SELECT 
  fav_added 
FROM 
  cscart_fav_merchants 
WHERE 
  `user_id` = 0 
  AND `merchant_id` = 1

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_fav_merchants",
      "access_type": "ALL",
      "rows": 132,
      "filtered": 100,
      "attached_condition": "cscart_fav_merchants.user_id = 0 and cscart_fav_merchants.merchant_id = 1"
    }
  }
}