• Rearrange the data for easy access and/or to avoid extra computation.
• Find a specific item/show it to me.
• Count the number of items that have a specific property.
• Rearrange the data—and don't take too long doing it.
• Find a specific item—fast!
• Count the number of items—quickly!
• Find an item that's probably the one I'm looking for.
• Approximate the number of items fairly accurately and very quickly.