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.