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

[Calin Pirtea\(RDS\) <pc...@rd...>]

Hi Ann,

Thanks for clarifying it for me. The way you described it, sounds awesome.

Cheers,
Calin.

----- Original Message ----- 
From: "Ann W. Harrison"

> Calin Pirtea(RDS) wrote:
>
>>
>> My argument is that metadata changes to a production database are meant 
>> to
>> be very rare hence a full table update is meant to be very rare for 
>> adding a
>> new field. Reading a table on production databases, on the other hand,
>> should be very often and if performance impact is 1% for these defaults,
>> then reading a record 100 times it's already worth the effort to update 
>> the
>> table to get that extra 1% performance back.
>
> The mechanism that Adriano has proposed will add almost no overhead.
> Reading a table and applying the default from the table definition
> may be cheaper than reading a record that's been modified to include the
> default value because the records are smaller and require fewer disk
> reads.  There's no computation involved in evaluating the default -
> it's part of the in-memory table definition.
>
> Maybe I'm wrong, but it seems to me that the only format that needs to
> store the default value is the format where the new not null field was
> added.  Only records of an older format could possibly have that field
> absent.
>
> Regards,
>
> Ann