table partition: add more warnings to inform the user that setting TiDB partition prune mode to dynamic at session level

sessionctx/variable/sysvar.go

@@ -1406,6 +1406,7 @@ var defaultSysVars = []*SysVar{
 		newMode := strings.ToLower(strings.TrimSpace(val))
 		if PartitionPruneMode(s.PartitionPruneMode.Load()) == Static && PartitionPruneMode(newMode) == Dynamic {
 			s.StmtCtx.AppendWarning(errors.New("Please analyze all partition tables again for consistency between partition and global stats"))
+			s.StmtCtx.AppendWarning(errors.New("Please avoid setting partition prune mode to dynamic at session level and set partition prune mode to dynamic at global level"))
 		}
 		s.PartitionPruneMode.Store(newMode)
 		return nil

table/tables/partition_test.go

@@ -729,3 +729,14 @@ func TestIssue31721(t *testing.T) {
 	tk.MustExec("insert into t_31721 values ('1')")
 	tk.MustExec("select * from t_31721 partition(p0, p1) where col1 != 2;")
 }
+
+func TestPruneMode(t *testing.T) {
+	store, _, clean := testkit.CreateMockStoreAndDomain(t)
+	defer clean()
+
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("set @@tidb_partition_prune_mode = 'static'")
+	tk.MustExec("set session tidb_partition_prune_mode = 'dynamic'")
+	tk.MustQuery("show warnings").Sort().Check(testkit.Rows("Warning 1105 Please analyze all partition tables again for consistency between partition and global stats",
+		"Warning 1105 Please avoid setting partition prune mode to dynamic at session level and set partition prune mode to dynamic at global level"))
+}