Here's the optimizer trace that indicates that MySQL thinks that the cost for the index scan is higher than the cost for a full table scan. This is of course wrong.

{
  "steps": [
    {
      "join_preparation": {
        "select#": 1,
        "steps": [
          {
            "expanded_query": "/* select#1 */ select `BIG_STORAGE_TOKU`.`EXPIRE_DATE` AS `EXPIRE_DATE` from `BIG_STORAGE_TOKU` where (`BIG_STORAGE_TOKU`.`HASH_ID` = sha2('70164',256))"
          }
        ]
      }
    },
    {
      "join_optimization": {
        "select#": 1,
        "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_dependencies": [
              {
                "table": "`BIG_STORAGE_TOKU`",
                "row_may_be_null": false,
                "map_bit": 0,
                "depends_on_map_bits": [
                ]
              }
            ]
          },
          {
            "ref_optimizer_key_uses": [
              {
                "table": "`BIG_STORAGE_TOKU`",
                "field": "HASH_ID",
                "equals": "sha2('70164',256)",
                "null_rejecting": false
              }
            ]
          },
          {
            "rows_estimation": [
              {
                "table": "`BIG_STORAGE_TOKU`",
                "range_analysis": {
                  "table_scan": {
                    "rows": 100030,
                    "cost": 53351
                  },
                  "potential_range_indices": [
                    {
                      "index": "HASH_ID_IX",
                      "usable": true,
                      "key_parts": [
                        "HASH_ID"
                      ]
                    },
                    {
                      "index": "EXPIRE_DATE_IX",
                      "usable": false,
                      "cause": "not_applicable"
                    }
                  ],
                  "setup_range_conditions": [
                  ],
                  "group_index_range": {
                    "chosen": false,
                    "cause": "not_group_by_or_distinct"
                  },
                  "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": 239680,
                        "cost": 287617,
                        "chosen": false,
                        "cause": "cost"
                      }
                    ],
                    "analyzing_roworder_intersect": {
                      "usable": false,
                      "cause": "too_few_roworder_scans"
                    }
                  }
                }
              }
            ]
          },
          {
            "considered_execution_plans": [
              {
                "plan_prefix": [
                ],
                "table": "`BIG_STORAGE_TOKU`",
                "best_access_path": {
                  "considered_access_paths": [
                    {
                      "access_type": "ref",
                      "index": "HASH_ID_IX",
                      "rows": 239680,
                      "cost": 57939,
                      "chosen": true
                    },
                    {
                      "access_type": "scan",
                      "rows": 75023,
                      "cost": 53349,
                      "chosen": true
                    }
                  ]
                },
                "cost_for_plan": 53349,
                "rows_for_plan": 75023,
                "chosen": true
              }
            ]
          },
          {
            "attaching_conditions_to_tables": {
              "original_condition": "(`BIG_STORAGE_TOKU`.`HASH_ID` = sha2('70164',256))",
              "attached_conditions_computation": [
              ],
              "attached_conditions_summary": [
                {
                  "table": "`BIG_STORAGE_TOKU`",
                  "attached": "(`BIG_STORAGE_TOKU`.`HASH_ID` = sha2('70164',256))"
                }
              ]
            }
          },
          {
            "refine_plan": [
              {
                "table": "`BIG_STORAGE_TOKU`",
                "access_type": "table_scan"
              }
            ]
          }
        ]
      }
    },
    {
      "join_execution": {
        "select#": 1,
        "steps": [
        ]
      }
    }
  ]
}