Dandandan commented on code in PR #7833:
URL: https://github.com/apache/arrow-datafusion/pull/7833#discussion_r1363678450


##########
datafusion/optimizer/src/push_down_projection.rs:
##########
@@ -423,7 +426,93 @@ pub fn collect_projection_expr(projection: &Projection) -> 
HashMap<String, Expr>
         .collect::<HashMap<_, _>>()
 }
 
-// Get the projection exprs from columns in the order of the schema
+/// Accumulate the memory size of a data type measured in bits.
+///
+/// Nested types are traversed and increment `nesting` on every level.
+fn nested_size(data_type: &DataType, nesting: &mut usize) -> usize {
+    use DataType::*;
+    if data_type.is_primitive() {
+        return data_type.primitive_width().unwrap_or(1) * 8;
+    }
+
+    if data_type.is_nested() {
+        *nesting += 1;
+    }
+
+    match data_type {
+        Null => 0,
+        Boolean => 1,
+        Binary | Utf8 => 128,
+        LargeBinary | LargeUtf8 => 256,
+        FixedSizeBinary(bytes) => (*bytes * 8) as usize,
+        // primitive types
+        Int8
+        | Int16
+        | Int32
+        | Int64
+        | UInt8
+        | UInt16
+        | UInt32
+        | UInt64
+        | Float16
+        | Float32
+        | Float64
+        | Timestamp(_, _)
+        | Date32
+        | Date64
+        | Time32(_)
+        | Time64(_)
+        | Duration(_)
+        | Interval(_)
+        | Dictionary(_, _)
+        | Decimal128(_, _)
+        | Decimal256(_, _) => data_type.primitive_width().unwrap_or(1) * 8,
+        // nested types
+        List(f) => nested_size(f.data_type(), nesting),
+        FixedSizeList(_, s) => (s * 8) as usize,
+        LargeList(f) => nested_size(f.data_type(), nesting),
+        Struct(fields) => fields
+            .iter()
+            .map(|f| nested_size(f.data_type(), nesting))

Review Comment:
   Yeah indeed :)



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