[jira] [Commented] (ARROW-7252) [Rust] Reading UTF-8/JSON/ENUM field results in a lot of vec allocation

["Wong Shek Hei (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ARROW-7252?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16983265#comment-16983265 ] 

Wong Shek Hei commented on ARROW-7252:
--------------------------------------

I have modified the Field::Str locally to hold the ByteArray instead, that removes the copying. Reading a 1.5MM, 1000 column file(440mb) gz.parquet file, on MacBook Pro (15-inch, 2019), improved from 3m20s to 2m20s.

 

But the problem is this will modify the signature of the Field::Str Variant.

> [Rust] Reading UTF-8/JSON/ENUM field results in a lot of vec allocation
> -----------------------------------------------------------------------
>
>                 Key: ARROW-7252
>                 URL: https://issues.apache.org/jira/browse/ARROW-7252
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: Rust
>            Reporter: Wong Shek Hei
>            Priority: Minor
>
> While reading a very large parquet file with basically all string fields was very slow(430MB gzipped), after profiling with osx instruments, I noticed that a lot of time is spent in "convert_byte_array", in particular, "reserving" and allocating Vec::with_capacity, which is done before String::from_utf8_unchecked.
> It seems like using String as the underlying storage is causing this(String uses Vec<u8> for its underlying storage), this also requires copying from slice to vec.
> "Field::Str<String>" is a pub enum so I am not sure how "refactorable" is the String part, for example, converting it into a &str(we can perhaps then defer the conversion from &[u8] to Vec<u8> until the user really needs a String)
> But of course, changing it to &str can result in quite a bit of interface changes... So I am wondering if there are already some plans or solution on the way to improve the handling of the "Field::Str" case?
>  



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