如何使用通配符投影别名? [英] How to project an alias using a wildcard?

问题描述

一旦我做了一个join A by id, B by id，我就会得到一个带有字段的别名A::f..., B::f...有没有办法只将它投射到 A 字段上?

Once I do a join A by id, B by id, I get an alias with fields A::f..., B::f... Is there a way to project it on only the A fields?

C = join A by id, B by id;
D = filter C by B::n < 1000;
E = foreach D generate A::*;

我明白

Unexpected character '*'

我想要的是 E 与模式 相同 到 A(即，describe E 和 describe A 应该打印完全相同的东西).

What I want is E with the schema identical to A (i.e., describe E and describe A should print the exact same things).

我该怎么做?

推荐答案

您可以使用 项目范围表达式 以参与其中.

You can use a project-range expression to get part of the way there.

不幸的是，没有办法系统地去除 A:: 前缀.如果你知道A的最后一个字段的名字(假设它是last)，你可以这样做:

Unfortunately, there is no way to systematically strip the A:: prefix. If you know the name of the last field of A (suppose it's last), you can do this:

E = foreach D generate .. A::last;

如果你只想要 B 中的字段，你会这样做

If you wanted just the fields from B you would do

E = foreach D generate B::first ..;

如果你真的需要应用一个特定的架构，也许你可以定义一个宏，在你需要的时候应用这个架构，这样你就可以覆盖来自分组、加入等的任何更改.

If you really need to apply a specific schema, perhaps you could just define a macro that applies this schema whenever you need it, so you can overwrite any of the changes that come from grouping, joining, etc.

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