Operations Manual
Set Semantics
One of the most common sources of non-determinism is the set semantics used by SQL.
E.g., if you run the following query repeatedly, you may get two different results:  
```sql
SELECT *
FROM (
SELECT 'A' AS x
UNION
SELECT 'B' AS x
);
```  
Both results `A`, `B` and `B`, `A` are correct.