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Questions about UDTF in flink SQL


Hi all,

When using user-defined table function in Flink SQL, it seems that the result type of a table function must be determinstic. 

If I want a UDTF whose result type is determined by its input parameters, what should I do?

What I want to do is like this:

```
SELECT input, f1, f2 length FROM MyTable, LATERAL TABLE(unnest_udtf(input, v1, v2)) as T(f1, f2), LATERAL TABLE(unnest_udtf(input, v3, v4, v5)) as T(f3, f4, f5)
```

I can surely register the same UDTF with different name and configuration, but I guess that’s not a good idea :(. 

If we can not make this in Flink SQL for now , may be we should consider this feature in future?

Best,
wangsan