SELECT 
  city_id, 
  city 
FROM 
  `cscart_custom_city` 
WHERE 
  `fias_id` = '0c5b2444-70a0-4932-980c-b4dc0d3f02b5'

Query time 0.00033

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.35"
    },
    "table": {
      "table_name": "cscart_custom_city",
      "access_type": "ref",
      "possible_keys": [
        "idx_custom_city_fias_id"
      ],
      "key": "idx_custom_city_fias_id",
      "used_key_parts": [
        "fias_id"
      ],
      "key_length": "386",
      "ref": [
        "const"
      ],
      "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": [
        "city_id",
        "city",
        "fias_id"
      ]
    }
  }
}