[Gen4] Merge SeenPredicates when creating route operator for join

go/vt/vtgate/planbuilder/physical/route.go

@@ -600,6 +600,8 @@ func (r *Route) planCompositeInOpRecursive(
 	return foundVindex
 }
 
+// Reset all vindex predicates on this route and re-build their options from
+// the list of seen routing predicates.
 func (r *Route) resetRoutingSelections(ctx *plancontext.PlanningContext) error {
 	switch r.RouteOpCode {
 	case engine.DBA, engine.Next, engine.Reference, engine.Unsharded:

go/vt/vtgate/planbuilder/physical/route_planning.go

@@ -562,6 +562,7 @@ func createRouteOperatorForJoin(aRoute, bRoute *Route, joinPredicates []sqlparse
 		Keyspace:            aRoute.Keyspace,
 		VindexPreds:         append(aRoute.VindexPreds, bRoute.VindexPreds...),
 		SysTableTableSchema: append(aRoute.SysTableTableSchema, bRoute.SysTableTableSchema...),
+		SeenPredicates:      append(aRoute.SeenPredicates, bRoute.SeenPredicates...),
 		SysTableTableName:   sysTableName,
 		Source: &ApplyJoin{
 			LHS:       aRoute.Source,

go/vt/vtgate/planbuilder/physical/subquery_planning.go

@@ -107,6 +107,22 @@ func mergeSubQueryOp(ctx *plancontext.PlanningContext, outer *Route, inner *Rout
 		outer.SysTableTableName[k] = v
 	}
 
+	// When merging an inner query with its outer query, we can remove the
+	// inner query from the list of predicates that can influence routing of
+	// the outer query.
+	//
+	// Note that not all inner queries necessarily are part of the routing
+	// predicates list, so this might be a no-op.
+	for i, predicate := range outer.SeenPredicates {
+		if sqlparser.EqualsExpr(predicate, subq.ExtractedSubquery) {
+			outer.SeenPredicates = append(outer.SeenPredicates[:i], outer.SeenPredicates[i+1:]...)
+
+			// The `ExtractedSubquery` of an inner query is unique (due to the uniqueness of bind variable names)
+			// so we can stop after the first match.
+			break
+		}
+	}
+
 	err = outer.resetRoutingSelections(ctx)
 	if err != nil {
 		return nil, err

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

@@ -4114,6 +4114,51 @@ Gen4 plan same as above
 "unsupported: cross-shard correlated subquery"
 Gen4 error: exists sub-queries are only supported with AND clause
 
+# correlated subquery that is dependent on one side of a join, fully mergeable
+"SELECT music.id FROM music INNER JOIN user ON music.user_id = user.id WHERE music.user_id = 5 AND music.id = (SELECT MAX(m2.id) FROM music m2 WHERE m2.user_id = user.id)"
+{
+  "QueryType": "SELECT",
+  "Original": "SELECT music.id FROM music INNER JOIN user ON music.user_id = user.id WHERE music.user_id = 5 AND music.id = (SELECT MAX(m2.id) FROM music m2 WHERE m2.user_id = user.id)",
+  "Instructions": {
+    "OperatorType": "Route",
+    "Variant": "EqualUnique",
+    "Keyspace": {
+      "Name": "user",
+      "Sharded": true
+    },
+    "FieldQuery": "select music.id from music join `user` on music.user_id = `user`.id where 1 != 1",
+    "Query": "select music.id from music join `user` on music.user_id = `user`.id where music.user_id = 5 and music.id = (select max(m2.id) from music as m2 where m2.user_id = `user`.id)",
+    "Table": "music, `user`",
+    "Values": [
+      "INT64(5)"
+    ],
+    "Vindex": "user_index"
+  }
+}
+{
+  "QueryType": "SELECT",
+  "Original": "SELECT music.id FROM music INNER JOIN user ON music.user_id = user.id WHERE music.user_id = 5 AND music.id = (SELECT MAX(m2.id) FROM music m2 WHERE m2.user_id = user.id)",
+  "Instructions": {
+    "OperatorType": "Route",
+    "Variant": "EqualUnique",
+    "Keyspace": {
+      "Name": "user",
+      "Sharded": true
+    },
+    "FieldQuery": "select music.id from music, `user` where 1 != 1",
+    "Query": "select music.id from music, `user` where music.user_id = 5 and music.id = (select max(m2.id) from music as m2 where m2.user_id = `user`.id) and music.user_id = `user`.id",
+    "Table": "`user`, music",
+    "Values": [
+      "INT64(5)"
+    ],
+    "Vindex": "user_index"
+  },
+  "TablesUsed": [
+    "user.music",
+    "user.user"
+  ]
+}
+
 # union as a derived table
 "select found from (select id as found from user union all (select id from unsharded)) as t"
 {