From 40a9abb6ea963eb6f7c1344b44da4cb454b937cb Mon Sep 17 00:00:00 2001
From: Andres Taylor <andres@planetscale.com>
Date: Thu, 3 Sep 2020 07:54:27 +0200
Subject: [PATCH] only take on simple dual queries in the vtgate

Signed-off-by: Andres Taylor <andres@planetscale.com>
---
 go/vt/vtgate/planbuilder/select.go             | 13 +++++++++----
 .../planbuilder/testdata/select_cases.txt      | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/go/vt/vtgate/planbuilder/select.go b/go/vt/vtgate/planbuilder/select.go
index 2a5f0c97933..66e6d3e8c9a 100644
--- a/go/vt/vtgate/planbuilder/select.go
+++ b/go/vt/vtgate/planbuilder/select.go
@@ -146,7 +146,7 @@ func (pb *primitiveBuilder) processSelect(sel *sqlparser.Select, outer *symtab)
 }
 
 func handleDualSelects(sel *sqlparser.Select, vschema ContextVSchema) (engine.Primitive, error) {
-	if !isOnlyDual(sel.From) {
+	if !isOnlyDual(sel) {
 		return nil, nil
 	}
 
@@ -191,11 +191,16 @@ func buildLockingPrimitive(sel *sqlparser.Select, vschema ContextVSchema) (engin
 	}, nil
 }
 
-func isOnlyDual(from sqlparser.TableExprs) bool {
-	if len(from) > 1 {
+func isOnlyDual(sel *sqlparser.Select) bool {
+	if sel.Where != nil || sel.GroupBy != nil || sel.Having != nil || sel.Limit != nil || sel.OrderBy != nil {
+		// we can only deal with queries without any other subclauses - just SELECT and FROM, nothing else is allowed
 		return false
 	}
-	table, ok := from[0].(*sqlparser.AliasedTableExpr)
+
+	if len(sel.From) > 1 {
+		return false
+	}
+	table, ok := sel.From[0].(*sqlparser.AliasedTableExpr)
 	if !ok {
 		return false
 	}
diff --git a/go/vt/vtgate/planbuilder/testdata/select_cases.txt b/go/vt/vtgate/planbuilder/testdata/select_cases.txt
index 17aa01ebe0c..35750f6e37d 100644
--- a/go/vt/vtgate/planbuilder/testdata/select_cases.txt
+++ b/go/vt/vtgate/planbuilder/testdata/select_cases.txt
@@ -1349,6 +1349,24 @@
   }
 }
 
+# don't filter on the vtgate
+"select 42 from dual where false"
+{
+  "QueryType": "SELECT",
+  "Original": "select 42 from dual where false",
+  "Instructions": {
+    "OperatorType": "Route",
+    "Variant": "SelectReference",
+    "Keyspace": {
+      "Name": "main",
+      "Sharded": false
+    },
+    "FieldQuery": "select 42 from dual where 1 != 1",
+    "Query": "select 42 from dual where false",
+    "Table": "dual"
+  }
+}
+
 # testing SingleRow Projection with arithmetics
 "select 42+2"
 {