With large data set we can run into memory issue. Here is an example.
>> Post.published.count
=> 25000
>> Post.where(published: true).each do |post|
post.archive!
end
# Loads 25000 posts in memory
Rails 5 adds warning when loading large data set
To mitigate issue shown above Rails 5 has added config.active_record.warn_on_records_fetched_greater_than. When this configuration is set to an integer value, any query that returns the number of records greater than the set limit, logs a warning.
config.active_record.warn_on_records_fetched_greater_than = 1500
>> Post.where(published: true).each do |post|
post.archive!
end
=> Query fetched 25000 Post records: SELECT "posts".* FROM "posts" WHERE "posts"."published" = ? [["published", true]]
[#<Post id: 1, title: 'Rails', user_id: 1, created_at: "2016-02-11 11:32:32", updated_at: "2016-02-11 11:32:32", published: true>, #<Post id: 2, title: 'Ruby', user_id: 2, created_at: "2016-02-11 11:36:05", updated_at: "2016-02-11 11:36:05", published: true>,....]
This helps us find areas where potential problems exist and then we can replace inefficient queries with better ones.
config.active_record.warn_on_records_fetched_greater_than = 1500
>> Post.where(published: true).find_each do |post|
post.archive!
end
# No warning is logged
Posted by Alexander M to Ruby and RoR knowledge base (2016-05-02 15:50)