Re: [Firebird-devel] Adding NOT NULL fields with DEFAULT

[Adriano d. S. F. <adr...@gm...>]

Vlad Khorsun wrote:
>> I'm implementing what's described in subj. Now, user can already do this, but it's not good.
> 
>     Not sure i understand - what is not good ?
>  
Current behavior is not good. Imagine a default expression with a generator being incremented every time a record is read.

>> When engine reads a NOT NULL field with a DEFAULT, it executed the DEFAULT expression and returns a value.
>>
>> For CURRENT_DATE/TIME/TIMESTAMP expressions, it returns the "0 value", with for dates means 1858-11-17. 
> 
>     Hmm, do you know why it returns 0 ? According to the said above it should evaluate expression
> at time of reading record (also not perfect, but better than return zero). Looks like a bug.
> 
It explicitly returns "0", I don't know why. See it in evl. But returning different dates/times everytime is what I said to be not good.

>> Only simple expressions are allowed for DEFAULT now, with means more problems if complex expressions are allowed.
> 
>     Is standard allow to reference fileds from the same table in default expression ?
>  
I don't know, but apparent don't make sense. But I see no problem in a function call being used, for example.

>> Initial verification to implement the thing in a good way has to populate the fields in DFW, after the new format has 
>> been created. This does not solve the problem completely. There can be snapshot transactions that see the new 
>> field but can't see its default value.
> 
>     It is performance killer "solution" and i don't like it very much.
> 
>> Related to above, I tried to update records in system transaction. Initially works in some condition, but I don't 
>> think it's good way to go either.
> 
>     Don't do it. System transaction should be read-only.
> 
Formats are updated using it. ;-)


Adriano