Use arrays in Pipeline#latest_builds_with_artifacts

This changes Ci::Pipeline#latest_builds_with_artifacts so it returns an
Array instead of a relation. Whenever we use this data we do so in two
steps:

1. Count the number of rows
2. If this number is greater than 0, iterate over the rows

By returning an Array instead we only execute 1 query of which the total
time/work is less than running either just a COUNT(*) or both queries
(in the worst case).

On GitLab.com this change should save us a few milliseconds per request
to ProjectsController#show.

app/models/ci/pipeline.rb

@@ -507,7 +507,10 @@ module Ci
     end
 
     def latest_builds_with_artifacts
-      @latest_builds_with_artifacts ||= builds.latest.with_artifacts
+      # We purposely cast the builds to an Array here. Because we always use the
+      # rows if there are more than 0 this prevents us from having to run two
+      # queries: one to get the count and one to get the rows.
+      @latest_builds_with_artifacts ||= builds.latest.with_artifacts.to_a
     end
 
     private

changelogs/unreleased/reduce-queries-for-artifacts-button.yml

+---
+title: Use arrays in Pipeline#latest_builds_with_artifacts
+merge_request:
+author:
+type: performance

spec/models/ci/pipeline_spec.rb

@@ -1502,6 +1502,10 @@ describe Ci::Pipeline, :mailer do
       create(:ci_build, :success, :artifacts, pipeline: pipeline)
     end
 
+    it 'returns an Array' do
+      expect(pipeline.latest_builds_with_artifacts).to be_an_instance_of(Array)
+    end
+
     it 'returns the latest builds' do
       expect(pipeline.latest_builds_with_artifacts).to eq([build])
     end