plan: push down constant filters over join properly (pingcap#9848)

plan/logical_plan_builder.go

@@ -190,6 +190,35 @@ func extractCorColumns(expr expression.Expression) (cols []*expression.Correlate
 	return
 }
 
+// pushDownConstExpr checks if the condition is from filter condition, if true, push it down to both
+// children of join, whatever the join type is; if false, push it down to inner child of outer join,
+// and both children of non-outer-join.
+func (p *LogicalJoin) pushDownConstExpr(expr expression.Expression, leftCond []expression.Expression,
+	rightCond []expression.Expression, filterCond bool) ([]expression.Expression, []expression.Expression) {
+	switch p.JoinType {
+	case LeftOuterJoin, LeftOuterSemiJoin, AntiLeftOuterSemiJoin:
+		if filterCond {
+			leftCond = append(leftCond, expr)
+			// Append the expr to right join condition instead of `rightCond`, to make it able to be
+			// pushed down to children of join.
+			p.RightConditions = append(p.RightConditions, expr)
+		} else {
+			rightCond = append(rightCond, expr)
+		}
+	case RightOuterJoin:
+		if filterCond {
+			rightCond = append(rightCond, expr)
+			p.LeftConditions = append(p.LeftConditions, expr)
+		} else {
+			leftCond = append(leftCond, expr)
+		}
+	case SemiJoin, AntiSemiJoin, InnerJoin:
+		leftCond = append(leftCond, expr)
+		rightCond = append(rightCond, expr)
+	}
+	return leftCond, rightCond
+}
+
 func extractOnCondition(conditions []expression.Expression, left LogicalPlan, right LogicalPlan) (
 	eqCond []*expression.ScalarFunction, leftCond []expression.Expression, rightCond []expression.Expression,
 	otherCond []expression.Expression) {
@@ -211,6 +240,12 @@ func extractOnCondition(conditions []expression.Expression, left LogicalPlan, ri
 			}
 		}
 		columns := expression.ExtractColumns(expr)
+		// `columns` may be empty, if the condition is like `correlated_column op constant`, or `constant`,
+		// push this kind of constant condition down according to join type.
+		if len(columns) == 0 {
+			leftCond, rightCond = p.pushDownConstExpr(expr, leftCond, rightCond, deriveLeft || deriveRight)
+			continue
+		}
 		allFromLeft, allFromRight := true, true
 		for _, col := range columns {
 			if !left.Schema().Contains(col) {