[ARROW-5153] [Rust] [Parquet] Use IntoIter trait for write_batch/write_mini_batch

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Invalid
• Component/s: Rust
• External issue URL: https://github.com/apache/arrow/issues/21633
• Assignee: Unassigned
• Reporter: Xavier Lange
• Created: 09/Apr/19 19:16
• Updated: 11/Jan/23 07:38
• Resolved: 26/Apr/21 11:23

Description

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.