Use upsert for MR metrics

app/models/merge_request.rb

@@ -1853,6 +1853,9 @@ class MergeRequest < ApplicationRecord
 
   override :ensure_metrics
   def ensure_metrics
+    if Feature.enabled?(:use_upsert_query_for_mr_metrics)
+      MergeRequest::Metrics.record!(self)
+    else
       # Backward compatibility: some merge request metrics records will not have target_project_id filled in.
       # In that case the first `safe_find_or_create_by` will return false.
       # The second finder call will be eliminated in https://gitlab.com/gitlab-org/gitlab/-/issues/233507
@@ -1874,6 +1877,7 @@ class MergeRequest < ApplicationRecord
         association(:metrics).target = metrics_record
       end
     end
+  end
 
   def allows_reviewers?
     true

app/models/merge_request/metrics.rb

@@ -14,10 +14,25 @@ class MergeRequest::Metrics < ApplicationRecord
   scope :with_valid_time_to_merge, -> { where(arel_table[:merged_at].gt(arel_table[:created_at])) }
   scope :by_target_project, ->(project) { where(target_project_id: project) }
 
-  def self.time_to_merge_expression
+  class << self
+    def time_to_merge_expression
       Arel.sql('EXTRACT(epoch FROM SUM(AGE(merge_request_metrics.merged_at, merge_request_metrics.created_at)))')
     end
 
+    def record!(mr)
+      sql = <<~SQL
+        INSERT INTO #{self.table_name} (merge_request_id, target_project_id, updated_at, created_at)
+        VALUES (#{mr.id}, #{mr.target_project_id}, NOW(), NOW())
+        ON CONFLICT (merge_request_id)
+        DO UPDATE SET
+        target_project_id = EXCLUDED.target_project_id,
+        updated_at = NOW()
+      SQL
+
+      connection.execute(sql)
+    end
+  end
+
   private
 
   def ensure_target_project_id

config/feature_flags/development/use_upsert_query_for_mr_metrics.yml

+---
+name: use_upsert_query_for_mr_metrics
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69240
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339677
+milestone: '14.3'
+type: development
+group: group::optimize
+default_enabled: false

ee/lib/analytics/merge_request_metrics_refresh.rb

@@ -8,7 +8,9 @@ module Analytics
 
     def execute(force: false)
       merge_requests.each do |mr|
-        metrics = mr.ensure_metrics
+        mr.ensure_metrics
+        mr.reset # clear already loaded (nil) metrics association
+        metrics = mr.metrics
 
         next if !force && metric_already_present?(metrics)