dual queries with LIMIT can still be handled on the vtgate

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/vt/vtgate/planbuilder/select.go

@@ -242,11 +242,27 @@ func createSelectOperator(ctx *plancontext.PlanningContext, selStmt sqlparser.Se
 }
 
 func isOnlyDual(sel *sqlparser.Select) bool {
-	if sel.Where != nil || sel.GroupBy != nil || sel.Having != nil || sel.Limit != nil || sel.OrderBy != nil {
+	if sel.Where != nil || sel.GroupBy != nil || sel.Having != nil || sel.OrderBy != nil {
 		// we can only deal with queries without any other subclauses - just SELECT and FROM, nothing else is allowed
 		return false
 	}
 
+	if sel.Limit != nil {
+		if sel.Limit.Offset != nil {
+			return false
+		}
+		limit := sel.Limit.Rowcount
+		switch limit := limit.(type) {
+		case nil:
+			return true
+		case *sqlparser.Literal:
+			if limit.Val == "0" {
+				// A limit with any value other than zero can still return a row
+				return false
+			}
+		}
+	}
+
 	if len(sel.From) > 1 {
 		return false
 	}

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

@@ -2717,6 +2717,28 @@
       ]
     }
   },
+  {
+    "comment": "Dual query should be handled on the vtgate even with a LIMIT",
+    "query": "select last_insert_id() limit 1",
+    "plan": {
+      "QueryType": "SELECT",
+      "Original": "select last_insert_id() limit 1",
+      "Instructions": {
+        "OperatorType": "Projection",
+        "Expressions": [
+          ":__lastInsertId as last_insert_id()"
+        ],
+        "Inputs": [
+          {
+            "OperatorType": "SingleRow"
+          }
+        ]
+      },
+      "TablesUsed": [
+        "main.dual"
+      ]
+    }
+  },
   {
     "comment": "PullOut subquery with an aggregation that should be typed in the final output",
     "query": "select (select sum(col) from user) from user_extra",