fix: handle table_schema = '' without failing (#15901)

Author: systay

go/test/endtoend/vtgate/queries/informationschema/informationschema_test.go

@@ -201,6 +201,10 @@ func TestMultipleSchemaPredicates(t *testing.T) {
 	_, err := mcmp.VtConn.ExecuteFetch(query, 1000, true)
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "specifying two different database in the query is not supported")
+
+	if utils.BinaryIsAtLeastAtVersion(20, "vtgate") {
+		_, _ = mcmp.ExecNoCompare("select * from information_schema.columns where table_schema = '' limit 1")
+	}
 }
 
 func TestInfrSchemaAndUnionAll(t *testing.T) {

go/vt/vtgate/engine/route_test.go

@@ -130,15 +130,15 @@ func TestInformationSchemaWithTableAndSchemaWithRoutedTables(t *testing.T) {
 		expectedLog: []string{
 			"FindTable(tableName)",
 			"ResolveDestinations routedKeyspace [] Destinations:DestinationAnyShard()",
-			"ExecuteMultiShard routedKeyspace.1: dummy_select {table_name: type:VARCHAR value:\"routedTable\"} false false"},
+			"ExecuteMultiShard routedKeyspace.1: dummy_select {__vtschemaname: type:VARCHAR table_name: type:VARCHAR value:\"routedTable\"} false false"},
 	}, {
 		testName:  "table name predicate - not routed",
 		tableName: map[string]evalengine.Expr{"table_name": evalengine.NewLiteralString([]byte("tableName"), collations.SystemCollation)},
 		routed:    false,
 		expectedLog: []string{
 			"FindTable(tableName)",
 			"ResolveDestinations ks [] Destinations:DestinationAnyShard()",
-			"ExecuteMultiShard ks.1: dummy_select {table_name: type:VARCHAR value:\"tableName\"} false false"},
+			"ExecuteMultiShard ks.1: dummy_select {__vtschemaname: type:VARCHAR table_name: type:VARCHAR value:\"tableName\"} false false"},
 	}, {
 		testName:    "schema predicate",
 		tableSchema: []string{"myKeyspace"},

go/vt/vtgate/engine/routing.go

@@ -192,22 +192,21 @@ func (rp *RoutingParameters) routeInfoSchemaQuery(ctx context.Context, vcursor V
 
 	env := evalengine.NewExpressionEnv(ctx, bindVars, vcursor)
 	var specifiedKS string
-	for _, tableSchema := range rp.SysTableTableSchema {
+	for idx, tableSchema := range rp.SysTableTableSchema {
 		result, err := env.Evaluate(tableSchema)
 		if err != nil {
 			return nil, err
 		}
 		ks := result.Value(vcursor.ConnCollation()).ToString()
-		if specifiedKS == "" {
+		switch {
+		case idx == 0:
 			specifiedKS = ks
-		}
-		if specifiedKS != ks {
+		case specifiedKS != ks:
 			return nil, vterrors.Errorf(vtrpcpb.Code_UNIMPLEMENTED, "specifying two different database in the query is not supported")
 		}
 	}
-	if specifiedKS != "" {
-		bindVars[sqltypes.BvSchemaName] = sqltypes.StringBindVariable(specifiedKS)
-	}
+
+	bindVars[sqltypes.BvSchemaName] = sqltypes.StringBindVariable(specifiedKS)
 
 	tableNames := map[string]string{}
 	for tblBvName, sysTableName := range rp.SysTableTableName {