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

[Ann W. H. <aha...@ib...>]

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