SELECT 
  nd.sf_id, 
  nd.name, 
  n.features_hash 
FROM 
  cscart_ab__sf_names n 
  INNER JOIN cscart_ab__sf_name_descriptions nd ON (
    n.sf_id = nd.sf_id 
    AND nd.lang_code = 'en'
  ) 
WHERE 
  n.category_id = 499 
  AND (
    LENGTH(n.features_hash)= 6 
    AND (
      LOCATE('1-12-9', n.features_hash)> 0 
      OR LOCATE('1-9-12', n.features_hash)> 0
    ) 
    AND n.fixed in ('F', 'U')
  )

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "n",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "fixed_category_hash"],
      "key": "fixed_category_hash",
      "key_length": "7",
      "used_key_parts": ["fixed", "category_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "n.category_id = 499 and octet_length(n.features_hash) = 6 and (locate('1-12-9',n.features_hash) > 0 or locate('1-9-12',n.features_hash) > 0) and n.`fixed` in ('F','U')",
      "using_index": true
    },
    "table": {
      "table_name": "nd",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "10",
      "used_key_parts": ["sf_id", "lang_code"],
      "ref": ["devmotorsbay.n.sf_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "nd.lang_code = 'en'"
    }
  }
}