[GitHub] [arrow] mqy commented on a change in pull request #9025: ARROW-10259: [Rust] Add custom metadata to Field

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mqy commented on a change in pull request #9025:
URL: https://github.com/apache/arrow/pull/9025#discussion_r549939736



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File path: rust/arrow/src/datatypes.rs
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@@ -22,6 +22,7 @@
 //!  * [`Field`](crate::datatypes::Field) to describe one field within a schema.
 //!  * [`DataType`](crate::datatypes::DataType) to describe the type of a field.
 
+use std::collections::BTreeMap;

Review comment:
       Copied from [rust doc std/collections](https://doc.rust-lang.org/std/collections/index.html):
   
   **`Use a HashMap when:**
   
   You want to associate arbitrary keys with an arbitrary value.
   You want a cache.
   You want a map, with no extra functionality.
   
   **Use a BTreeMap when:**
   
   You want a map sorted by its keys.
   You want to be able to get a range of entries on-demand.
   You're interested in what the smallest or largest key-value pair is.
   You want to find the largest or smallest key that is smaller or larger than something.`
   
   The only reason that use `BTreeMap` for `Field` is: don't break the `'Hash`, `PartialOrd`, `Ord`.
   I almost can't image any situation that will sort schemas, unless compare/sort a schema by versions.




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