statistics: allow using not-loaded stats when ndv is 0 (pingcap#33241)

ref pingcap#32758, close pingcap#33280

statistics/histogram.go

@@ -1130,7 +1130,7 @@ func (c *Column) IsInvalid(sctx sessionctx.Context, collPseudo bool) bool {
 			}
 		}
 	}
-	return c.TotalRowCount() == 0 || !c.IsLoaded()
+	return c.TotalRowCount() == 0 || (!c.IsLoaded() && c.Histogram.NDV > 0)
 }
 
 // IsHistNeeded checks if this column needs histogram to be loaded

statistics/integration_test.go

@@ -559,3 +559,69 @@ func hasPseudoStats(rows [][]interface{}) bool {
 	}
 	return false
 }
+
+// TestNotLoadedStatsOnAllNULLCol makes sure that stats on a column that only contains NULLs can be used even when it's
+// not loaded. This is reasonable because it makes no difference whether it's loaded or not.
+func TestNotLoadedStatsOnAllNULLCol(t *testing.T) {
+	store, dom, clean := testkit.CreateMockStoreAndDomain(t)
+	defer clean()
+	h := dom.StatsHandle()
+	oriLease := h.Lease()
+	h.SetLease(1000)
+	defer func() {
+		h.SetLease(oriLease)
+	}()
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("drop table if exists t2")
+	tk.MustExec("create table t1(a int)")
+	tk.MustExec("create table t2(a int)")
+	tk.MustExec("insert into t1 values(null), (null), (null), (null)")
+	tk.MustExec("insert into t2 values(null), (null)")
+	tk.MustExec("analyze table t1;")
+	tk.MustExec("analyze table t2;")
+
+	res := tk.MustQuery("explain format = 'brief' select * from t1 left join t2 on t1.a=t2.a order by t1.a, t2.a")
+	res.Check(testkit.Rows(
+		"Sort 4.00 root  test.t1.a, test.t2.a",
+		"└─HashJoin 4.00 root  left outer join, equal:[eq(test.t1.a, test.t2.a)]",
+		"  ├─TableReader(Build) 0.00 root  data:Selection",
+		// If we are not using stats on this column (which means we use pseudo estimation), the row count for the Selection will become 2.
+		"  │ └─Selection 0.00 cop[tikv]  not(isnull(test.t2.a))",
+		"  │   └─TableFullScan 2.00 cop[tikv] table:t2 keep order:false",
+		"  └─TableReader(Probe) 4.00 root  data:TableFullScan",
+		"    └─TableFullScan 4.00 cop[tikv] table:t1 keep order:false"))
+
+	res = tk.MustQuery("explain format = 'brief' select * from t2 left join t1 on t1.a=t2.a order by t1.a, t2.a")
+	res.Check(testkit.Rows(
+		"Sort 2.00 root  test.t1.a, test.t2.a",
+		"└─HashJoin 2.00 root  left outer join, equal:[eq(test.t2.a, test.t1.a)]",
+		// If we are not using stats on this column, the build side will become t2 because of smaller row count.
+		"  ├─TableReader(Build) 0.00 root  data:Selection",
+		// If we are not using stats on this column, the row count for the Selection will become 4.
+		"  │ └─Selection 0.00 cop[tikv]  not(isnull(test.t1.a))",
+		"  │   └─TableFullScan 4.00 cop[tikv] table:t1 keep order:false",
+		"  └─TableReader(Probe) 2.00 root  data:TableFullScan",
+		"    └─TableFullScan 2.00 cop[tikv] table:t2 keep order:false"))
+
+	res = tk.MustQuery("explain format = 'brief' select * from t1 right join t2 on t1.a=t2.a order by t1.a, t2.a")
+	res.Check(testkit.Rows(
+		"Sort 2.00 root  test.t1.a, test.t2.a",
+		"└─HashJoin 2.00 root  right outer join, equal:[eq(test.t1.a, test.t2.a)]",
+		"  ├─TableReader(Build) 0.00 root  data:Selection",
+		"  │ └─Selection 0.00 cop[tikv]  not(isnull(test.t1.a))",
+		"  │   └─TableFullScan 4.00 cop[tikv] table:t1 keep order:false",
+		"  └─TableReader(Probe) 2.00 root  data:TableFullScan",
+		"    └─TableFullScan 2.00 cop[tikv] table:t2 keep order:false"))
+
+	res = tk.MustQuery("explain format = 'brief' select * from t2 right join t1 on t1.a=t2.a order by t1.a, t2.a")
+	res.Check(testkit.Rows(
+		"Sort 4.00 root  test.t1.a, test.t2.a",
+		"└─HashJoin 4.00 root  right outer join, equal:[eq(test.t2.a, test.t1.a)]",
+		"  ├─TableReader(Build) 0.00 root  data:Selection",
+		"  │ └─Selection 0.00 cop[tikv]  not(isnull(test.t2.a))",
+		"  │   └─TableFullScan 2.00 cop[tikv] table:t2 keep order:false",
+		"  └─TableReader(Probe) 4.00 root  data:TableFullScan",
+		"    └─TableFullScan 4.00 cop[tikv] table:t1 keep order:false"))
+}