plan: always chose the smaller child as outer for IndexJoin

plan/exhaust_physical_plans.go

@@ -519,22 +519,29 @@ func (p *LogicalJoin) tryToGetIndexJoin(prop *requiredProp) ([]PhysicalPlan, boo
 			plans = append(plans, join...)
 		}
 	case InnerJoin:
-		join := p.getIndexJoinByOuterIdx(prop, 0)
-		if join != nil {
-			// If the plan is not nil and matches the hint, return it directly.
-			if leftOuter {
-				return join, true
-			}
-			plans = append(plans, join...)
+		lhsCardinality := p.Children()[0].cardinality()
+		rhsCardinality := p.Children()[1].cardinality()
+
+		leftJoins := p.getIndexJoinByOuterIdx(prop, 0)
+		if leftOuter && leftJoins != nil {
+			return leftJoins, true
 		}
-		join = p.getIndexJoinByOuterIdx(prop, 1)
-		if join != nil {
-			// If the plan is not nil and matches the hint, return it directly.
-			if rightOuter {
-				return join, true
-			}
-			plans = append(plans, join...)
+
+		rightJoins := p.getIndexJoinByOuterIdx(prop, 1)
+		if rightOuter && rightJoins != nil {
+			return rightJoins, true
 		}
+
+		if leftJoins != nil && lhsCardinality < rhsCardinality {
+			return leftJoins, leftOuter
+		}
+
+		if rightJoins != nil && rhsCardinality < lhsCardinality {
+			return rightJoins, rightOuter
+		}
+
+		plans = append(plans, leftJoins...)
+		plans = append(plans, rightJoins...)
 	}
 	return plans, false
 }

plan/plan.go

@@ -184,6 +184,8 @@ type LogicalPlan interface {
 	// deriveStats derives statistic info between plans.
 	deriveStats() (*statsInfo, error)
 
+	cardinality() float64
+
 	// preparePossibleProperties is only used for join and aggregation. Like group by a,b,c, all permutation of (a,b,c) is
 	// valid, but the ordered indices in leaf plan is limited. So we can get all possible order properties by a pre-walking.
 	// Please make sure that children's method is called though we may not need its return value,
@@ -324,6 +326,20 @@ func (p *baseLogicalPlan) extractCorrelatedCols() []*expression.CorrelatedColumn
 	return corCols
 }
 
+func (p *DataSource) cardinality() float64 {
+	if p.statsAfterSelect == nil {
+		return math.MaxFloat64
+	}
+	return p.statsAfterSelect.count
+}
+
+func (p *baseLogicalPlan) cardinality() float64 {
+	if p.stats == nil {
+		return math.MaxFloat64
+	}
+	return p.stats.count
+}
+
 // PruneColumns implements LogicalPlan interface.
 func (p *baseLogicalPlan) PruneColumns(parentUsedCols []*expression.Column) {
 	if len(p.children) == 0 {