Re: [Firebird-devel] "cannot update erased record" - handling

[Dmitry Y. <fir...@ya...>]

Ivan Prenosil wrote:
> 
> I think that it can occur in the following situation:
> 
>   Updating (or deleting) a record consists of two stages
>   - old record version is read into buffer
>   - new record version is created/stored
> 
>   If it is done in READ COMMITED transaction,
>   and some other transaction deletes that record and
>   manages to commit it between those two stages,
>   then "cannot update erased record" is raised.

Correct.

> * Somehow, it seems to me that this error is more artifact of the way
>   the command is internally processed than real error.

I'd say it's an artifact of the read-committed implementation.

> * It can be raised in the situation one would not expect it - normally
>   the DELETE/UPDATE command either succeeds because
>   it deletes/updates required record(s), or it succeeds because there is nothing
>   to delete/update (e.g. delete from tab where 1=2;)
>   But in described situation the DELETE can complain that record
>   was already deleted - looks strange.

I kinda agree. But I think that all related issues should be solved as a 
whole. For example, if the concurrent transaction has modified the 
record, we need to re-fetch it and re-evaluate booleans. In other words, 
read-committed transactions should (almost?) never experience an update 
conflict. But it's not very easy to do.

> * Could this error by avoided at all ? If the record was deleted,
>   it means there is nothing to delete/update, so from users point of view
>   the error has no sense.
>   The only problem I see is with Before triggers that were already fired,
>   can't they be just silently undone ?

I fail to see how the trigger actions could be undone for a single 
record, sorry.


Dmitry