Postgres 12

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create table demo as with q as (select 42 x union all select 42) select 42 x from q a cross join q b cross join q c cross join q d cross join q e cross join q f cross join q g cross join q h cross join q i cross join q j cross join q k cross join q l cross join q m cross join q n cross join q o  cross join q p  cross join q r cross join q s cross join q t  cross join q u
1048576 rows affected

select pg_size_pretty( pg_total_relation_size('demo') );

pg_size_pretty

36 MB

-- adding a column does not add space :)
alter table demo add column b numeric ;
select pg_size_pretty( pg_total_relation_size('demo') );

pg_size_pretty

36 MB

-- even with a default value in it :))
alter table demo add c numeric default 42 not null;
select pg_size_pretty( pg_total_relation_size('demo') );

pg_size_pretty

36 MB

-- proof that table would grow if it updates the rows
update demo set b=42;
1048576 rows affected

select pg_size_pretty( pg_total_relation_size('demo') );

pg_size_pretty

81 MB