I’d vote for showing both (with RETURNING and without), since without it the second argument to SPI_exec has no effect in this example, which may not be obvious. That seems to be one of the subtle points illustrated by this example.
I concur:
=> SELECT execq('CREATE TABLE a (x integer)', 0); // start at 0 => INSERT INTO a VALUES (execq('INSERT INTO a VALUES (0)', 0)); // insert 2 => SELECT execq('SELECT * FROM a', 0);
+ => SELECT execq('INSERT INTO a SELECT x + 2 FROM a returning x', 1); -- one more
+ -- (IIUC non-deterministically regardless of observed behavior even if there were a limit in the SELECT)
=> SELECT execq('INSERT INTO a SELECT x + 2 FROM a', 1); // three more => SELECT execq('SELECT * FROM a', 10); // 6 as 10 is a limit
