bugfix: don't panic when missing schema information (#14787)

go/vt/vtgate/planbuilder/testdata/from_cases.json

@@ -4256,5 +4256,28 @@
         "user.user",
         "user.user_extra"
       ]
-    }  }
+    }
+  },
+  {
+    "comment": "unexpanded columns are fine if we can push down into single route",
+    "query": "select x from (select t.*, 1 as x from unsharded t union select t.*, 1 as x from unsharded t) as x",
+    "plan": {
+      "QueryType": "SELECT",
+      "Original": "select x from (select t.*, 1 as x from unsharded t union select t.*, 1 as x from unsharded t) as x",
+      "Instructions": {
+        "OperatorType": "Route",
+        "Variant": "Unsharded",
+        "Keyspace": {
+          "Name": "main",
+          "Sharded": false
+        },
+        "FieldQuery": "select x from (select t.*, 1 as x from unsharded as t where 1 != 1 union select t.*, 1 as x from unsharded as t where 1 != 1) as x where 1 != 1",
+        "Query": "select x from (select t.*, 1 as x from unsharded as t union select t.*, 1 as x from unsharded as t) as x",
+        "Table": "unsharded"
+      },
+      "TablesUsed": [
+        "main.unsharded"
+      ]
+    }
+  }
 ]

go/vt/vtgate/planbuilder/testdata/unsupported_cases.json

@@ -383,5 +383,10 @@
     "comment": "correlated subqueries in select expressions are unsupported",
     "query": "SELECT (SELECT sum(user.name) FROM music LIMIT 1) FROM user",
     "plan": "VT12001: unsupported: correlated subquery is only supported for EXISTS"
+  },
+  {
+    "comment": "We need schema tracking to allow unexpanded columns inside UNION",
+    "query": "select x from (select t.*, 0 as x from user t union select t.*, 1 as x from user_extra t) AS t",
+    "plan": "VT09015: schema tracking required"
   }
 ]

go/vt/vtgate/semantics/derived_table.go

@@ -107,6 +107,10 @@ func (dt *DerivedTable) dependencies(colName string, org originable) (dependenci
 		if !strings.EqualFold(name, colName) {
 			continue
 		}
+		if len(dt.recursive) == 0 {
+			// we have unexpanded columns and can't figure this out
+			return nil, ShardedError{Inner: vterrors.VT09015()}
+		}
 		recursiveDeps, qt := dt.recursive[i], dt.types[i]
 
 		return createCertain(directDeps, recursiveDeps, qt), nil