SELECT 
  COUNT(*) 
FROM 
  cscart_product_options_inventory 
WHERE 
  product_id = 9841

Query time 0.00082

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_options_inventory ref pc pc 3 const 1189 Using index

Result

COUNT(*)
1792