Make a user owner of the whole database.

ownership.sql

DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT * FROM pg_tables 
    WHERE NOT schemaname = ANY(ARRAY['pg_catalog', 'information_schema'])
    AND tableowner <> 'db_user'
    LOOP
        EXECUTE 'ALTER TABLE '|| this.schemaname || '.' || this.tablename ||' OWNER TO db_user;';
    END LOOP;
END
$$
LANGUAGE plpgsql;

DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT oid::regclass::text as mat_view
    FROM   pg_class
    WHERE  relkind = 'm'
    LOOP
        EXECUTE 'ALTER TABLE '|| this.mat_view ||' OWNER TO db_user;';
    END LOOP;
END
$$
LANGUAGE plpgsql;

DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT 'ALTER '
        || CASE WHEN p.proisagg THEN 'AGGREGATE ' ELSE 'FUNCTION ' END
        || quote_ident(n.nspname) || '.' || quote_ident(p.proname) || '(' 
        || pg_catalog.pg_get_function_identity_arguments(p.oid) || ') OWNER TO db_user;' AS sql
    FROM   pg_catalog.pg_proc p
    JOIN   pg_catalog.pg_namespace n ON n.oid = p.pronamespace
    WHERE  NOT n.nspname = ANY(ARRAY['pg_catalog', 'information_schema'])
    LOOP        
        EXECUTE this.sql;
    END LOOP;
END
$$
LANGUAGE plpgsql;


DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT * FROM pg_views
    WHERE NOT schemaname = ANY(ARRAY['pg_catalog', 'information_schema'])
    AND viewowner <> 'db_user'
    LOOP
        EXECUTE 'ALTER VIEW '|| this.schemaname || '.' || this.viewname ||' OWNER TO db_user;';
    END LOOP;
END
$$
LANGUAGE plpgsql;


DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT 'ALTER SCHEMA ' || nspname || ' OWNER TO db_user;' AS sql FROM pg_namespace
    WHERE nspname NOT LIKE 'pg_%'
    AND nspname <> 'information_schema'
    LOOP
        EXECUTE this.sql;
    END LOOP;
END
$$
LANGUAGE plpgsql;



DO
$$
    DECLARE this record;
BEGIN
    IF(CURRENT_USER = 'db_user') THEN
        RETURN;
    END IF;

    FOR this IN 
    SELECT      'ALTER TYPE ' || n.nspname || '.' || t.typname || ' OWNER TO db_user;' AS sql
    FROM        pg_type t 
    LEFT JOIN   pg_catalog.pg_namespace n ON n.oid = t.typnamespace 
    WHERE       (t.typrelid = 0 OR (SELECT c.relkind = 'c' FROM pg_catalog.pg_class c WHERE c.oid = t.typrelid)) 
    AND         NOT EXISTS(SELECT 1 FROM pg_catalog.pg_type el WHERE el.oid = t.typelem AND el.typarray = t.oid)
    AND         typtype NOT IN ('b')
    AND         n.nspname NOT IN ('pg_catalog', 'information_schema')
    LOOP
        EXECUTE this.sql;
    END LOOP;
END
$$
LANGUAGE plpgsql;