diff --git a/crates/fidc-core/src/data.rs b/crates/fidc-core/src/data.rs
index ccf2fa9..4269569 100644
--- a/crates/fidc-core/src/data.rs
+++ b/crates/fidc-core/src/data.rs
@@ -4513,8 +4513,14 @@ fn group_rows_by_symbol_id<'a, T>(
     symbol_ids_by_date: &BTreeMap<NaiveDate, Vec<u32>>,
     symbol_count: usize,
 ) -> Result<Vec<Vec<&'a T>>, DataSetError> {
-    let mut rows_by_symbol_id = (0..symbol_count)
-        .map(|_| Vec::<&T>::new())
+    let mut row_counts_by_symbol_id = vec![0usize; symbol_count];
+    for symbol_id in symbol_ids_by_date.values().flatten() {
+        row_counts_by_symbol_id[*symbol_id as usize] =
+            row_counts_by_symbol_id[*symbol_id as usize].saturating_add(1);
+    }
+    let mut rows_by_symbol_id = row_counts_by_symbol_id
+        .into_iter()
+        .map(Vec::<&T>::with_capacity)
         .collect::<Vec<_>>();
     for (date, rows) in groups {
         let symbol_ids = symbol_ids_by_date