Add comment explaining the rationale for using .get_slice_memory_size()

Author: gabotechs

datafusion/functions-aggregate/src/array_agg.rs

@@ -393,6 +393,17 @@ impl Accumulator for ArrayAggAccumulator {
             + self
                 .values
                 .iter()
+                // Each ArrayRef might be just a reference to a bigger array, and many
+                // ArrayRefs here might be referencing exactly the same array, so if we
+                // were to call `arr.get_array_memory_size()`, we would be double-counting
+                // the same underlying data many times.
+                //
+                // Instead, we do an approximation by estimating how much memory each
+                // ArrayRef would occupy if its underlying data was fully owned by this
+                // accumulator.
+                //
+                // Note that this is just an estimation, but the reality is that this
+                // accumulator might not own any data.
                 .map(|arr| arr.to_data().get_slice_memory_size().unwrap_or_default())
                 .sum::<usize>()
             + self.datatype.size()