[GitHub] [arrow-datafusion] gandronchik commented on pull request #2177: User Defined Table Function (udtf) support

[GitBox <gi...@apache.org>]

gandronchik commented on PR #2177:
URL: https://github.com/apache/arrow-datafusion/pull/2177#issuecomment-1118507143

   > @gandronchik thank you for the explanation in this PR's description. It helps though I will admit I still don't fully understand what is going o.
   > 
   > I agree with @doki23 -- I expect a table function to logically return a table (that something with both rows and columns)
   > 
   > > Regarding signature, I decided to use a single vector and vector with sizes of sections instead of vec of vecs to have better performance. If we use Vec, this will require a lot of memory in case of a request for millions of rows.
   > 
   > The way the rest of DataFusion avoids buffering all the intermediate results at once int memory is with `Stream`s but then that requires interacting with rust's `async` ecosystem which is non trivial
   > 
   > If you wanted a streaming solution, that would mean the signature might look something like the following (maybe)
   > 
   > ```rust
   > Arc<dyn Fn(Box<dyn SendableRecordBatchStream>) -> Result<Box<dyn SendableRecordBatchStream>> + Send + Sync>;
   > ```
   
   Looks like I got the title wrong. I have implemented a function that returns many rows, probably it is not a table function. If I rename it, will it be fine?
   
   Regarding the function signature, I think my solution is a compromise between vec<vec> and streaming. Actually, I don't think that function can return so many rows. However, of course, I will rewrite it if you want. So which solution do we choose: current `Result<(ArrayRef, Vec<usize>)> + Send + Sync>`, `Result<Vec<ColumnarValue>> + Send + Sync>` or `Result<Box< dyn SendableRecordBatchStream>> + Send + Sync>` ?


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