SELECT 
  cscart_user_session_products.item_id, 
  cscart_user_session_products.item_type, 
  cscart_user_session_products.product_id, 
  cscart_user_session_products.amount, 
  cscart_user_session_products.price, 
  cscart_user_session_products.extra, 
  cscart_product_descriptions.product 
FROM 
  cscart_user_session_products 
  LEFT JOIN cscart_product_descriptions ON cscart_user_session_products.product_id = cscart_product_descriptions.product_id 
  AND cscart_product_descriptions.lang_code = 'uk' 
WHERE 
  cscart_user_session_products.user_id = 686829980 
  AND cscart_user_session_products.type = 'C'

Query time 0.00030

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_user_session_products",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "type"],
          "key": "PRIMARY",
          "key_length": "7",
          "used_key_parts": ["user_id", "type"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "cscart_user_session_products.`type` = 'C'"
        }
      },
      {
        "table": {
          "table_name": "cscart_product_descriptions",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "product_id"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["product_id", "lang_code"],
          "ref": [
            "lillyshop_lillyshop.cscart_user_session_products.product_id",
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "trigcond(cscart_product_descriptions.lang_code = 'uk')"
        }
      }
    ]
  }
}