address review comments

datafusion/expr/src/expr.rs

@@ -1425,7 +1425,7 @@ impl Expr {
     /// Adds references to all columns and their occurrence counts in the expression to
     /// the map.
     ///
-    /// See [`Self::column_refs`] for details
+    /// See [`Self::column_refs_counts`] for details
     pub fn add_column_ref_counts<'a>(&'a self, map: &mut HashMap<&'a Column, usize>) {
         self.apply(|expr| {
             if let Expr::Column(col) = expr {

datafusion/optimizer/src/common_subexpr_eliminate.rs

@@ -899,7 +899,9 @@ struct ExprIdentifierVisitor<'a, 'n> {
     random_state: &'a RandomState,
     // a flag to indicate that common expression found
     found_common: bool,
-    // if we are in a conditional branch
+    // if we are in a conditional branch. A conditional branch means that the expression
+    // might not be executed depending on the runtime values of other expressions, and
+    // thus can not be extracted as a common expression.
     conditional: bool,
 }
 
@@ -1053,7 +1055,7 @@ impl<'n> TreeNodeVisitor<'n> for ExprIdentifierVisitor<'_, 'n> {
             } else {
                 *count += 1;
             }
-            if *count > 1 || *count == 1 && *conditional_count > 0 {
+            if *count > 1 || (*count == 1 && *conditional_count > 0) {
                 self.found_common = true;
             }
         }