planner: Choose index with statistics vs one without

pkg/planner/cardinality/BUILD.bazel

@@ -59,7 +59,7 @@ go_test(
     data = glob(["testdata/**"]),
     embed = [":cardinality"],
     flaky = True,
-    shard_count = 29,
+    shard_count = 30,
     deps = [
         "//pkg/config",
         "//pkg/domain",

pkg/planner/cardinality/selectivity_test.go

@@ -341,6 +341,28 @@ func TestEstimationForUnknownValuesAfterModify(t *testing.T) {
 	require.Truef(t, count > 20, "expected: between 20 to 40, got: %v", count)
 }
 
+func TestNewIndexWithoutStats(t *testing.T) {
+	store, _ := testkit.CreateMockStoreAndDomain(t)
+	testKit := testkit.NewTestKit(t, store)
+	testKit.MustExec("use test")
+	testKit.MustExec("drop table if exists t")
+	testKit.MustExec("create table t(a int, b int, c int, index idxa(a))")
+	testKit.MustExec("set @@tidb_analyze_version=2")
+	testKit.MustExec("set @@global.tidb_enable_auto_analyze='OFF'")
+	testKit.MustExec("insert into t values (1, 1, 1)")
+	testKit.MustExec("insert into t select mod(a,250), mod(a,10), mod(a,100) from (with recursive x as (select 1 as a union all select a + 1 AS a from x where a < 500) select a from x) as subquery")
+	testKit.MustExec("analyze table t")
+	testKit.MustExec("create index idxb on t(b)")
+	// Create index after ANALYZE. SkyLine pruning should ensure that idxa is chosen because it has statistics
+	testKit.MustQuery("explain format='brief' select * from t where a = 5 and b = 5").CheckContain("idxa(a)")
+	testKit.MustExec("analyze table t")
+	// idxa should still win after statistics
+	testKit.MustQuery("explain format='brief' select * from t where a = 5 and b = 5").CheckContain("idxa(a)")
+	testKit.MustExec("create index idxab on t(a, b)")
+	// New index idxab should win due to having the most matching equal predicates - regardless of no statistics
+	testKit.MustQuery("explain format='brief' select * from t where a = 5 and b = 5").CheckContain("idxab(a, b)")
+}
+
 func TestEstimationUniqueKeyEqualConds(t *testing.T) {
 	store, dom := testkit.CreateMockStoreAndDomain(t)
 	testKit := testkit.NewTestKit(t, store)

pkg/planner/core/find_best_task.go

@@ -711,13 +711,37 @@ func compareGlobalIndex(lhs, rhs *candidatePath) int {
 
 // compareCandidates is the core of skyline pruning, which is used to decide which candidate path is better.
 // The return value is 1 if lhs is better, -1 if rhs is better, 0 if they are equivalent or not comparable.
-func compareCandidates(sctx base.PlanContext, prop *property.PhysicalProperty, lhs, rhs *candidatePath) int {
+func compareCandidates(sctx base.PlanContext, statsTbl *statistics.Table, prop *property.PhysicalProperty, lhs, rhs *candidatePath) int {
 	// Due to #50125, full scan on MVIndex has been disabled, so MVIndex path might lead to 'can't find a proper plan' error at the end.
 	// Avoid MVIndex path to exclude all other paths and leading to 'can't find a proper plan' error, see #49438 for an example.
 	if isMVIndexPath(lhs.path) || isMVIndexPath(rhs.path) {
 		return 0
 	}
 
+	// If one index has statistics and the other does not, choose the index with statistics if it
+	// has the same or higher number of equal/IN predicates.
+	lhsHasStatistics := statsTbl.Pseudo
+	if statsTbl != nil && lhs.path.Index != nil {
+		lhsHasStatistics = statsTbl.ColAndIdxExistenceMap.HasAnalyzed(lhs.path.Index.ID, true)
+	}
+	rhsHasStatistics := statsTbl.Pseudo
+	if statsTbl != nil && rhs.path.Index != nil {
+		rhsHasStatistics = statsTbl.ColAndIdxExistenceMap.HasAnalyzed(rhs.path.Index.ID, true)
+	}
+	if !lhs.path.IsTablePath() && !rhs.path.IsTablePath() && // Not a table scan
+		(lhsHasStatistics || rhsHasStatistics) && // At least one index has statistics
+		(!lhsHasStatistics || !rhsHasStatistics) && // At least one index doesn't have statistics
+		len(lhs.path.PartialIndexPaths) == 0 && len(rhs.path.PartialIndexPaths) == 0 { // not IndexMerge due to unreliability
+		lhsTotalEqual := lhs.path.EqCondCount + lhs.path.EqOrInCondCount
+		rhsTotalEqual := rhs.path.EqCondCount + rhs.path.EqOrInCondCount
+		if lhsHasStatistics && lhsTotalEqual > 0 && lhsTotalEqual >= rhsTotalEqual {
+			return 1
+		}
+		if rhsHasStatistics && rhsTotalEqual > 0 && rhsTotalEqual >= lhsTotalEqual {
+			return -1
+		}
+	}
+
 	// This rule is empirical but not always correct.
 	// If x's range row count is significantly lower than y's, for example, 1000 times, we think x is better.
 	if lhs.path.CountAfterAccess > 100 && rhs.path.CountAfterAccess > 100 && // to prevent some extreme cases, e.g. 0.01 : 10
@@ -1140,7 +1164,7 @@ func skylinePruning(ds *logicalop.DataSource, prop *property.PhysicalProperty) [
 			if candidates[i].path.StoreType == kv.TiFlash {
 				continue
 			}
-			result := compareCandidates(ds.SCtx(), prop, candidates[i], currentCandidate)
+			result := compareCandidates(ds.SCtx(), ds.StatisticTable, prop, candidates[i], currentCandidate)
 			if result == 1 {
 				pruned = true
 				// We can break here because the current candidate cannot prune others anymore.