[doc] improve comments

rdettai · Sep 24, 2021 · 2ef3233 · 2ef3233
1 parent ec24a3b
commit 2ef3233
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Showing 2 changed files with 5 additions and 3 deletions.
diff --git a/datafusion/src/datasource/object_store/local.rs b/datafusion/src/datasource/object_store/local.rs
@@ -70,6 +70,8 @@ impl ObjectReader for LocalFileReader {
         start: u64,
         length: usize,
     ) -> Result<Box<dyn Read + Send + Sync>> {
+        // A new file descriptor is opened for each chunk reader.
+        // This okay because chunks are usually fairly large.
         let mut file = File::open(&self.file.path)?;
         file.seek(SeekFrom::Start(start))?;
         Ok(Box::new(file.take(length as u64)))

diff --git a/datafusion/src/datasource/object_store/mod.rs b/datafusion/src/datasource/object_store/mod.rs
@@ -32,9 +32,9 @@ use local::LocalFileSystem;
 
 use crate::error::{DataFusionError, Result};
 
-/**
-Object Reader for one file in an object store
-*/
+/// Object Reader for one file in an object store
+/// Note that the dynamic dispatch on the reader might
+/// have some performance impacts.
 #[async_trait]
 pub trait ObjectReader {
     /// Get reader for a part [start, start + length] in the file