CREATE INDEX ON test (date_trunc('day', updated_at));

/*If updated_at is a timestamp without time zone, that will work fine. 
For a timestamp with time zone, you'll have to specify a time zone, 
because otherwise the result will depend on the session time zone, 
which makes it unusable for an index:*/

CREATE INDEX ON test (date_trunc('day', updated_at AT TIME ZONE 'UTC'));