Here are 2 options equivalent to your original query which will only perform a single scan of table `foo`:
Using a temp table
```sql
CREATE TEMPORARY TABLE x AS
SELECT x, min(y) min_y, max(y) max_y FROM foo GROUP BY (x/10);
SELECT x, min_y
FROM x
UNION
SELECT x, max_y
FROM x;
```
In a single query
```sql
SELECT DISTINCT x,
CASE a
WHEN 1 THEN min_y
WHEN 2 THEN max_y
END y
FROM (SELECT x, min(y) min_y, max(y) max_y FROM foo GROUP BY (x/10))
JOIN (SELECT 1 a UNION ALL SELECT 2);
```