fix(sql): keep one cell per duplicate constant column

nodedb/src/control/planner/sql_plan_convert/output_schema/build.rs

@@ -173,17 +173,26 @@ pub fn build_output_schema<C: SqlCatalog + ?Sized>(
             let types = super::join_types::join_column_types(left, right, catalog, database_id);
             schema_from_projection(projection, &types, &[])
         }
-        SqlPlan::ConstantResult { columns, .. } => OutputSchema {
-            columns: columns
-                .iter()
-                .map(|c| OutputColumn {
-                    display_name: c.clone(),
-                    lookup_key: c.clone(),
-                    ty: DdlColType::Text,
-                })
-                .collect(),
-            is_star: false,
-        },
+        SqlPlan::ConstantResult { columns, .. } => {
+            // The row payload keys each cell by the unique per-column key
+            // (`cell_keys`), not the raw display name: two constant columns may
+            // share a name (`SELECT nextval('s'), nextval('s')`), and a single
+            // JSON object would collapse them. `display_name` keeps the
+            // client-facing name; `lookup_key` is the cell key.
+            let lookup_keys = crate::control::server::response_shape::project::cell_keys(columns);
+            OutputSchema {
+                columns: columns
+                    .iter()
+                    .zip(lookup_keys)
+                    .map(|(c, lookup_key)| OutputColumn {
+                        display_name: c.clone(),
+                        lookup_key,
+                        ty: DdlColType::Text,
+                    })
+                    .collect(),
+                is_star: false,
+            }
+        }
         SqlPlan::Aggregate {
             input,
             group_by,

nodedb/src/control/planner/sql_plan_convert/set_ops.rs

@@ -19,13 +19,19 @@ pub(super) fn convert_constant_result(
     tenant_id: TenantId,
     ctx: &ConvertContext,
 ) -> crate::Result<Vec<PhysicalTask>> {
+    // A constant row is one JSON object, which cannot hold two cells under one
+    // key. `SELECT nextval('s'), nextval('s')` legally repeats an output name;
+    // keying both cells by the name would collapse them to the last value. Use
+    // the same unique per-column keys every response encoder derives, so each
+    // column keeps its own cell.
+    let cell_keys = crate::control::server::response_shape::project::cell_keys(columns);
     let mut obj = serde_json::Map::new();
-    for (col, val) in columns.iter().zip(values.iter()) {
+    for ((_col, val), key) in columns.iter().zip(values.iter()).zip(cell_keys.iter()) {
         let json_val = match val {
             SqlValue::Null => serde_json::Value::Null,
             other => serde_json::Value::String(sql_value_to_string(other)),
         };
-        obj.insert(col.clone(), json_val);
+        obj.insert(key.clone(), json_val);
     }
     let arr = serde_json::Value::Array(vec![serde_json::Value::Object(obj)]);
     let payload = nodedb_types::json_to_msgpack(&arr).map_err(|e| crate::Error::Serialization {

nodedb/tests/wire/cases/sql_sequences.rs

@@ -426,3 +426,34 @@ async fn a_refused_accessor_produces_no_row_and_no_allocation() {
         "the refused statement must not have allocated, got {first:?}"
     );
 }
+
+/// A from-less projection legally repeats an output name
+/// (`SELECT nextval('s'), nextval('s')`). The constant row is a single JSON
+/// object, so keying both cells by the display name collapsed them to the
+/// last value and both wire columns rendered it. Each column must keep its
+/// own cell.
+#[tokio::test(flavor = "multi_thread", worker_threads = 4)]
+async fn duplicate_constant_column_names_keep_their_own_cells() {
+    let server = TestServer::start().await;
+    server
+        .exec("CREATE SEQUENCE dup_cell_seq START 1 INCREMENT 1")
+        .await
+        .unwrap();
+
+    let rows = server
+        .query_rows("SELECT nextval('dup_cell_seq'), nextval('dup_cell_seq')")
+        .await
+        .expect("a from-less projection repeating an output name must return one row");
+
+    assert_eq!(
+        rows.len(),
+        1,
+        "expected exactly one constant row, got {rows:?}"
+    );
+    assert_eq!(
+        rows[0],
+        vec!["1".to_string(), "2".to_string()],
+        "each duplicate column must keep its own cell; keying both by the \
+         display name collapses them to the last value"
+    );
+}