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diff --git a/doc/development/database/query_recorder.md b/doc/development/database/query_recorder.md new file mode 100644 index 00000000000..da5c6c8e6cb --- /dev/null +++ b/doc/development/database/query_recorder.md @@ -0,0 +1,145 @@ +--- +stage: Data Stores +group: Database +info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments +--- + +# QueryRecorder + +QueryRecorder is a tool for detecting the [N+1 queries problem](https://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/-/blob/master/spec/support/helpers/query_recorder.rb) via [9c623e3e](https://gitlab.com/gitlab-org/gitlab-foss/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 can 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 = ActiveRecord::QueryRecorder.new { visit_some_page } + create_list(:issue, 5) + expect { visit_some_page }.not_to exceed_query_limit(control) +end +``` + +You can if you wish, have both the expectation and the control as +`QueryRecorder` instances: + +```ruby +it "avoids N+1 database queries" do + control = ActiveRecord::QueryRecorder.new { visit_some_page } + create_list(:issue, 5) + action = ActiveRecord::QueryRecorder.new { visit_some_page } + + expect(action).not_to exceed_query_limit(control) +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. + +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. + +If this test fails, and the control was passed as a `QueryRecorder`, then the +failure message indicates where the extra queries are by matching queries on +the longest common prefix, grouping similar queries together. + +## Cached queries + +By default, QueryRecorder ignores [cached queries](../merge_request_performance_guidelines.md#cached-queries) in the count. However, it may be better to count +all queries to avoid introducing an N+1 query that may be masked by the statement cache. +To do this, this requires the `:use_sql_query_cache` flag to be set. +You should pass the `skip_cached` variable to `QueryRecorder` and use the `exceed_all_query_limit` matcher: + +```ruby +it "avoids N+1 database queries", :use_sql_query_cache do + control = ActiveRecord::QueryRecorder.new(skip_cached: false) { visit_some_page } + create_list(:issue, 5) + expect { visit_some_page }.not_to exceed_all_query_limit(control) +end +``` + +## Use request specs instead of controller specs + +Use a [request spec](https://gitlab.com/gitlab-org/gitlab-foss/tree/master/spec/requests) when writing a N+1 test on the controller level. + +Controller specs should not be used to write N+1 tests as the controller is only initialized once per example. +This could lead to false successes where subsequent "requests" could have queries reduced (for example, because of memoization). + +## Finding the source of the query + +There are multiple ways to find the source of queries. + +- Inspect the `QueryRecorder` `data` attribute. It stores queries by `file_name:line_number:method_name`. + Each entry is a `hash` with the following fields: + + - `count`: the number of times a query from this `file_name:line_number:method_name` was called + - `occurrences`: the actual `SQL` of each call + - `backtrace`: the stack trace of each call (if either of the two following options were enabled) + + `QueryRecorder#find_query` allows filtering queries by their `file_name:line_number:method_name` and + `count` attributes. For example: + + ```ruby + control = ActiveRecord::QueryRecorder.new(skip_cached: false) { visit_some_page } + control.find_query(/.*note.rb.*/, 0, first_only: true) + ``` + + `QueryRecorder#occurrences_by_line_method` returns a sorted array based on `data`, sorted by `count`. + +- View the call backtrace for the specific `QueryRecorder` instance you want + by using `ActiveRecord::QueryRecorder.new(query_recorder_debug: true)`. The output + is stored in file `test.log`. + +- Enable the call backtrace for all tests using the `QUERY_RECORDER_DEBUG` environment variable. + + To enable this, run the specs with the `QUERY_RECORDER_DEBUG` environment variable set. For example: + + ```shell + QUERY_RECORDER_DEBUG=1 bundle exec rspec spec/requests/api/projects_spec.rb + ``` + + This logs calls to QueryRecorder into the `test.log` file. For example: + + ```sql + QueryRecorder SQL: SELECT COUNT(*) FROM "issues" WHERE "issues"."deleted_at" IS NULL AND "issues"."project_id" = $1 AND ("issues"."state" IN ('opened')) AND "issues"."confidential" = $2 + --> /home/user/gitlab/gdk/gitlab/spec/support/query_recorder.rb:19:in `callback' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/fanout.rb:127:in `finish' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/fanout.rb:46:in `block in finish' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/fanout.rb:46:in `each' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/fanout.rb:46:in `finish' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/instrumenter.rb:36:in `finish' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications/instrumenter.rb:25:in `instrument' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract_adapter.rb:478:in `log' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:601:in `exec_cache' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/postgresql_adapter.rb:585:in `execute_and_clear' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/postgresql/database_statements.rb:160:in `exec_query' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract/database_statements.rb:356:in `select' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract/database_statements.rb:32:in `select_all' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract/query_cache.rb:68:in `block in select_all' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract/query_cache.rb:83:in `cache_sql' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/connection_adapters/abstract/query_cache.rb:68:in `select_all' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/relation/calculations.rb:270:in `execute_simple_calculation' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/relation/calculations.rb:227:in `perform_calculation' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/relation/calculations.rb:133:in `calculate' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activerecord-4.2.8/lib/active_record/relation/calculations.rb:48:in `count' + --> /home/user/gitlab/gdk/gitlab/app/services/base_count_service.rb:20:in `uncached_count' + --> /home/user/gitlab/gdk/gitlab/app/services/base_count_service.rb:12:in `block in count' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:299:in `block in fetch' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:585:in `block in save_block_result_to_cache' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:547:in `block in instrument' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/notifications.rb:166:in `instrument' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:547:in `instrument' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:584:in `save_block_result_to_cache' + --> /home/user/.rbenv/versions/2.3.5/lib/ruby/gems/2.3.0/gems/activesupport-4.2.8/lib/active_support/cache.rb:299:in `fetch' + --> /home/user/gitlab/gdk/gitlab/app/services/base_count_service.rb:12:in `count' + --> /home/user/gitlab/gdk/gitlab/app/models/project.rb:1296:in `open_issues_count' + ``` + +## See also + +- [Bullet](../profiling.md#bullet) For finding `N+1` query problems +- [Performance guidelines](../performance.md) +- [Merge request performance guidelines - Query counts](../merge_request_performance_guidelines.md#query-counts) +- [Merge request performance guidelines - Cached queries](../merge_request_performance_guidelines.md#cached-queries) |