SELECT 
  COUNT(*) 
FROM 
  cscart_pages 
  INNER JOIN cscart_ult_objects_sharing ON (
    cscart_ult_objects_sharing.share_object_id = cscart_pages.page_id 
    AND cscart_ult_objects_sharing.share_company_id = 2 
    AND cscart_ult_objects_sharing.share_object_type = 'pages'
  ) 
WHERE 
  page_id = 51

Query time 0.00019

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE Impossible WHERE noticed after reading const tables

Result

COUNT(*)
0