mysql - Counting the number of queries performed

Question

I'd like to test that a certain piece of code performs as few SQL queries as possible.

ActiveRecord::TestCase seems to have its own assert_queries method, which will do just that. But since I'm not patching ActiveRecord, it's of little use to me.

Does RSpec or ActiveRecord provide any official, public means of counting the number of SQL queries performed in a block of code?

Answer 1

I think you answered your own question by mentioning assert_queries, but here goes:

I would recommend taking a look at the code behind assert_queries and using that to build your own method which you can use to count queries. The main magic involved here is this line:

ActiveSupport::Notifications.subscribe('sql.active_record', SQLCounter.new)

I had a bit of a tinker this morning and ripped out the parts of ActiveRecord that do the query counting and came up with this:

module ActiveRecord
  class QueryCounter
    cattr_accessor :query_count do
      0
    end

    IGNORED_SQL = [/^PRAGMA (?!(table_info))/, /^SELECT currval/, /^SELECT CAST/, /^SELECT @@IDENTITY/, /^SELECT @@ROWCOUNT/, /^SAVEPOINT/, /^ROLLBACK TO SAVEPOINT/, /^RELEASE SAVEPOINT/, /^SHOW max_identifier_length/]

    def call(name, start, finish, message_id, values)
      # FIXME: this seems bad. we should probably have a better way to indicate
      # the query was cached
      unless 'CACHE' == values[:name]
        self.class.query_count += 1 unless IGNORED_SQL.any? { |r| values[:sql] =~ r }
      end
    end
  end
end

ActiveSupport::Notifications.subscribe('sql.active_record', ActiveRecord::QueryCounter.new)

module ActiveRecord
  class Base
    def self.count_queries(&block)
      ActiveRecord::QueryCounter.query_count = 0
      yield
      ActiveRecord::QueryCounter.query_count
    end
  end
end

You will be able to reference the ActiveRecord::Base.count_queries method anywhere. Pass it a block wherein your queries are run and it will return the number of queries that have been executed:

ActiveRecord::Base.count_queries do
  Ticket.first
end

Returns "1" for me. To make this work: put it in a file at lib/active_record/query_counter.rb and require it in your config/application.rb file like this:

require 'active_record/query_counter'

Hey presto!

---

A little bit of explanation probably is required. When we call this line:

    ActiveSupport::Notifications.subscribe('sql.active_record', ActiveRecord::QueryCounter.new)

We hook into Rails 3's little notifications framework. It's a shiny little addition to the latest major version of Rails that nobody really knows about. It allows us to subscribe to notifications of events within Rails by using the subscribe method. We pass in the event we want to subscribe to as the first argument then any object that responds to call as the second.

In this case when a query is executed our little query counter will dutifully increment the ActiveRecord::QueryCounter.query_count variable, but only for the real queries.

Anyway, this was fun. I hope it comes useful to you.