Re: ALTER TABLE ADD COLUMN fast default

On 03/29/2018 11:31 PM, Andres Freund wrote:
> Hi,
> 
> On 2018-03-29 17:27:47 -0400, Tom Lane wrote:
>> Andres Freund <andres@anarazel.de> writes:
>>> There's plenty databases with pg_attribute being many gigabytes large,
>>> and this is going to make that even worse.
>>
>> Only if you imagine that a sizable fraction of the columns have fast
>> default values, which seems somewhat unlikely.
> 
> 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.

cheers

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services