SELECT 
  COUNT(*) 
FROM 
  cscart_vendor_following 
WHERE 
  user_id = 0 
  AND vendor_id = 120

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}