alamb commented on code in PR #7015:
URL: https://github.com/apache/arrow-rs/pull/7015#discussion_r1992264897


##########
arrow-json/src/writer/encoder.rs:
##########
@@ -196,9 +236,18 @@ impl Encoder for StructArrayEncoder<'_> {
         let mut is_first = true;
         // Nulls can only be dropped in explicit mode
         let drop_nulls = (self.struct_mode == StructMode::ObjectOnly) && 
!self.explicit_nulls;
-        for field_encoder in &mut self.encoders {
-            let is_null = is_some_and(field_encoder.nulls.as_ref(), |n| 
n.is_null(idx));
-            if drop_nulls && is_null {
+
+        // Collect all the field nulls buffers up front to avoid dynamic 
dispatch in the loop
+        // This creates a temporary Vec, but avoids repeated virtual calls 
which should be a net win

Review Comment:
   Let's just rn
   ```shell
   cargo bench --bench json_writer
   ```
   
   And see what the numbers say?



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