planner: can't use primary as an index in IndexMerge for predicate id=? and (a=? or b=?) #66668

create table if not exists t (
    id int,
    a int,
    b int,
    primary key (id, a),
    key ka (id, a),
    key kb (id, b)
);
explain select /*+ use_index_merge(t, primary, kb) */ * from t where id=1 and (a=1 or b=1);
explain select /*+ use_index_merge(t, ka, kb) */ * from t where id=1 and (a=1 or b=1);

mysql> explain select /*+ use_index_merge(t, primary, kb) */ * from t where id=1 and (a=1 or b=1);
+--------------------------+---------+-----------+---------------+---------------------------------------------+
| id                       | estRows | task      | access object | operator info                               |
+--------------------------+---------+-----------+---------------+---------------------------------------------+
| TableReader_8            | 1.00    | root      |               | data:Selection_7                            |
| └─Selection_7            | 1.00    | cop[tikv] |               | or(eq(test.t.a, 1), eq(test.t.b, 1))        |
|   └─TableRangeScan_6     | 10.00   | cop[tikv] | table:t       | range:[1,1], keep order:false, stats:pseudo |
+--------------------------+---------+-----------+---------------+---------------------------------------------+
3 rows in set, 1 warning (0.002 sec)

mysql> explain select /*+ use_index_merge(t, ka, kb) */ * from t where id=1 and (a=1 or b=1);
+-------------------------------+---------+-----------+--------------------------+--------------------------------------------------------+
| id                            | estRows | task      | access object            | operator info                                          |
+-------------------------------+---------+-----------+--------------------------+--------------------------------------------------------+
| IndexMerge_10                 | 0.00    | root      |                          | type: union                                            |
| ├─IndexRangeScan_6(Build)     | 0.10    | cop[tikv] | table:t, index:ka(id, a) | range:[1 1,1 1], keep order:false, stats:pseudo        |
| ├─IndexRangeScan_7(Build)     | 0.10    | cop[tikv] | table:t, index:kb(id, b) | range:[1 1,1 1], keep order:false, stats:pseudo        |
| └─Selection_9(Probe)          | 0.00    | cop[tikv] |                          | eq(test.t.id, 1), or(eq(test.t.a, 1), eq(test.t.b, 1)) |
|   └─TableRowIDScan_8          | 8.00    | cop[tikv] | table:t                  | keep order:false, stats:pseudo                         |
+-------------------------------+---------+-----------+--------------------------+--------------------------------------------------------+