Re: Adding a nullable DOMAIN column w/ CHECK

On Sat, Sep 06, 2014 at 02:01:32AM +0200, Marko Tiikkaja wrote:
> First of all, sorry about breaking the thread; I don't subscribe to
> -general so I can't copy the original email. This is in response to
> the problem here: http://www.postgresql.org/message-id/CACfv+p+8dToaR7h06_M_YMjpV5Na-CQq7kN=KgJq_k84H7UHWA@mail.gmail.com

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The old thread was fuzzy concerning how the system works today. Adding a
domain-type column rewrites the table unless the domain has no constraints.
Simultaneously adding an all-NULL column and a CHECK constraint to a table
scans the table, even if the CHECK constraint references only the new column.

> The patch is obviously a load of horse crap, but does anyone have
> any objections to the general approach of making this pattern
> faster?

+1 in general.

> To do this optimization we do have to assume that CHECKs in
> DOMAINs are at least STABLE, but I don't see that as a problem;
> those should be IMMUTABLE anyway, I think.

The system has such assumptions already.