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

问题描述

一旦我执行了join A by id, B by id，我将获得一个别名，其中包含字段A::f...，B::f... 有没有一种方法可以将其仅投影在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::*;

我知道

Unexpected character '*'

我想要的是E，其架构为相同到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获取其中的一部分.

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

不幸的是，没有办法系统地剥离A::前缀.如果您知道A的最后一个字段的名称(假设它是last)，则可以执行以下操作:

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;

如果只需要B中的字段，则可以

If you wanted just the fields from B you would do

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

如果您确实需要应用特定的架构，也许您可​​以只定义一个在需要时应用该架构的宏，这样您就可以覆盖来自分组，联接等的任何更改.

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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