No equality substitution for references out of the subqueries.

== Test dataset ==
create table t10 (a int, b int, c int);
insert into t10 select seq, seq, seq from seq_1_to_10;
create table t11 like t10;
insert into t11 select seq, seq, seq from seq_1_to_20;

create table t20 as select * from t10;
create table t21 as select * from t11;


== Example 1 ==

- "t11.a > (SELECT ... )" refers to t11
- The subquery refers to t10.

Note that "t11.a > (SELECT...)" became "t10.a > (SELECT...)"

which allowed to attach the subquery to table t10.

explain format=json
select * from t10, t11 
where
  t10.a=t11.a and
  t11.a >  (select max(t21.a)
            from 
              t20, t21
            where
              t20.b=t21.b and
              t20.c >=t10.a)\G

EXPLAIN: {
  "query_block": {
    "select_id": 1,
    "cost": 0.04578146,
    "nested_loop": [
      {
        "table": {
          "table_name": "t10",
          "access_type": "ALL",
          "loops": 1,
          "rows": 10,
          "cost": 0.0124848,
          "filtered": 100,
          "attached_condition": "t10.a > (subquery#2)"
        }
      },
      {
        "block-nl-join": {
          "table": {
            "table_name": "t11",
            "access_type": "ALL",
            "loops": 10,
            "rows": 20,
            "cost": 0.03329666,
            "filtered": 100
          },
          "buffer_type": "flat",
          "buffer_size": "173",
          "join_type": "BNL",
          "attached_condition": "t11.a = t10.a"
        }
      }
    ],
    "subqueries": [
      {
        "expression_cache": {
          "state": "uninitialized",
          "query_block": {
            "select_id": 2,
            "cost": 0.04578146,
            "nested_loop": [
              {
                "table": {
                  "table_name": "t20",
                  "access_type": "ALL",
                  "loops": 1,
                  "rows": 10,
                  "cost": 0.0124848,
                  "filtered": 100,
                  "attached_condition": "t20.c >= t10.a"
                }
              },
              {
                "block-nl-join": {
                  "table": {
                    "table_name": "t21",
                    "access_type": "ALL",
                    "loops": 10,
                    "rows": 20,
                    "cost": 0.03329666,
                    "filtered": 100
                  },
                  "buffer_type": "flat",
                  "buffer_size": "119",
                  "join_type": "BNL",
                  "attached_condition": "t21.b = t20.b"
                }
              }
            ]
          }
        }
      }
    ]
  }
}
1 row in set (0,001 sec)

== Example 2 ==
- "t11.a > (SELECT ... )" refers to t11
- The subquery refers to t11.

In EXPLAIN, note that 
- "t11.a > (SELECT...)" became "t10.a > (SELECT...)"
- But the subquery still has  t21.c >= t11.a.
It did NOT became "t21.c >= t10.a"...

explain format=json
select * from t10, t11 
where
  t10.a=t11.a and
  t11.a >  (select max(t21.a)
            from 
              t20, t21
            where
              t20.b=t21.b and
              t21.c >=t11.a)\G

EXPLAIN: {
  "query_block": {
    "select_id": 1,
    "cost": 0.04578146,
    "nested_loop": [
      {
        "table": {
          "table_name": "t10",
          "access_type": "ALL",
          "loops": 1,
          "rows": 10,
          "cost": 0.0124848,
          "filtered": 100
        }
      },
      {
        "block-nl-join": {
          "table": {
            "table_name": "t11",
            "access_type": "ALL",
            "loops": 10,
            "rows": 20,
            "cost": 0.03329666,
            "filtered": 100
          },
          "buffer_type": "flat",
          "buffer_size": "173",
          "join_type": "BNL",
          "attached_condition": "t11.a = t10.a and t10.a > (subquery#2)"
        }
      }
    ],
    "subqueries": [
      {
        "expression_cache": {
          "state": "uninitialized",
          "query_block": {
            "select_id": 2,
            "cost": 0.04578146,
            "nested_loop": [
              {
                "table": {
                  "table_name": "t20",
                  "access_type": "ALL",
                  "loops": 1,
                  "rows": 10,
                  "cost": 0.0124848,
                  "filtered": 100
                }
              },
              {
                "block-nl-join": {
                  "table": {
                    "table_name": "t21",
                    "access_type": "ALL",
                    "loops": 10,
                    "rows": 20,
                    "cost": 0.03329666,
                    "filtered": 100
                  },
                  "buffer_type": "flat",
                  "buffer_size": "65",
                  "join_type": "BNL",
                  "attached_condition": "t21.b = t20.b and t21.c >= t11.a"
                }
              }
            ]
          }
        }
      }
    ]
  }
}
1 row in set (0,001 sec)