perf: use individual field access for maybe_null_field_by_idx (#1881)

Part of #1749

This notably improves compression speed for tables with wide schemas.

Author: a10y

vortex-array/src/array/struct_/mod.rs

@@ -150,11 +150,24 @@ impl VariantsVTable<StructArray> for StructEncoding {
 
 impl StructArrayTrait for StructArray {
     fn maybe_null_field_by_idx(&self, idx: usize) -> Option<ArrayData> {
-        self.dtypes().get(idx).map(|dtype| {
-            self.as_ref()
-                .child(idx, dtype, self.len())
-                .unwrap_or_else(|e| vortex_panic!(e, "StructArray: field {} not found", idx))
-        })
+        Some(
+            self.field_info(&Field::Index(idx))
+                .map(|field_info| {
+                    self.as_ref()
+                        .child(
+                            idx,
+                            &field_info
+                                .dtype
+                                .value()
+                                .vortex_expect("FieldInfo could not access dtype"),
+                            self.len(),
+                        )
+                        .unwrap_or_else(|e| {
+                            vortex_panic!(e, "StructArray: field {} not found", idx)
+                        })
+                })
+                .unwrap_or_else(|e| vortex_panic!(e, "StructArray: field {} not found", idx)),
+        )
     }
 
     fn project(&self, projection: &[Field]) -> VortexResult<ArrayData> {

vortex-array/src/variants.rs

@@ -5,7 +5,7 @@
 
 use std::sync::Arc;
 
-use vortex_dtype::{DType, ExtDType, Field, FieldNames, PType};
+use vortex_dtype::{DType, ExtDType, Field, FieldInfo, FieldNames, PType};
 use vortex_error::{vortex_panic, VortexError, VortexExpect as _, VortexResult};
 
 use crate::encoding::Encoding;
@@ -189,6 +189,14 @@ pub trait StructArrayTrait: ArrayTrait {
         st.names()
     }
 
+    fn field_info(&self, field: &Field) -> VortexResult<FieldInfo> {
+        let DType::Struct(st, _) = self.dtype() else {
+            unreachable!()
+        };
+
+        st.field_info(field)
+    }
+
     fn dtypes(&self) -> Vec<DType> {
         let DType::Struct(st, _) = self.dtype() else {
             unreachable!()