Adds docs for QueryRecorder tests

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+# QueryRecorder
+
+QueryRecorder is a tool for detecting the [N+1 queries problem](http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations) from tests.
+
+> Implemented in [spec/support/query_recorder.rb](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/support/query_recorder.rb) via [9c623e3e](https://gitlab.com/gitlab-org/gitlab-ce/commit/9c623e3e5d7434f2e30f7c389d13e5af4ede770a)
+
+As a rule, merge requests [should not increase query counts](merge_request_performance_guidelines.md#query-counts). If you find yourself adding something like `.includes(:author, :assignee)` to avoid having `N+1` queries, consider using QueryRecorder to enforce this with a test. Without this, a new feature which causes an additional model to be accessed will silently reintroduce the problem.
+
+## How it works
+
+This style of test works by counting the number of SQL queries executed by ActiveRecord. First a control count is taken, then you add new records to the database and rerun the count. If the number of queries has significantly increased then an `N+1` queries problem exists.
+
+```ruby
+it "avoids N+1 database queries" do
+  control_count = ActiveRecord::QueryRecorder.new { visit_some_page }.count
+  create_list(:issue, 5)
+  expect { visit_some_page }.not_to exceed_query_limit(control_count)
+end
+```
+
+As an example you might create 5 issues in between counts, which would cause the query count to increase by 5 if an N+1 problem exists.
+
+> **Note:** In some cases the query count might change slightly between runs for unrelated reasons. In this case you might need to test `exceed_query_limit(control_count + acceptable_change)`, but this should be avoided if possible.
+
+## See also
+
+- [Bullet](profiling.md#Bullet) For finding `N+1` query problems
+- [Performance guidelines](performance.md)
+- [Merge request performance guidelines](merge_request_performance_guidelines.md#query-counts)
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