Re: ALTER TABLE ADD COLUMN fast default

On Fri, Mar 30, 2018 at 10:08 AM, Andrew Dunstan
<andrew(dot)dunstan(at)2ndquadrant(dot)com> wrote:
> On Fri, Mar 30, 2018 at 5:00 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:

[ missing values are loaded in the TupleDesc by RelationBuildTupleDesc ]

>>> > I'm still strongly object to do doing this unconditionally for queries
>>> > that never need any of this. We're can end up with a significant number
>>> > of large tuples in memory here, and then copy that through dozens of
>>> > tupledesc copies in queries.
>>
>>> We're just doing the same thing we do for default values.
>>
>> That's really not a defense. It's hurting us there too.
>>
>
>
> I will take a look and see if it can be avoided easily.
>
>

For missing values there are three places where we would need to load
them on demand: getmissingattr(), slot_getmissingattrs() and
expand_tuple(). However, none of those functions have obvious access
to the information required to load the missing values. We could
probably do something very ugly like getting the relid from the
TupleDesc's first attribute. Maybe someone else has a better option.
But I can't see a simple and clean way to do this.

cheers

andrew

--
Andrew Dunstan https://www.2ndQuadrant.com
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