SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(273, 912, 278, 276)

Query time 0.00057

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_categories range parent parent 3 11 Using index condition

Result

category_id parent_id
274 273
296 273
275 273
277 273
352 273
383 276
914 276
370 278
873 278
905 278
906 278
913 912