Count() versus keeping a column with a total count Possible Duplicate:
  Storing vs calculating aggregate values  




Say I have a table full of photos that each have an albumId that is a foreign key to an album table. To get the amount of photos in each album, I could simply do a Count() on the photo table where the albumId = the id from the album table. However, to accomplish this same task, I could keep a photoCount column in the album table. Is it worth keeping that extra column in the album table instead of using the Count() function?
You can also consider materialized views (not all RDMS support this feature though)