planner: fix invalid pointer caused by a recursive CTE query (#54450)

pkg/executor/cte_test.go

@@ -604,3 +604,53 @@ func TestCTEIterationMemTracker(t *testing.T) {
 	}()
 	tk.MustQuery(fmt.Sprintf("explain analyze with recursive cte1 as (select c1 from t1 union all select c1 + 1 c1 from cte1 where c1 < %d) select * from cte1", maxIter))
 }
+
+func TestCTETableInvaildTask(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("CREATE TABLE p ( groupid bigint(20) DEFAULT NULL, KEY k1 (groupid));")
+	tk.MustExec(`CREATE TABLE g (groupid bigint(20) DEFAULT NULL,parentid bigint(20) NOT NULL,KEY k1 (parentid),KEY k2 (groupid,parentid));`)
+	tk.MustExec(`set tidb_opt_enable_hash_join=off;`)
+	tk.MustQuery(`explain WITH RECURSIVE w(gid) AS (
+  SELECT
+    groupId
+  FROM
+    p
+  UNION
+  SELECT
+    g.groupId
+  FROM
+    g
+    JOIN w ON g.parentId = w.gid
+)
+SELECT
+  1
+FROM
+  g
+WHERE
+  g.groupId IN (
+    SELECT
+      gid
+    FROM
+      w
+  );`).Check(testkit.Rows(
+		"Projection_54 9990.00 root  1->Column#17",
+		"└─IndexJoin_59 9990.00 root  inner join, inner:IndexReader_58, outer key:test.p.groupid, inner key:test.g.groupid, equal cond:eq(test.p.groupid, test.g.groupid)",
+		"  ├─HashAgg_75(Build) 12800.00 root  group by:test.p.groupid, funcs:firstrow(test.p.groupid)->test.p.groupid",
+		"  │ └─Selection_72 12800.00 root  not(isnull(test.p.groupid))",
+		"  │   └─CTEFullScan_73 16000.00 root CTE:w data:CTE_0",
+		"  └─IndexReader_58(Probe) 9990.00 root  index:Selection_57",
+		"    └─Selection_57 9990.00 cop[tikv]  not(isnull(test.g.groupid))",
+		"      └─IndexRangeScan_56 10000.00 cop[tikv] table:g, index:k2(groupid, parentid) range: decided by [eq(test.g.groupid, test.p.groupid)], keep order:false, stats:pseudo",
+		"CTE_0 16000.00 root  Recursive CTE",
+		"├─IndexReader_24(Seed Part) 10000.00 root  index:IndexFullScan_23",
+		"│ └─IndexFullScan_23 10000.00 cop[tikv] table:p, index:k1(groupid) keep order:false, stats:pseudo",
+		"└─IndexHashJoin_34(Recursive Part) 10000.00 root  inner join, inner:IndexLookUp_31, outer key:test.p.groupid, inner key:test.g.parentid, equal cond:eq(test.p.groupid, test.g.parentid)",
+		"  ├─Selection_51(Build) 8000.00 root  not(isnull(test.p.groupid))",
+		"  │ └─CTETable_52 10000.00 root  Scan on CTE_0",
+		"  └─IndexLookUp_31(Probe) 10000.00 root  ",
+		"    ├─IndexRangeScan_29(Build) 10000.00 cop[tikv] table:g, index:k1(parentid) range: decided by [eq(test.g.parentid, test.p.groupid)], keep order:false, stats:pseudo",
+		"    └─TableRowIDScan_30(Probe) 10000.00 cop[tikv] table:g keep order:false, stats:pseudo"))
+	tk.MustQuery(`show warnings`).Check(testkit.Rows())
+}

pkg/planner/core/find_best_task.go

@@ -2616,7 +2616,7 @@ func (p *LogicalCTE) findBestTask(prop *property.PhysicalProperty, counter *Plan
 
 func (p *LogicalCTETable) findBestTask(prop *property.PhysicalProperty, _ *PlanCounterTp, _ *physicalOptimizeOp) (t task, cntPlan int64, err error) {
 	if !prop.IsSortItemEmpty() {
-		return nil, 1, nil
+		return invalidTask, 0, nil
 	}
 
 	pcteTable := PhysicalCTETable{IDForStorage: p.idForStorage}.Init(p.SCtx(), p.StatsInfo())