Fix dual merging in outer join queries

go/vt/sqlparser/ast_funcs.go

@@ -1938,6 +1938,26 @@ func (joinType JoinType) IsCommutative() bool {
  }
 }
 
+// IsLeft returns whether the join type is left join type or not.
+func (joinType JoinType) IsLeft() bool {
+ switch joinType {
+ case LeftJoinType, NaturalLeftJoinType:
+ return true
+ default:
+ return false
+ }
+}
+
+// IsRight returns whether the join type is right join type or not.
+func (joinType JoinType) IsRight() bool {
+ switch joinType {
+ case RightJoinType, NaturalRightJoinType:
+ return true
+ default:
+ return false
+ }
+}
+
 // IsInner returns whether the join type is an inner join or not.
 func (joinType JoinType) IsInner() bool {
  switch joinType {

go/vt/vtgate/planbuilder/operators/join_merging.go

@@ -27,17 +27,24 @@ import (
 // mergeJoinInputs checks whether two operators can be merged into a single one.
 // If they can be merged, a new operator with the merged routing is returned
 // If they cannot be merged, nil is returned.
-func mergeJoinInputs(ctx *plancontext.PlanningContext, lhs, rhs Operator, joinPredicates []sqlparser.Expr, m merger) *Route {
+func mergeJoinInputs(ctx *plancontext.PlanningContext, lhs, rhs Operator, joinPredicates []sqlparser.Expr, m *joinMerger) *Route {
  lhsRoute, rhsRoute, routingA, routingB, a, b, sameKeyspace := prepareInputRoutes(lhs, rhs)
  if lhsRoute == nil {
  return nil
  }
 
  switch {
- // if either side is a dual query, we can always merge them together
  case a == dual:
+ // If a dual is on the left side and it is a left join, then we can only merge if the right side is a single sharded routing.
+ if m.joinType.IsLeft() && !routingB.OpCode().IsSingleShard() {
+ return nil
+ }
  return m.merge(ctx, lhsRoute, rhsRoute, routingB)
  case b == dual:
+ // If a dual is on the right side and it is a right join, then we can only merge if the left side is a single sharded routing.
+ if m.joinType.IsRight() && !routingA.OpCode().IsSingleShard() {
+ return nil
+ }
  return m.merge(ctx, lhsRoute, rhsRoute, routingA)
 
  // an unsharded/reference route can be merged with anything going to that keyspace
@@ -74,12 +81,6 @@ func prepareInputRoutes(lhs Operator, rhs Operator) (*Route, *Route, Routing, Ro
  lhsRoute, rhsRoute, routingA, routingB, sameKeyspace := getRoutesOrAlternates(lhsRoute, rhsRoute)
 
  a, b := getRoutingType(routingA), getRoutingType(routingB)
- if getTypeName(routingA) < getTypeName(routingB) {
- // while deciding if two routes can be merged, the LHS/RHS order of the routes is not important.
- // for the actual merging, we still need to remember which side was inner and which was outer for subqueries
- a, b = b, a
- routingA, routingB = routingB, routingA
- }
 
  return lhsRoute, rhsRoute, routingA, routingB, a, b, sameKeyspace
 }
@@ -178,7 +179,7 @@ func getRoutingType(r Routing) routingType {
  panic(fmt.Sprintf("switch should be exhaustive, got %T", r))
 }
 
-func newJoinMerge(predicates []sqlparser.Expr, joinType sqlparser.JoinType) merger {
+func newJoinMerge(predicates []sqlparser.Expr, joinType sqlparser.JoinType) *joinMerger {
  return &joinMerger{
  predicates: predicates,
  joinType: joinType,

go/vt/vtgate/planbuilder/testdata/select_cases.json

@@ -2043,6 +2043,121 @@
  ]
  }
  },
+ {
+ "comment": "left join with a dual table on left",
+ "query": "select t.title, user.col from (select 'hello' as title) as t left join user on user.id=1",
+ "plan": {
+ "QueryType": "SELECT",
+ "Original": "select t.title, user.col from (select 'hello' as title) as t left join user on user.id=1",
+ "Instructions": {
+ "OperatorType": "Join",
+ "Variant": "LeftJoin",
+ "JoinColumnIndexes": "L:0,R:0",
+ "TableName": "dual_`user`",
+ "Inputs": [
+ {
+ "OperatorType": "Route",
+ "Variant": "Reference",
+ "Keyspace": {
+ "Name": "main",
+ "Sharded": false
+ },
+ "FieldQuery": "select t.title from (select 'hello' as title from dual where 1 != 1) as t where 1 != 1",
+ "Query": "select t.title from (select 'hello' as title from dual) as t",
+ "Table": "dual"
+ },
+ {
+ "OperatorType": "Route",
+ "Variant": "EqualUnique",
+ "Keyspace": {
+ "Name": "user",
+ "Sharded": true
+ },
+ "FieldQuery": "select `user`.col from `user` where 1 != 1",
+ "Query": "select `user`.col from `user` where `user`.id = 1",
+ "Table": "`user`",
+ "Values": [
+ "1"
+ ],
+ "Vindex": "user_index"
+ }
+ ]
+ },
+ "TablesUsed": [
+ "main.dual",
+ "user.user"
+ ]
+ }
+ },
+ {
+ "comment": "right join with a dual table on left",
+ "query": "select t.title, user.col from (select 'hello' as title) as t right join user on user.id=1",
+ "plan": {
+ "QueryType": "SELECT",
+ "Original": "select t.title, user.col from (select 'hello' as title) as t right join user on user.id=1",
+ "Instructions": {
+ "OperatorType": "Route",
+ "Variant": "Scatter",
+ "Keyspace": {
+ "Name": "user",
+ "Sharded": true
+ },
+ "FieldQuery": "select t.title, `user`.col from `user` left join (select 'hello' as title from dual where 1 != 1) as t on `user`.id = 1 where 1 != 1",
+ "Query": "select t.title, `user`.col from `user` left join (select 'hello' as title from dual) as t on `user`.id = 1",
+ "Table": "`user`, dual"
+ },
+ "TablesUsed": [
+ "main.dual",
+ "user.user"
+ ]
+ }
+ },
+ {
+ "comment": "right join with a dual table on right",
+ "query": "select t.title, user.col from user right join (select 'hello' as title) as t on user.id=1",
+ "plan": {
+ "QueryType": "SELECT",
+ "Original": "select t.title, user.col from user right join (select 'hello' as title) as t on user.id=1",
+ "Instructions": {
+ "OperatorType": "Join",
+ "Variant": "LeftJoin",
+ "JoinColumnIndexes": "L:0,R:0",
+ "TableName": "dual_`user`",
+ "Inputs": [
+ {
+ "OperatorType": "Route",
+ "Variant": "Reference",
+ "Keyspace": {
+ "Name": "main",
+ "Sharded": false
+ },
+ "FieldQuery": "select t.title from (select 'hello' as title from dual where 1 != 1) as t where 1 != 1",
+ "Query": "select t.title from (select 'hello' as title from dual) as t",
+ "Table": "dual"
+ },
+ {
+ "OperatorType": "Route",
+ "Variant": "EqualUnique",
+ "Keyspace": {
+ "Name": "user",
+ "Sharded": true
+ },
+ "FieldQuery": "select `user`.col from `user` where 1 != 1",
+ "Query": "select `user`.col from `user` where `user`.id = 1",
+ "Table": "`user`",
+ "Values": [
+ "1"
+ ],
+ "Vindex": "user_index"
+ }
+ ]
+ },
+ "TablesUsed": [
+ "main.dual",
+ "user.user"
+ ]
+ }
+ },
  {
  "comment": "Union after into outfile is incorrect",
  "query": "select id from user into outfile 'out_file_name' union all select id from music",