planner: show un-cacheable reasons for execute statements (#40651)

ref #36598

Author: qw4990

planner/core/plan_cache.go

@@ -127,7 +127,7 @@ func GetPlanFromSessionPlanCache(ctx context.Context, sctx sessionctx.Context,
 	sessVars := sctx.GetSessionVars()
 	stmtCtx := sessVars.StmtCtx
 	stmtAst := stmt.PreparedAst
-	stmtCtx.UseCache = stmt.StmtCacheable
+	stmtCtx.UseCache = true
 	if !stmt.StmtCacheable {
 		stmtCtx.SetSkipPlanCache(errors.Errorf("skip plan-cache: %s", stmt.UncacheableReason))
 	}

planner/core/plan_cache_test.go

@@ -503,3 +503,18 @@ func TestPlanCacheWithLimit(t *testing.T) {
 	tk.MustExec("execute stmt using @a")
 	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: limit count more than 10000"))
 }
+
+func TestUncacheableReason(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table t(a int)")
+
+	tk.MustExec("prepare st from 'select /*+ ignore_plan_cache() */ * from t'")
+	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: ignore plan cache by hint"))
+	tk.MustExec("execute st") // show the un-cacheable reason when executing the statement
+	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: ignore plan cache by hint"))
+	tk.MustExec("execute st")
+	tk.MustQuery("select @@last_plan_from_cache").Check(testkit.Rows("0"))
+}