Improve performance of the cycle analytics page.

1. These changes bring down page load time for 100 issues from more than
   a minute to about 1.5 seconds.

2. This entire commit is composed of these types of performance
   enhancements:

     - Cache relevant data in `IssueMetrics` wherever possible.
     - Cache relevant data in `MergeRequestMetrics` wherever possible.
     - Preload metrics

3. Given these improvements, we now only need to make 4 SQL calls:

    - Load all issues
    - Load all merge requests
    - Load all metrics for the issues
    - Load all metrics for the merge requests

4. A list of all the data points that are now being pre-calculated:

    a. The first time an issue is mentioned in a commit

      - In `GitPushService`, find all issues mentioned by the given commit
        using `ReferenceExtractor`. Set the `first_mentioned_in_commit_at`
        flag for each of them.

      - There seems to be a (pre-existing) bug here - files (and
        therefore commits) created using the Web CI don't have
        cross-references created, and issues are not closed even when
        the commit title is "Fixes #xx".

    b. The first time a merge request is deployed to production

      When a `Deployment` is created, find all merge requests that
      were merged in before the deployment, and set the
      `first_deployed_to_production_at` flag for each of them.

    c. The start / end time for a merge request pipeline

      Hook into the `Pipeline` state machine. When the `status` moves to
      `running`, find the merge requests whose tip commit matches the
      pipeline, and record the `latest_build_started_at` time for each
      of them. When the `status` moves to `success`, record the
      `latest_build_finished_at` time.

    d. The merge requests that close an issue

      - This was a big cause of the performance problems we were having
        with Cycle Analytics. We need to use `ReferenceExtractor` to make
        this calculation, which is slow when we have to run it on a large
        number of merge requests.

      - When a merge request is created, updated, or refreshed, find the
        issues it closes, and create an instance of
        `MergeRequestsClosingIssues`, which acts as a join model between
        merge requests and issues.

      - If a `MergeRequestsClosingIssues` instance links a merge request
        and an issue, that issue closes that merge request.

5. The `Queries` module was changed into a class, so we can cache the
   results of `issues` and `merge_requests_closing_issues` across
   various cycle analytics stages.

6. The code added in this commit is untested. Tests will be added in the
   next commit.

1 files changed, 18 insertions, 18 deletions

diff --git a/app/models/cycle_analytics.rb b/app/models/cycle_analytics.rb
index 9444f1f31a8..17a80115fe5 100644
--- a/app/models/cycle_analytics.rb
+++ b/app/models/cycle_analytics.rb
@@ -2,8 +2,8 @@ class CycleAnalytics
   attr_reader :from
 
   def initialize(project, from:)
-    @project = project
     @from = from
+    @queries = Queries.new(project)
   end
 
   def as_json(options = {})
@@ -14,45 +14,45 @@ class CycleAnalytics
   end
 
   def issue
-    calculate_metric(Queries::issues(@project, created_after: @from),
+    calculate_metric(@queries.issues(created_after: @from),
                      -> (data_point) { data_point[:issue].created_at },
-                     [Queries::issue_first_associated_with_milestone_at, Queries::issue_first_added_to_list_label_at])
+                     [@queries.issue_first_associated_with_milestone_at, @queries.issue_first_added_to_list_label_at])
   end
 
   def plan
-    calculate_metric(Queries::issues(@project, created_after: @from),
-                     [Queries::issue_first_associated_with_milestone_at, Queries::issue_first_added_to_list_label_at],
-                     Queries::issue_first_mentioned_in_commit_at)
+    calculate_metric(@queries.issues(created_after: @from),
+                     [@queries.issue_first_associated_with_milestone_at, @queries.issue_first_added_to_list_label_at],
+                     @queries.issue_first_mentioned_in_commit_at)
   end
 
   def code
-    calculate_metric(Queries::merge_requests_closing_issues(@project, created_after: @from),
-                     Queries::issue_first_mentioned_in_commit_at,
+    calculate_metric(@queries.merge_requests_closing_issues(created_after: @from),
+                     @queries.issue_first_mentioned_in_commit_at,
                      -> (data_point) { data_point[:merge_request].created_at })
   end
 
   def test
-    calculate_metric(Queries::merge_requests_closing_issues(@project, created_after: @from),
-                     Queries::merge_request_build_started_at,
-                     Queries::merge_request_build_finished_at)
+    calculate_metric(@queries.merge_requests_closing_issues(created_after: @from),
+                     @queries.merge_request_build_started_at,
+                     @queries.merge_request_build_finished_at)
   end
 
   def review
-    calculate_metric(Queries::merge_requests_closing_issues(@project, created_after: @from),
+    calculate_metric(@queries.merge_requests_closing_issues(created_after: @from),
                      -> (data_point) { data_point[:merge_request].created_at },
-                     Queries::merge_request_merged_at)
+                     @queries.merge_request_merged_at)
   end
 
   def staging
-    calculate_metric(Queries::merge_requests_closing_issues(@project, created_after: @from),
-                     Queries::merge_request_merged_at,
-                     Queries::merge_request_deployed_to_production_at)
+    calculate_metric(@queries.merge_requests_closing_issues(created_after: @from),
+                     @queries.merge_request_merged_at,
+                     @queries.merge_request_deployed_to_production_at)
   end
 
   def production
-    calculate_metric(Queries::merge_requests_closing_issues(@project, created_after: @from),
+    calculate_metric(@queries.merge_requests_closing_issues(created_after: @from),
                      -> (data_point) { data_point[:issue].created_at },
-                     Queries::merge_request_deployed_to_production_at)
+                     @queries.merge_request_deployed_to_production_at)
   end
 
   private