CREATE TABLE t1 ( a INT );

INSERT INTO t1 VALUES (1),(5);

# t2 must be MyISAM or Aria and contain 1 row

CREATE TABLE t2 ( b INT ) ENGINE=MyISAM;

INSERT INTO t2 VALUES (1);

CREATE TABLE t3 ( c INT );

INSERT INTO t3 VALUES (4),(5);

SET optimizer_switch='subquery_cache=off';

SELECT ( SELECT b FROM t2 WHERE b = a OR EXISTS ( SELECT c FROM t3 WHERE c = b ) ) FROM t1;

mysql> explain format=json SELECT ( SELECT b FROM t2 WHERE b = a OR EXISTS ( SELECT c FROM t3 WHERE c = b ) ) FROM t1\G

*************************** 1. row ***************************

EXPLAIN: {

  "query_block": {

    "select_id": 1,

    "table": {

      "table_name": "t1",

      "access_type": "ALL",

      "rows": 2,

      "filtered": 100

    },

    "subqueries": [

      {

        "query_block": {

          "select_id": 2,

          "table": {

            "table_name": "t2",

            "access_type": "system",

            "rows": 1,

            "filtered": 100

          },

          "subqueries": [

            {

              "query_block": {

                "select_id": 3,

                "table": {

                  "table_name": "t3",

                  "access_type": "ALL",

                  "rows": 2,

                  "filtered": 100,

                  "attached_condition": "1 = t3.c"

                }

              }

            }

          ]

        }

      }

    ]

  }

}