create table t0 (a int) engine=myisam;
INSERT INTO t0 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table t1 (a int) engine=myisam;
INSERT INTO t1 select * from t0;

MariaDB [j2]> analyze (select * from t1 A where a<5) union (select * from t1 B where a in (2,3));
+------+--------------+------------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+
| id   | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | r_rows | filtered | r_filtered | Extra       |
+------+--------------+------------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+
|    1 | PRIMARY      | A          | ALL  | NULL          | NULL | NULL    | NULL |   10 |  10.00 |   100.00 |      50.00 | Using where |
|    2 | UNION        | B          | ALL  | NULL          | NULL | NULL    | NULL |   10 |  10.00 |   100.00 |      20.00 | Using where |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL |   5.00 |     NULL |       NULL |             |
+------+--------------+------------+------+---------------+------+---------+------+------+--------+----------+------------+-------------+

MariaDB [j2]> analyze format=json (select * from t1 A where a<5) union (select * from t1 B where a in (2,3))\G
*************************** 1. row ***************************
ANALYZE: {
  "query_block": {
    "union_result": {
      "table_name": "<union1,2>",
      "access_type": "ALL",
      "query_specifications": [
        {
          "query_block": {
            "select_id": 1,
            "r_loops": 1,
            "r_total_time_ms": 0.584,
            "table": {
              "table_name": "A",
              "access_type": "ALL",
              "r_loops": 1,
              "rows": 10,
              "r_rows": 10,
              "r_total_time_ms": 0.4224,
              "filtered": 100,
              "r_filtered": 50,
              "attached_condition": "(A.a < 5)"
            }
          }
        },
        {
          "query_block": {
            "select_id": 2,
            "r_loops": 1,
            "r_total_time_ms": 0.382,
            "table": {
              "table_name": "B",
              "access_type": "ALL",
              "r_loops": 1,
              "rows": 10,
              "r_rows": 10,
              "r_total_time_ms": 0.3193,
              "filtered": 100,
              "r_filtered": 20,
              "attached_condition": "(B.a in (2,3))"
            }
          }
        }
      ]
    }
  }
}