Fix percentile query: field is mandatory (should not crash), fieldnames can be unquoted

executor.go

@@ -1239,10 +1239,10 @@ func (e *executor) executePercentile(ctx context.Context, qcx *Qcx, index string
 	}
 
 	// get field
-	if fieldArg := c.Args["field"]; fieldArg == "" {
+	fieldName, err := c.FirstStringArg("field", "_field")
+	if err != nil {
 		return ValCount{}, errors.New("Percentile(): field required")
 	}
-	fieldName, _, _ := c.StringArg("field")
 
 	// filter call for min & max
 	var filterCall *pql.Call

executor_test.go

@@ -7026,6 +7026,11 @@ func variousQueriesOnPercentiles(t *testing.T, c *test.Cluster) {
 			query:       query,
 			csvVerifier: fmt.Sprintf("%d,1\n", expectedPercentile),
 		})
+		query2 := fmt.Sprintf(`Percentile(field=net_worth, filter=Row(val="foo"), nth=%d)`, nth)
+		tests = append(tests, testCase{
+			query:       query2,
+			csvVerifier: fmt.Sprintf("%d,1\n", expectedPercentile),
+		})
 	}
 
 	for i, tst := range tests {

pql/ast.go

@@ -481,6 +481,7 @@ var callInfoByFunc = map[string]callInfo{
 		allowUnknown: false,
 		prototypes: map[string]interface{}{
 			"field":  "",
+			"_field": "",
 			"filter": nil,
 			"nth":    nil,
 		},