convert unique index to primary key

less_downtime_unique_to_primary_key.sql

--https://dba.stackexchange.com/questions/324803/add-composite-primary-key-on-existing-postgresql-table/324815#324815

CREATE TABLE demo (
    a int,
    b int,
    CONSTRAINT a_b UNIQUE (a, b)
);

INSERT INTO demo VALUES (1, 2),(2, 3);

------------------------------------------------------
ALTER TABLE public.demo
    ALTER COLUMN a SET NOT NULL,
    ALTER COLUMN b SET NOT NULL;

-- must run on its own:
CREATE UNIQUE INDEX CONCURRENTLY a_b_temp ON demo (a, b);

ALTER TABLE demo
    DROP CONSTRAINT a_b,
    ADD CONSTRAINT demo_pkey PRIMARY KEY USING INDEX a_b_temp;