diff --git a/lib/src/revset.rs b/lib/src/revset.rs
index d88a584d4..357cc48a5 100644
--- a/lib/src/revset.rs
+++ b/lib/src/revset.rs
@@ -824,13 +824,19 @@ pub fn lower_expression(
             let lhs = lower_expression(lhs_node, context)?;
             let rhs = lower_expression(rhs_node, context)?;
             match op {
-                BinaryOp::Union => Ok(lhs.union(&rhs)),
                 BinaryOp::Intersection => Ok(lhs.intersection(&rhs)),
                 BinaryOp::Difference => Ok(lhs.minus(&rhs)),
                 BinaryOp::DagRange => Ok(lhs.dag_range_to(&rhs)),
                 BinaryOp::Range => Ok(lhs.range(&rhs)),
             }
         }
+        ExpressionKind::UnionAll(nodes) => {
+            let expressions: Vec<_> = nodes
+                .iter()
+                .map(|node| lower_expression(node, context))
+                .try_collect()?;
+            Ok(RevsetExpression::union_all(&expressions))
+        }
         ExpressionKind::FunctionCall(function) => lower_function_call(function, context),
         ExpressionKind::Modifier(modifier) => {
             let name = modifier.name;
diff --git a/lib/src/revset_parser.rs b/lib/src/revset_parser.rs
index 6c0c315e0..b860f5041 100644
--- a/lib/src/revset_parser.rs
+++ b/lib/src/revset_parser.rs
@@ -311,6 +311,8 @@ pub enum ExpressionKind<'i> {
     RangeAll,
     Unary(UnaryOp, Box<ExpressionNode<'i>>),
     Binary(BinaryOp, Box<ExpressionNode<'i>>, Box<ExpressionNode<'i>>),
+    /// `x | y | ..`
+    UnionAll(Vec<ExpressionNode<'i>>),
     FunctionCall(Box<FunctionCallNode<'i>>),
     /// `name: body`
     Modifier(Box<ModifierNode<'i>>),
@@ -341,6 +343,10 @@ impl<'i> FoldableExpression<'i> for ExpressionKind<'i> {
                 let rhs = Box::new(folder.fold_expression(*rhs)?);
                 Ok(ExpressionKind::Binary(op, lhs, rhs))
             }
+            ExpressionKind::UnionAll(nodes) => {
+                let nodes = dsl_util::fold_expression_nodes(folder, nodes)?;
+                Ok(ExpressionKind::UnionAll(nodes))
+            }
             ExpressionKind::FunctionCall(function) => folder.fold_function_call(function, span),
             ExpressionKind::Modifier(modifier) => {
                 let modifier = Box::new(ModifierNode {
@@ -392,8 +398,6 @@ pub enum UnaryOp {
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub enum BinaryOp {
-    /// `|`
-    Union,
     /// `&`
     Intersection,
     /// `~`
@@ -418,6 +422,20 @@ pub struct ModifierNode<'i> {
     pub body: ExpressionNode<'i>,
 }
 
+fn union_nodes<'i>(lhs: ExpressionNode<'i>, rhs: ExpressionNode<'i>) -> ExpressionNode<'i> {
+    let span = lhs.span.start_pos().span(&rhs.span.end_pos());
+    let expr = match lhs.kind {
+        // Flatten "x | y | z" to save recursion stack. Machine-generated query
+        // might have long chain of unions.
+        ExpressionKind::UnionAll(mut nodes) => {
+            nodes.push(rhs);
+            ExpressionKind::UnionAll(nodes)
+        }
+        _ => ExpressionKind::UnionAll(vec![lhs, rhs]),
+    };
+    ExpressionNode::new(expr, span)
+}
+
 pub(super) fn parse_program(revset_str: &str) -> Result<ExpressionNode, RevsetParseError> {
     let mut pairs = RevsetParser::parse(Rule::program, revset_str)?;
     let first = pairs.next().unwrap();
@@ -551,7 +569,7 @@ fn parse_expression_node(pairs: Pairs<Rule>) -> Result<ExpressionNode, RevsetPar
         })
         .map_infix(|lhs, op, rhs| {
             let op_kind = match op.as_rule() {
-                Rule::union_op => BinaryOp::Union,
+                Rule::union_op => return Ok(union_nodes(lhs?, rhs?)),
                 Rule::compat_add_op => Err(not_infix_op(&op, "|", "union"))?,
                 Rule::intersection_op => BinaryOp::Intersection,
                 Rule::difference_op => BinaryOp::Difference,
@@ -883,6 +901,10 @@ mod tests {
                 let rhs = Box::new(normalize_tree(*rhs));
                 ExpressionKind::Binary(op, lhs, rhs)
             }
+            ExpressionKind::UnionAll(nodes) => {
+                let nodes = normalize_list(nodes);
+                ExpressionKind::UnionAll(nodes)
+            }
             ExpressionKind::FunctionCall(function) => {
                 let function = Box::new(normalize_function_call(*function));
                 ExpressionKind::FunctionCall(function)
@@ -1067,7 +1089,11 @@ mod tests {
         // Parse the "union" operator
         assert_matches!(
             parse_into_kind("foo | bar"),
-            Ok(ExpressionKind::Binary(BinaryOp::Union, _, _))
+            Ok(ExpressionKind::UnionAll(nodes)) if nodes.len() == 2
+        );
+        assert_matches!(
+            parse_into_kind("foo | bar | baz"),
+            Ok(ExpressionKind::UnionAll(nodes)) if nodes.len() == 3
         );
         // Parse the "difference" operator
         assert_matches!(
@@ -1479,8 +1505,8 @@ mod tests {
     #[test]
     fn test_expand_symbol_alias() {
         assert_eq!(
-            with_aliases([("AB", "a|b")]).parse_normalized("AB|c"),
-            parse_normalized("(a|b)|c")
+            with_aliases([("AB", "a&b")]).parse_normalized("AB|c"),
+            parse_normalized("(a&b)|c")
         );
         assert_eq!(
             with_aliases([("AB", "a|b")]).parse_normalized("AB::heads(AB)"),
diff --git a/lib/tests/test_revset.rs b/lib/tests/test_revset.rs
index 86ca9260d..91e091f6b 100644
--- a/lib/tests/test_revset.rs
+++ b/lib/tests/test_revset.rs
@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::iter;
 use std::path::Path;
 
 use assert_matches::assert_matches;
@@ -2641,6 +2642,29 @@ fn test_evaluate_expression_union() {
     );
 }
 
+#[test]
+fn test_evaluate_expression_machine_generated_union() {
+    let settings = testutils::user_settings();
+    let test_repo = TestRepo::init();
+    let repo = &test_repo.repo;
+
+    let mut tx = repo.start_transaction(&settings);
+    let mut_repo = tx.mut_repo();
+    let mut graph_builder = CommitGraphBuilder::new(&settings, mut_repo);
+    let commit1 = graph_builder.initial_commit();
+    let commit2 = graph_builder.commit_with_parents(&[&commit1]);
+
+    // This query shouldn't trigger stack overflow. Here we use "x::y" in case
+    // we had optimization path for trivial "commit_id|.." expression.
+    let revset_str = iter::repeat(format!("({}::{})", commit1.id().hex(), commit2.id().hex()))
+        .take(5000)
+        .join("|");
+    assert_eq!(
+        resolve_commit_ids(mut_repo, &revset_str),
+        vec![commit2.id().clone(), commit1.id().clone()]
+    );
+}
+
 #[test]
 fn test_evaluate_expression_intersection() {
     let settings = testutils::user_settings();