fix empty filter

columnar/src/columnar/merge/mod.rs

@@ -454,10 +454,17 @@ fn group_columns_for_merge_iter<'a>(
                 group,
             )
         });
-        let iter = group_column_iter.filter(|res| {
+        let iter = group_column_iter.filter(move |res| {
             // Filter out empty columns.
             res.as_ref()
-                .map(|(_, _, group)| !group.columns.iter().all(|column| column.is_none()))
+                .map(|(_, _, group)| {
+                    let column_is_required = force_type.is_some();
+                    if column_is_required {
+                        return true;
+                    }
+                    let all_columns_none = group.columns.iter().all(|column| column.is_none());
+                    !all_columns_none
+                })
                 .unwrap_or(true)
         });
         Box::new(iter)
@@ -470,16 +477,13 @@ fn generate_require_column<'a>(
     required_columns_map: &HashMap<String, ColumnType>,
 ) -> MergeIter<'a> {
     let mut grouped_columns = GroupedColumns::new(columnar_readers.len());
-    let force_type: ColumnType = required_columns_map
-        .get(&*column_name)
-        .map(|el| (*el).into())
-        .unwrap();
+    let force_type: ColumnType = required_columns_map.get(&*column_name).cloned().unwrap();
     grouped_columns.require_type(force_type).unwrap(); // Can't panic
-    return Box::new(std::iter::once(Ok((
+    Box::new(std::iter::once(Ok((
         column_name,
         force_type,
         grouped_columns,
-    )))) as MergeIter<'a>;
+    )))) as MergeIter<'a>
 }
 
 fn group_columns_iter<'a>(

src/indexer/index_writer.rs

@@ -2426,6 +2426,13 @@ mod tests {
         test_operation_strategy(&ops[..], false, true).unwrap();
     }
 
+    #[test]
+    fn test_merge_regression_1() {
+        use IndexingOp::*;
+        let ops = &[AddDoc { id: 15 }, Commit, AddDoc { id: 9 }, Commit, Merge];
+        test_operation_strategy(&ops[..], false, true).unwrap();
+    }
+
     #[test]
     fn test_range_query_bug_1() {
         use IndexingOp::*;