fix:修复了被open调用时，set_schema会重复添加数据的问题

[Fischer0522]

What problem were solved in this pull request?

Issue Number: close #311

Problem: 在NLJ里面会遍历多次右表，就会调用很多次close和open，而open当中会调用set_schema，从而导致join的时候就会出来一个几十个field和value的tuple，后续的话会被select的字段给过滤掉，不影响正确性

What is changed and how it works?

在set_schema当中每次先清空原本tuple当中的field即可。

Other information

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