Re: adding a nullable column of type domain w/ check constraint runs checks?

[Joe Van Dyk <joe@xxxxxxxxx>]

On Tue, Aug 19, 2014 at 3:16 PM, Joe Van Dyk <joe@xxxxxxxxx> wrote:

On Tue, Aug 19, 2014 at 3:10 PM, Joe Van Dyk <joe@xxxxxxxxx> wrote:

I have a large table that I don't want to lock for more than couple seconds. I want to add a nullable column to the table, the type of the column is a domain with a check constraint.

It appears that the check constraint is being checked for each row, even though the column can be nullable? Is there a way around this?

BEGIN
  Timing is on.

create domain test_enum numeric check (value > 0);
  CREATE DOMAIN
  Time: 1.817 ms

create table test_enum_table (id serial primary key);
  CREATE TABLE
  Time: 2.213 ms

insert into test_enum_table select * from generate_series(1, 2000000);
  INSERT 0 2000000
  Time: 4299.000 ms

alter table test_enum_table add column t test_enum;
  ALTER TABLE
  Time: 3165.869 ms -- Takes 3 seconds in this test example

Also:

alter table test_enum_table add column t1 numeric check (t1 > 0);
  ALTER TABLE
  Time: 140.185 ms

which is much more reasonable.

johnto on irc says:

"I'm not sure why it's done this way. it seems like it could test the domain once against NULL and see whether that's rejected or not.  instead it just forces a rewrite :-("