>
> Clay Jackson
>
>
>
>
>
> --
> +353-86-0695383

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