planner: check whether plan-cache is disabled each time when getting …

…plan from the plan cache (#49373) (#49386)

close #49344

planner/core/plan_cache.go

@@ -152,13 +152,16 @@ func GetPlanFromSessionPlanCache(ctx context.Context, sctx sessionctx.Context,
 	sessVars := sctx.GetSessionVars()
 	stmtCtx := sessVars.StmtCtx
 	stmtAst := stmt.PreparedAst
-	stmtCtx.UseCache = stmt.StmtCacheable
+	cacheEnabled := false
 	if isNonPrepared {
 		stmtCtx.CacheType = stmtctx.SessionNonPrepared
+		cacheEnabled = sctx.GetSessionVars().EnableNonPreparedPlanCache // plan-cache might be disabled after prepare.
 	} else {
 		stmtCtx.CacheType = stmtctx.SessionPrepared
+		cacheEnabled = sctx.GetSessionVars().EnablePreparedPlanCache
 	}
-	if !stmt.StmtCacheable {
+	stmtCtx.UseCache = stmt.StmtCacheable && cacheEnabled
+	if !stmt.StmtCacheable && stmt.UncacheableReason != "" {
 		stmtCtx.SetSkipPlanCache(errors.New(stmt.UncacheableReason))
 	}
 

planner/core/plan_cache_test.go

@@ -1171,6 +1171,17 @@ func TestIssue38533(t *testing.T) {
 	tk.MustQuery("select @@last_plan_from_cache").Check(testkit.Rows("0"))
 }
 
+func TestIssue49344(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec(`create table t(a int)`)
+	tk.MustExec(`set @@tidb_enable_prepared_plan_cache=1`)
+	tk.MustExec(`prepare s from "select * from t"`)
+	tk.MustExec(`set @@tidb_enable_prepared_plan_cache=0`)
+	tk.MustExec(`execute s`) // no error
+}
+
 func TestInvalidRange(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)