March 2021
Intermediate to advanced
265 pages
6h 50m
Chinese
Content preview from Presto实战
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160
｜
第
9
章
9.14
解嵌套复杂数据类型
通过
UNNEST
操作，可以将
8.9.1
节中讨论的复杂集合数据类型展开成关系。对于大数据和
嵌套结构数据类型，这是一个非常强大的功能。通过将数据展开到关系中，你可以更容易
地在查询中访问嵌套在结构中的值。
例如，你存储了一些访问控制策略，并希望查询这些策略。首先，我们有一个用户，他可
以与一个或多个角色关联，而一个角色可以与一个或多个权限集关联。也许这些逻辑是由
schema
定义的，像下面这样：
SELECT * FROM permissions;
user | roles
--------+----------------------------------------------
matt | [[WebService_ReadWrite, Storage_ReadWrite],
[Billing_Read]]
martin | [[WebService_ReadWrite, Storage_ReadWrite],
[Billing_ReadWrite, Audit_Read]]
(2 rows)
我们可以通过使用
UNNEST
来展开每个角色，然后与用户关联：
SELECT user, t.roles
FROM permissions,
UNNEST(permissions.roles) AS t(roles);
user | roles
--------+------------------------------------------- ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.