explicitly set the stateful set update strategy to onDelete

pkg/cluster/k8sres.go

@@ -858,6 +858,11 @@ func (c *Cluster) generateStatefulSet(spec *acidv1.PostgresSpec) (*v1beta1.State
 
 	numberOfInstances := c.getNumberOfInstances(spec)
 
+	// the operator has domain-specific logic on how to do rolling updates of PG clusters
+	// so we do not use default rolling updates implemented by stateful sets
+	// that leaves "OnDelete" update strategy as the only option
+	updateStrategy := v1beta1.StatefulSetUpdateStrategy{Type: v1beta1.OnDeleteStatefulSetStrategyType}
+
 	statefulSet := &v1beta1.StatefulSet{
 		ObjectMeta: metav1.ObjectMeta{
 			Name:        c.statefulSetName(),
@@ -871,6 +876,7 @@ func (c *Cluster) generateStatefulSet(spec *acidv1.PostgresSpec) (*v1beta1.State
 			ServiceName:          c.serviceName(Master),
 			Template:             *podTemplate,
 			VolumeClaimTemplates: []v1.PersistentVolumeClaim{*volumeClaimTemplate},
+			UpdateStrategy:       updateStrategy,
 		},
 	}