There are some records in the dataset which trigger the table scan effect, and other which do not:

dbcache05>explain DELETE FROM BIG_STORAGE_TOKU  WHERE HASH_ID = SHA2('30398', 256);
+----+-------------+------------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table            | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+------------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | BIG_STORAGE_TOKU | ALL  | HASH_ID_IX    | NULL | NULL    | NULL | 100014 | Using where |
+----+-------------+------------------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

dbcache05>explain DELETE FROM BIG_STORAGE_TOKU  WHERE HASH_ID = SHA2('70164', 256);
+----+-------------+------------------+-------+---------------+------------+---------+-------+------+-------------+
| id | select_type | table            | type  | possible_keys | key        | key_len | ref   | rows | Extra       |
+----+-------------+------------------+-------+---------------+------------+---------+-------+------+-------------+
|  1 | SIMPLE      | BIG_STORAGE_TOKU | range | HASH_ID_IX    | HASH_ID_IX | 64      | const |   14 | Using where |
+----+-------------+------------------+-------+---------------+------------+---------+-------+------+-------------+
1 row in set (0.00 sec)



dbcache05>SET optimizer_trace='enabled=on';
Query OK, 0 rows affected (0.00 sec)

dbcache05>DELETE FROM BIG_STORAGE_TOKU  WHERE HASH_ID = SHA2('70164', 256);
Query OK, 1 row affected (0.00 sec)

dbcache05>SELECT trace FROM information_schema.optimizer_trace \G
*************************** 1. row ***************************
trace: {
  "steps": [
    {
      "condition_processing": {
        "condition": "WHERE",
        "original_condition": "(`BIG_STORAGE_TOKU`.`HASH_ID` = sha2('70164',256))",
        "steps": [
          {
            "transformation": "equality_propagation",
            "resulting_condition": "multiple equal(sha2('70164',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          },
          {
            "transformation": "constant_propagation",
            "resulting_condition": "multiple equal(sha2('70164',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          },
          {
            "transformation": "trivial_condition_removal",
            "resulting_condition": "multiple equal(sha2('70164',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          }
        ]
      }
    },
    {
      "table": "`BIG_STORAGE_TOKU`",
      "range_analysis": {
        "table_scan": {
          "rows": 100022,
          "cost": 53347
        },
        "potential_range_indices": [
          {
            "index": "HASH_ID_IX",
            "usable": true,
            "key_parts": [
              "HASH_ID"
            ]
          },
          {
            "index": "EXPIRE_DATE_IX2",
            "usable": true,
            "key_parts": [
              "EXPIRE_DATE"
            ]
          }
        ],
        "setup_range_conditions": [
        ],
        "group_index_range": {
          "chosen": false,
          "cause": "no_join"
        },
        "analyzing_range_alternatives": {
          "range_scan_alternatives": [
            {
              "index": "HASH_ID_IX",
              "ranges": [
                "ffff925a44dfc908915841bdafe81d3ee3b5238610e4901d0d806e4740881084 <= HASH_ID <= ffff925a44dfc908915841bdafe81d3ee3b5238610e4901d0d806e4740881084"
              ],
              "index_dives_for_eq_ranges": true,
              "rowid_ordered": true,
              "using_mrr": false,
              "index_only": false,
              "rows": 14,
              "cost": 17.81,
              "chosen": true
            }
          ]
        },
        "chosen_range_access_summary": {
          "range_access_plan": {
            "type": "range_scan",
            "index": "HASH_ID_IX",
            "rows": 14,
            "ranges": [
              "ffff925a44dfc908915841bdafe81d3ee3b5238610e4901d0d806e4740881084 <= HASH_ID <= ffff925a44dfc908915841bdafe81d3ee3b5238610e4901d0d806e4740881084"
            ]
          },
          "rows_for_plan": 14,
          "cost_for_plan": 17.81,
          "chosen": true
        }
      }
    }
  ]
}
1 row in set (0.00 sec)

dbcache05>DELETE FROM BIG_STORAGE_TOKU  WHERE HASH_ID = SHA2('30398', 256);
Query OK, 1 row affected (2.40 sec)

dbcache05>SELECT trace FROM information_schema.optimizer_trace \G
*************************** 1. row ***************************
trace: {
  "steps": [
    {
      "condition_processing": {
        "condition": "WHERE",
        "original_condition": "(`BIG_STORAGE_TOKU`.`HASH_ID` = sha2('30398',256))",
        "steps": [
          {
            "transformation": "equality_propagation",
            "resulting_condition": "multiple equal(sha2('30398',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          },
          {
            "transformation": "constant_propagation",
            "resulting_condition": "multiple equal(sha2('30398',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          },
          {
            "transformation": "trivial_condition_removal",
            "resulting_condition": "multiple equal(sha2('30398',256), `BIG_STORAGE_TOKU`.`HASH_ID`)"
          }
        ]
      }
    },
    {
      "table": "`BIG_STORAGE_TOKU`",
      "range_analysis": {
        "table_scan": {
          "rows": 100021,
          "cost": 53347
        },
        "potential_range_indices": [
          {
            "index": "HASH_ID_IX",
            "usable": true,
            "key_parts": [
              "HASH_ID"
            ]
          },
          {
            "index": "EXPIRE_DATE_IX2",
            "usable": true,
            "key_parts": [
              "EXPIRE_DATE"
            ]
          }
        ],
        "setup_range_conditions": [
        ],
        "group_index_range": {
          "chosen": false,
          "cause": "no_join"
        },
        "analyzing_range_alternatives": {
          "range_scan_alternatives": [
            {
              "index": "HASH_ID_IX",
              "ranges": [
                "ffff8ed56f65caf0019f90d65b7f158b862efcab5c4517a76c01c73acf92d99b <= HASH_ID <= ffff8ed56f65caf0019f90d65b7f158b862efcab5c4517a76c01c73acf92d99b"
              ],
              "index_dives_for_eq_ranges": true,
              "rowid_ordered": true,
              "using_mrr": false,
              "index_only": false,
              "rows": 181860,
              "cost": 218233,
              "chosen": false,
              "cause": "cost"
            }
          ]
        }
      }
    }
  ]
}
1 row in set (0.01 sec)