[Rails] each vs each_slice vs find_each vs find_in_batches vs in_batches

each

When you call #each, all records will be loaded into memory.

• Loop through an array

a = [ "a", "b", "c" ]
a.each {|x| print x, " -- " }

each_slice

• Loop through an enumerable

• Kind of like each but in batches

(1..10).each_slice(3) { |a| p a }
# outputs below
[1, 2, 3]
[4, 5, 6]
[7, 8, 9]
[10]

find_each

• find_each loads batches (1000 by default) and yield single record in loop
• Each record is just an ActiveRecord

• ActiveRecord method

• Basically, find_each = find_in_batches + each

• Used when too many records that the client can’t load or process

When you call #find_each, records will be loaded into memory in batches of the given batch size.

Person.find_each(:conditions => "age > 21") do |person|
  person.party_all_night!
end

find_in_batches

• find_in_batches loads batches and yield batches in loop
• Each batch is an array

• Used when too many records that the client can’t load or process

Person.where("age > 21").find_in_batches do |group|
  sleep(50) # Make sure it doesn't get too crowded in there!
  group.each { |person| person.party_all_night! }
end

in_batches

• Yield ActiveRecord::Relation

• Might be good if you need to process the entire batch at once but lack of memory

Person.where("age > 21").in_batches do |relation|
  relation.delete_all
  sleep(10) # Throttle the delete queries
end

Source