Fix UNION column mapping bug in nested multi-way operations

Fixed column scrambling in complex UNION queries with 3+ branches and 10+ columns. The bug caused duplicate rows with wrong column positions when processing nested UNION operations like (A UNION B) UNION C.

Root cause: colIdsForRel() used sorted ColSet.ForEach() for SetOp nodes, losing original SELECT column order. This caused wrong column mappings in mergeSetOpSchemas() for nested UNIONs.

Solution: SetOp nodes now preserve schema order by recursing to left child instead of using sorted column set, matching MySQL behavior.

Added regression test that reproduces the exact threshold where bug occurs.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

enginetest/queries/script_queries.go

@@ -11237,6 +11237,38 @@ where
 			},
 		},
 	},
+	{
+		Name: "UNION column mapping bug - issue 9628",
+		SetUpScript: []string{
+			"CREATE TABLE report_card (id INT PRIMARY KEY, name VARCHAR(255), entity_id INT, dashboard_id INT, description TEXT, display TEXT, collection_preview TEXT, dataset_query TEXT, collection_id INT, archived_directly BOOLEAN DEFAULT FALSE, collection_position INT, database_id INT, archived BOOLEAN DEFAULT FALSE, last_used_at DATETIME, table_id INT, query_type VARCHAR(50), type VARCHAR(50))",
+			"CREATE TABLE collection (id INT PRIMARY KEY, name VARCHAR(255), entity_id INT, location VARCHAR(500), authority_level VARCHAR(50), personal_owner_id INT, archived_directly BOOLEAN DEFAULT FALSE, type VARCHAR(50), archived BOOLEAN DEFAULT FALSE, namespace VARCHAR(100))",
+			"CREATE TABLE report_dashboard (id INT PRIMARY KEY, name VARCHAR(255), entity_id INT, description TEXT, collection_id INT, archived_directly BOOLEAN DEFAULT FALSE, collection_position INT, archived BOOLEAN DEFAULT FALSE, last_viewed_at DATETIME)",
+			"INSERT INTO report_card (id, name, entity_id, dashboard_id, type, archived_directly, archived, collection_position) VALUES (2, 'Card 2', 1002, NULL, 'question', FALSE, FALSE, NULL), (3, 'Card 3', 1003, NULL, 'question', FALSE, FALSE, NULL)",
+			"INSERT INTO collection (id, name, entity_id, location, type, archived, personal_owner_id, namespace) VALUES (2, 'Collection 2', 4002, '/test2/', 'normal', FALSE, NULL, NULL), (3, 'Collection 3', 4003, '/test3/', 'normal', FALSE, NULL, NULL)",
+			"INSERT INTO report_dashboard (id, name, entity_id, description, collection_id, archived_directly, archived, collection_position) VALUES (1, 'Dashboard 1', 3001, 'Test dashboard description', NULL, FALSE, FALSE, NULL), (2, 'Dashboard 2', 3002, 'Test dashboard 2 description', NULL, FALSE, FALSE, NULL)",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: `WITH visible_collection_ids AS (SELECT id FROM collection AS c WHERE (1 <> c.id)) 
+				SELECT 5 AS model_ranking, c.id, c.name, c.description, c.entity_id, c.display, c.collection_preview, c.dataset_query, c.collection_id, 'card' AS model 
+				FROM report_card AS c WHERE (c.dashboard_id IS NULL) AND (archived = FALSE) AND (c.type = 'question') 
+				UNION 
+				SELECT 7 AS model_ranking, id, name, name AS description, entity_id, NULL AS display, NULL AS collection_preview, NULL AS dataset_query, id AS collection_id, 'collection' AS model 
+				FROM collection AS col WHERE (archived = FALSE) AND (id <> 1) AND (personal_owner_id IS NULL) AND (namespace IS NULL) 
+				UNION 
+				SELECT 1 AS model_ranking, d.id, d.name, d.description, d.entity_id, NULL AS display, NULL AS collection_preview, NULL AS dataset_query, NULL AS collection_id, 'dashboard' AS model 
+				FROM report_dashboard AS d WHERE (archived = FALSE)`,
+				Expected: []sql.Row{
+					{5, 2, "Card 2", nil, 1002, nil, nil, nil, nil, "card"},
+					{5, 3, "Card 3", nil, 1003, nil, nil, nil, nil, "card"},
+					{7, 2, "Collection 2", "Collection 2", 4002, nil, nil, nil, 2, "collection"},
+					{7, 3, "Collection 3", "Collection 3", 4003, nil, nil, nil, 3, "collection"},
+					{1, 1, "Dashboard 1", "Test dashboard description", 3001, nil, nil, nil, nil, "dashboard"},
+					{1, 2, "Dashboard 2", "Test dashboard 2 description", 3002, nil, nil, nil, nil, "dashboard"},
+				},
+			},
+		},
+	},
 }
 
 var SpatialScriptTests = []ScriptTest{

sql/planbuilder/set_op.go

@@ -228,6 +228,11 @@ func colIdsForRel(n sql.Node) []sql.ColumnId {
 		}
 		return ids
 	case plan.TableIdNode:
+		// SetOp nodes need to preserve original schema order to avoid column scrambling in nested UNIONs
+		if setOp, ok := n.(*plan.SetOp); ok {
+			return colIdsForRel(setOp.Left())
+		}
+		
 		cols := n.Columns()
 		if tn, ok := n.(sql.TableNode); ok {
 			if pkt, ok := tn.UnderlyingTable().(sql.PrimaryKeyTable); ok && len(pkt.PrimaryKeySchema().Schema) != len(n.Schema()) {