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 = 510 
  AND (
    LENGTH(n.features_hash)= 7 
    AND LOCATE('29-1856', n.features_hash)> 0 
    AND n.fixed in ('F', 'U')
  ) 
  AND n.company_id = 1

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 61,
          "filtered": 100,
          "index_condition": "n.category_id = 510 and octet_length(n.features_hash) = 7 and locate('29-1856',n.features_hash) > 0 and n.`fixed` in ('F','U')",
          "attached_condition": "n.company_id = 1"
        }
      },
      {
        "table": {
          "table_name": "nd",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "10",
          "used_key_parts": ["sf_id", "lang_code"],
          "ref": ["lillyshop_lillyshop.n.sf_id", "const"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "nd.lang_code = 'en'"
        }
      }
    ]
  }
}