About the shift / doctors examples, the right design should have two tables:

```

CREATE TABLE shifts (id int PRIMARY KEY);

CREATE TABLE shift_doctors (id int REFERENCES shifts, name text, on_call boolean);

```

and counting should lock the shift:

```

SELECT COUNT(*) FROM (

SELECT *

FROM shifts NATURAL JOIN shift_doctors

WHERE id = 1 AND on_call

FOR UPDATE OF shifts

) shift_doctors;

```

This is bug-free and has the advantage to wait on conflict rather than failing