Is adding a column to a typed table in PostgreSQL instant?

Today at the SOUG Day I did some little demos and one of them was about creating typed tables. In the demo the two tables did not contain any rows and one of the questions was: When these tables contain a million of rows would adding a column be instant as well? Lets do a quick test.

Same setup as in the post referenced above: Two schemas, one type, two tables based on the type:

postgres=# select version();
                                                          version                                                           
----------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 10.4 build on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-28), 64-bit
(1 row)
postgres=# create schema a;
CREATE SCHEMA
postgres=# create schema b;
CREATE SCHEMA
postgres=# create type typ1 as ( a int, b text );
CREATE TYPE
postgres=# create table a.t1 of typ1;
CREATE TABLE
postgres=# create table b.t1 of typ1;
CREATE TABLE
postgres=# insert into a.t1
postgres-# select a.*
postgres-#      , md5(a::text)
postgres-#   from generate_series(1,1000000) a;
INSERT 0 1000000
postgres=# insert into b.t1 select * from a.t1;
INSERT 0 1000000
postgres=# 

Both of the tables contain 1’000’000 rows so how much time would a modification of the type take?

postgres=# timing
Timing is on.
postgres=# alter type typ1 add attribute c timestamp cascade;;
ALTER TYPE
Time: 9.338 ms
Time: 0.867 ms
postgres=# d a.t1
                             Table "a.t1"
 Column |            Type             | Collation | Nullable | Default 
--------+-----------------------------+-----------+----------+---------
 a      | integer                     |           |          | 
 b      | text                        |           |          | 
 c      | timestamp without time zone |           |          | 
Typed table of type: typ1

postgres=# d b.t1
                             Table "b.t1"
 Column |            Type             | Collation | Nullable | Default 
--------+-----------------------------+-----------+----------+---------
 a      | integer                     |           |          | 
 b      | text                        |           |          | 
 c      | timestamp without time zone |           |          | 
Typed table of type: typ1

Almost instant. Btw: Of course you can also remove an attribute from the type:

postgres=# alter type typ1 drop attribute c cascade;
ALTER TYPE
Time: 14.417 ms
postgres=# d a.t1
                   Table "a.t1"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 a      | integer |           |          | 
 b      | text    |           |          | 
Typed table of type: typ1

postgres=# d b.t1
                   Table "b.t1"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 a      | integer |           |          | 
 b      | text    |           |          | 
Typed table of type: typ1