Neither optimizer will hit the database until it has exhausted its in-memory group values. The above example hits the database every 5 identifier values.
直到内存组中的值耗尽后，两个优化器才会去访问数据库，上面的例子每5个标识值符访问一次数据库。
2
If the optimizer joins two tables first, the join results could be huge and may use a large amount of system resources such as CPU and memory.
如果优化器首先连接两个表，那么连接结果可能十分庞大，并可能使用大量系统资源，如CPU和内存。
3
The optimizer avoids generating access plans using the hash distinct algorithm if it detects that a low memory situation may occur during query execution.