plan: fix cost estimation (#3896)

plan/cbo_test.go

@@ -108,6 +108,54 @@ func (s *testAnalyzeSuite) TestIndexRead(c *C) {
 	}
 }
 
+func (s *testAnalyzeSuite) TestEmptyTable(c *C) {
+	defer func() {
+		testleak.AfterTest(c)()
+	}()
+	store, err := newStoreWithBootstrap()
+	c.Assert(err, IsNil)
+	testKit := testkit.NewTestKit(c, store)
+	defer func() {
+		store.Close()
+	}()
+	testKit.MustExec("use test")
+	testKit.MustExec("drop table if exists t, t1")
+	testKit.MustExec("create table t (c1 int)")
+	testKit.MustExec("create table t1 (c1 int)")
+	testKit.MustExec("analyze table t, t1")
+	tests := []struct {
+		sql  string
+		best string
+	}{
+		{
+			sql:  "select * from t where t.c1 <= 50",
+			best: "TableReader(Table(t)->Sel([le(test.t.c1, 50)]))",
+		},
+		{
+			sql:  "select * from t where c1 in (select c1 from t1)",
+			best: "SemiJoin{TableReader(Table(t))->TableReader(Table(t1))}(test.t.c1,test.t1.c1)",
+		},
+		{
+			sql:  "select * from t, t1 where t.c1 = t1.c1",
+			best: "LeftHashJoin{TableReader(Table(t))->TableReader(Table(t1))}(test.t.c1,test.t1.c1)",
+		},
+	}
+	for _, tt := range tests {
+		ctx := testKit.Se.(context.Context)
+		stmts, err := tidb.Parse(ctx, tt.sql)
+		c.Assert(err, IsNil)
+		c.Assert(stmts, HasLen, 1)
+		stmt := stmts[0]
+		is := sessionctx.GetDomain(ctx).InfoSchema()
+		err = plan.ResolveName(stmt, is, ctx)
+		c.Assert(err, IsNil)
+		err = expression.InferType(ctx.GetSessionVars().StmtCtx, stmt)
+		c.Assert(err, IsNil)
+		p, err := plan.Optimize(ctx, stmt, is)
+		c.Assert(plan.ToString(p), Equals, tt.best, Commentf("for %s", tt.sql))
+	}
+}
+
 func (s *testAnalyzeSuite) TestAnalyze(c *C) {
 	defer func() {
 		testleak.AfterTest(c)()

plan/task.go

@@ -150,6 +150,9 @@ func (p *PhysicalHashJoin) getCost(lCnt, rCnt float64) float64 {
 	if p.SmallTable == 1 {
 		smallTableCnt = rCnt
 	}
+	if smallTableCnt <= 1 {
+		smallTableCnt = 1
+	}
 	return (lCnt + rCnt) * (1 + math.Log2(smallTableCnt))
 }
 
@@ -180,6 +183,9 @@ func (p *PhysicalMergeJoin) attach2Task(tasks ...task) task {
 }
 
 func (p *PhysicalHashSemiJoin) getCost(lCnt, rCnt float64) float64 {
+	if rCnt <= 1 {
+		rCnt = 1
+	}
 	return (lCnt + rCnt) * (1 + math.Log2(rCnt))
 }