planner: skip plan cache if plans have MemTableScan operator (#41830)

close pingcap/tidb#41829

planner/core/plan_cache_test.go

@@ -1025,6 +1025,21 @@ func TestPlanCacheWithLimit(t *testing.T) {
 	tk.MustQuery("show warnings").Check(testkit.Rows("Warning 1105 skip plan-cache: limit count more than 10000"))
 }
 
+func TestPlanCacheMemoryTable(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec(`create table t1 (a int)`)
+	tk.MustExec(`create table t2 (a int, b int)`)
+
+	tk.MustExec(`prepare st from 'select count(*) from information_schema.COLUMNS where table_name=?'`)
+	tk.MustExec(`set @a='t1'`)
+	tk.MustQuery(`execute st using @a`).Check(testkit.Rows("1")) // 1 column
+	tk.MustExec(`set @a='t2'`)
+	tk.MustQuery(`execute st using @a`).Check(testkit.Rows("2"))           // 2 columns
+	tk.MustQuery(`select @@last_plan_from_cache`).Check(testkit.Rows("0")) // plan accessing memory tables cannot hit the cache
+}
+
 func TestPlanCacheWithSubquery(t *testing.T) {
 	store := testkit.CreateMockStore(t)
 	tk := testkit.NewTestKit(t, store)

planner/core/plan_cacheable_checker.go

@@ -392,6 +392,8 @@ func isPhysicalPlanCacheable(sctx sessionctx.Context, p PhysicalPlan, paramNum,
 		}
 	case *PhysicalShuffle, *PhysicalShuffleReceiverStub:
 		return false, "skip plan-cache: get a Shuffle plan"
+	case *PhysicalMemTable:
+		return false, "skip plan-cache: PhysicalMemTable plan is un-cacheable"
 	case *PhysicalIndexMergeReader:
 		if x.AccessMVIndex {
 			return false, "skip plan-cache: the plan with IndexMerge accessing Multi-Valued Index is un-cacheable"