SELECT 
  sf_id 
FROM 
  cscart_gr__sf_names n 
WHERE 
  n.category_id = 712 
  AND (
    LENGTH(n.features_hash)= 9 
    AND LOCATE('744-13865', n.features_hash)> 0 
    AND n.fixed in ('F', 'U')
  )

Query time 0.00046

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.07"
    },
    "table": {
      "table_name": "n",
      "access_type": "ref",
      "possible_keys": [
        "category_id"
      ],
      "key": "category_id",
      "used_key_parts": [
        "category_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 0,
      "filtered": "20.00",
      "cost_info": {
        "read_cost": "0.97",
        "eval_cost": "0.02",
        "prefix_cost": "1.07",
        "data_read_per_join": "68"
      },
      "used_columns": [
        "sf_id",
        "category_id",
        "features_hash",
        "fixed"
      ],
      "attached_condition": "((length(`dev2`.`n`.`features_hash`) = 9) and (locate('744-13865',`dev2`.`n`.`features_hash`) > 0) and (`dev2`.`n`.`fixed` in ('F','U')))"
    }
  }
}