How to optimize total value calculation while retrieving all items in Rails

I'm working on a Rails application where I need to display all items and the total value of those items.
Curently, in my ItemsController#index, I retrieve all items with @items = current_user.items and I want to calculate the total value.

I have considered two approaches:

1. Using a query like @total_value = @items.sum(:value)
2. Calculating the sum in Ruby with @total_value = @items.sum(&:value)

I want to understand the pros and cons of these apporaches in terms of performance and efficiency.

Is it better to use an additional query for summing values or is it acceptable to calculate the sum in Ruby?

Are there any best practices or alternative solutions to optimize this process?

I would appreciate any answer. Thanks!

Welcome to SO!

I think, given the question you're asking, you should back up one step.

@items = current_user.items is loading all item records into memory after a query of the database.

Once they are all in memory, you are considering two very similar options: re-query or loop.

Both those options are essentially the same in terms of big O notation. You've already fetched all the records, now you want to act on all of them.

If the only thing you need from @items is the sum of the value, your best approach is to only get that:

current_user.items.sum(:value) is a single SQL call:

> current_user.items.sum(:value).to_sql
=> SELECT SUM("items."value") FROM "items" where "items"."user_id" = 1

In terms of the two options, the factors at play are:

• your database and how fast it is at SUM vs. how fast Ruby is at #sum.
  • in deployment, you'll need to also think about the additional overhead of the round-trip call between the server and the database and whether that outweighs any speed boost from SUM
• the number of records you're loading into memory, as that will have an effect on #sum (and later garbage collection)