July 2023
Intermediate to advanced
278 pages
5h 40m
Chinese
Content preview from Trino 权威指南：原书第2版
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Trino
架构
|
63
示例 4-7：semi-join（IN）的查询案例
SELECT DISTINCT o.orderkey
FROM lineitem l
JOIN orders o ON o.orderkey = l.orderkey
JOIN customer c ON o.custkey = c.custkey
WHERE c.nationkey IN (
-- subquery invoked multiple times
SELECT s.nationkey
FROM part p
JOIN partsupp ps ON p.partkey = ps.partkey
JOIN supplier s ON ps.suppkey = s.suppkey
WHERE p.partkey = l.partkey
);
像
lateral join
一样，它也可以通过循环遍历外部查询的行，并多次调用子查询来实现。
这些子查询会查找一个物品所有的供应商，以及这些供应商所在的所有国家（或地区）。
Trino
并没有这样做，它用去关联化子查询来实现
—
子查询在去除关联条件的情况下
被求值一次，之后再用关联条件与外部查询连接在一起。这么做，最复杂的部分是确保
Join
不会产生重复结果行（因此使用了去重聚合），以及变换可以正确地保留
IN
的三值
逻辑（
true
、
false
或
null
其中之一）。
在这里，去重聚合使用了和
Join
相同的分区方式。因此，它无须网络数据交换并使内存
开销降到最低，就可以流式地执行。
4.8 ...
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