[jira] [Created] (ARROW-5153) [Rust] Use IntoIter trait for write_batch/write_mini_batch

["Xavier Lange (JIRA)" <ji...@apache.org>]

Xavier Lange created ARROW-5153:
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             Summary: [Rust] Use IntoIter trait for write_batch/write_mini_batch
                 Key: ARROW-5153
                 URL: https://issues.apache.org/jira/browse/ARROW-5153
             Project: Apache Arrow
          Issue Type: Improvement
            Reporter: Xavier Lange


Writing data to a parquet file requires a lot of copying and intermediate Vec creation. Take a record struct like:

{{struct MyData {}}{{  name: String,}}{{  address: Option<String>}}{{}}}

Over the course of working sets of this data, you'll have the bulk data Vec<MyData>,  the names column in a Vec<&String>, the address column in a Vec<Option<String>>. This puts extra memory pressure on the system, at the minimum we have to allocate a Vec the same size as the bulk data even if we are using references.

What I'm proposing is to use an IntoIter style. This will maintain backward compat as a slice automatically implements IntoIter. Where ColumnWriterImpl#write_batch goes from "values: &[T::T]"to values "values: IntoIter<Item=T::T>". Then you can do things like

{{  write_batch(bulk.iter().map(|x| x.name), None, None)}}{{  write_batch(bulk.iter().map(|x| x.address), Some(bulk.iter().map(|x| x.is_some())), None)}}

and you can see there's no need for an intermediate Vec, so no short-term allocations to write out the data.

I am writing data with many columns and I think this would really help to speed things up.



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