SELECT 
  `hash` 
FROM 
  `hash_filters` 
WHERE 
  `link` = 'gaz-sobol-nn-bortovoy-gaz-a21s12-.'

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "hash_filters",
      "access_type": "ALL",
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.10",
        "prefix_cost": "0.35",
        "data_read_per_join": "5K"
      },
      "used_columns": [
        "hash",
        "link"
      ],
      "attached_condition": "(`dev2`.`hash_filters`.`link` = 'gaz-sobol-nn-bortovoy-gaz-a21s12-.')"
    }
  }
}