SELECT 
  * 
FROM 
  cscart_companies AS com 
  LEFT JOIN cscart_products AS prod ON com.company_id = prod.company_id 
WHERE 
  prod.product_id = 348

Query time 0.00056

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "com",
      "access_type": "system",
      "possible_keys": ["PRIMARY"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "prod",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}