Re: ALTER TABLE ADD COLUMN fast default

Hi,

On 2018-03-30 00:28:43 +0200, Tomas Vondra wrote:
> > Why is that unlikely?  In the field it's definitely not uncommon to
> > define default values for just about every column. And in a lot of cases
> > that'll mean we'll end up with pg_attribute containing default values
> > for most columns but the ones defined at table creation. A lot of
> > frameworks make it a habit to add columns near exclusively in
> > incremental steps.  You'd only get rid of them if you force an operation
> > that does a full table rewrite, which often enough is impractical.
> > 
> 
> I don't quite see how moving that gets solved by moving the info into a
> different catalog? We would need to fetch it whenever attribute
> meta-data from pg_attribute are loaded.

I'm also complaining nearby that the "stored default" values should only
be loaded on demand. We very regularly build relcache entries that'll
never have their default values queried. Even moreso with partitioned
tables.

Greetings,

Andres Freund