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

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

Hi Ann,

While I understand and mostly agree with what you say I argue that not 
performing a full table update can be as good as it is bad. Implementation 
can make it good if done realy well.

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.

Please correct my logic if wrong.

Cheers,

Calin.

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

> Calin Pirtea(RDS) wrote:
>>
>> Sounds to me like what you suggest is to create the not null field and on
>> next read make it look like it has a value. That sounds sooo wrong to me.
>> Currently, every time I create a new not null field I follow it with a 
>> full
>> table update.
>>
>
> It may sound wrong to you, but it is the general style that Firebird
> and InterBase have used for years.  When you add a column to a table,
> Firebird generates a new record in RDB$FORMATS, describing the new
> physical structure.  Existing records are unchanged.  When Firebird
> reads a record, it compares the format version in the record header
> with the format requested - which may not be the most recent, depending
> on what's happening and how old the request is - and, if necessary,
> moves the record forward or back to the requested format.  So if you
> add a field FOO to a table and read that table, you'll see a bunch
> of null FOO's.  They're not stored, but are created on reference.
> If you update those records, they'll be stored in the new format
> with the FOO field.
>
> What Adriano is proposing is a mechanism to add new fields with a
> NOT NULL constraint without doing a full table update.  That would
> be a good thing.
>
>
> Cheers,
>
> Ann
>
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