postgres=# SELECT atan2(1, 0) AS positive_x,
postgres-# atan2(-1, 0) AS negative_x,
postgres-# atan2(0, 0) AS zero_x,
postgres-# pi() / 2 AS pi_over_two;
positive_x | negative_x | zero_x | pi_over_two
-----------------+------------------+--------+-----------------
1.5707963267949 | -1.5707963267949 | 0 | 1.5707963267949
(1 row)
postgres=#