plan: return table dual when filter is false/null (#7756)

plan/logical_plan_test.go

@@ -426,6 +426,11 @@ func (s *testPlanSuite) TestPredicatePushDown(c *C) {
  sql: "select t1.a, t2.a from t as t1 left join t as t2 on t1.a = t2.a where t1.a < 1.0",
  best: "Join{DataScan(t1)->DataScan(t2)}(t1.a,t2.a)->Projection",
  },
+ // issue #7728
+ {
+ sql: "select * from t t1 join t t2 on t1.a = t2.a where t2.a = null",
+ best: "Dual->Projection",
+ },
  }
  for _, ca := range tests {
  comment := Commentf("for %s", ca.sql)

plan/rule_predicate_push_down.go

@@ -34,6 +34,12 @@ func addSelection(p LogicalPlan, child LogicalPlan, conditions []expression.Expr
  return
  }
  conditions = expression.PropagateConstant(p.context(), conditions)
+ // Return table dual when filter is constant false or null.
+ dual := conds2TableDual(child, conditions)
+ if dual != nil {
+ p.Children()[chIdx] = dual
+ return
+ }
  selection := LogicalSelection{Conditions: conditions}.init(p.context())
  selection.SetChildren(child)
  p.Children()[chIdx] = selection
@@ -55,6 +61,11 @@ func (p *LogicalSelection) PredicatePushDown(predicates []expression.Expression)
  retConditions, child := p.children[0].PredicatePushDown(append(p.Conditions, predicates...))
  if len(retConditions) > 0 {
  p.Conditions = expression.PropagateConstant(p.ctx, retConditions)
+ // Return table dual when filter is constant false or null.
+ dual := conds2TableDual(p, p.Conditions)
+ if dual != nil {
+ return nil, dual
+ }
  return nil, p
  }
  return nil, child
@@ -129,7 +140,13 @@ func (p *LogicalJoin) PredicatePushDown(predicates []expression.Expression) (ret
  tempCond = append(tempCond, p.OtherConditions...)
  tempCond = append(tempCond, predicates...)
  tempCond = expression.ExtractFiltersFromDNFs(p.ctx, tempCond)
- equalCond, leftPushCond, rightPushCond, otherCond = extractOnCondition(expression.PropagateConstant(p.ctx, tempCond), leftPlan, rightPlan, true, true)
+ tempCond = expression.PropagateConstant(p.ctx, tempCond)
+ // Return table dual when filter is constant false or null.
+ dual := conds2TableDual(p, tempCond)
+ if dual != nil {
+ return ret, dual
+ }
+ equalCond, leftPushCond, rightPushCond, otherCond = extractOnCondition(tempCond, leftPlan, rightPlan, true, true)
  p.LeftConditions = nil
  p.RightConditions = nil
  p.EqualConditions = equalCond
@@ -378,3 +395,21 @@ func deriveOtherConditions(p *LogicalJoin, deriveLeft bool, deriveRight bool) (l
  }
  return
 }
+
+// conds2TableDual builds a LogicalTableDual if cond is constant false or null.
+func conds2TableDual(p LogicalPlan, conds []expression.Expression) LogicalPlan {
+ if len(conds) != 1 {
+ return nil
+ }
+ con, ok := conds[0].(*expression.Constant)
+ if !ok {
+ return nil
+ }
+ sc := p.context().GetSessionVars().StmtCtx
+ if isTrue, err := con.Value.ToBool(sc); (err == nil && isTrue == 0) || con.Value.IsNull() {
+ dual := LogicalTableDual{}.init(p.context())
+ dual.SetSchema(p.Schema())
+ return dual
+ }
+ return nil
+}