A PostrgeSQL function to define a materialized view column as `not null`.

alter_materialized_view_alter_column_set_not_null.sql

create or replace function alter_materialized_view_alter_column_set_not_null(mv regclass, col_name text, not_null bool)
returns setof bool
returns null on null input
security definer
language sql
as $$
    with to_update as (
        select
            attrelid as attrelid_u, attnum as attnum_u
        from pg_catalog.pg_attribute as attr
            left outer join pg_catalog.pg_type as tp on tp.typelem = attr.atttypid
        where
            attr.attrelid = mv::oid and
            attr.attname = col_name and
            not attr.attisdropped and
            (tp.typanalyze is null or cast(tp.typanalyze as text) = 'array_typanalyze')
            and attr.attnum > 0
    )
    update pg_catalog.pg_attribute
    set attnotnull = not_null
    from to_update t
    where attrelid_u = attrelid and attnum_u = attnum
    returning attnotnull
$$
;

-- Aknowledgment : https://dba.stackexchange.com/a/255798

usage.md

Usage

-- Set a column as non-nullable
select alter_materialized_view_alter_column_set_not_null('my_materialized_view', 'my_column', true);

Note on permissions

Because modifying pg_catalog.pg_attribute requires superuser, but making a column's nullability is something a non-superuser may want to do, the function is defined with security definer and should be created as superuser. security definer works a bit like the setuid bit on Unix, it makes the function run with its definer's permissions.