bugfix: make sure to ignore merged predicates

Signed-off-by: Andres Taylor <andres@planetscale.com>

go/vt/vtgate/planbuilder/operators/SQL_builder.go

@@ -233,7 +233,9 @@ func (qb *queryBuilder) joinWith(other *queryBuilder, onCondition sqlparser.Expr
 	switch joinType {
 	case sqlparser.NormalJoinType:
 		newFromClause = append(stmt.GetFrom(), otherStmt.GetFrom()...)
-		qb.addPredicate(onCondition)
+		for _, pred := range sqlparser.SplitAndExpression(nil, onCondition) {
+			qb.addPredicate(pred)
+		}
 	default:
 		newFromClause = []sqlparser.TableExpr{buildJoin(stmt, otherStmt, onCondition, joinType)}
 	}

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

@@ -1824,6 +1824,30 @@
       ]
     }
   },
+  {
+    "comment": "three table join with join predicate touching all tables",
+    "query": "select 42 from user u join user_extra ue on u.id = ue.user_id join music m on m.user_id = u.id where u.foo or m.foo or ue.foo",
+    "plan": {
+      "QueryType": "SELECT",
+      "Original": "select 42 from user u join user_extra ue on u.id = ue.user_id join music m on m.user_id = u.id where u.foo or m.foo or ue.foo",
+      "Instructions": {
+        "OperatorType": "Route",
+        "Variant": "Scatter",
+        "Keyspace": {
+          "Name": "user",
+          "Sharded": true
+        },
+        "FieldQuery": "select 42 from `user` as u, user_extra as ue, music as m where 1 != 1",
+        "Query": "select 42 from `user` as u, user_extra as ue, music as m where u.id = ue.user_id and m.user_id = u.id and (u.foo or m.foo or ue.foo)",
+        "Table": "`user`, music, user_extra"
+      },
+      "TablesUsed": [
+        "user.music",
+        "user.user",
+        "user.user_extra"
+      ]
+    }
+  },
   {
     "comment": "join of normal table with information_schema",
     "query": "select unsharded.foo from unsharded join information_schema.CHARACTER_SETS",