Improve query performance of merged_at filter

This change significantly improves the performance of productivity
analytics and throughput analytics features.

app/finders/concerns/merged_at_filter.rb

@@ -11,7 +11,9 @@ module MergedAtFilter
     mr_metrics_scope = mr_metrics_scope.merged_after(merged_after) if merged_after.present?
     mr_metrics_scope = mr_metrics_scope.merged_before(merged_before) if merged_before.present?
 
-    items.joins(:metrics).merge(mr_metrics_scope)
+    scope = items.joins(:metrics).merge(mr_metrics_scope)
+    scope = target_project_id_filter_on_metrics(scope) if Feature.enabled?(:improved_mr_merged_at_queries)
+    scope
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
@@ -22,4 +24,10 @@ module MergedAtFilter
   def merged_before
     params[:merged_before]
   end
+
+  # rubocop: disable CodeReuse/ActiveRecord
+  def target_project_id_filter_on_metrics(scope)
+    scope.where(MergeRequest.arel_table[:target_project_id].eq(MergeRequest::Metrics.arel_table[:target_project_id]))
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
 end

config/feature_flags/development/improved_mr_merged_at_queries.yml

+---
+name: improved_mr_merged_at_queries
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39329
+rollout_issue_url: 
+group: group::analytics
+type: development
+default_enabled: false

db/migrate/20200812112204_add_index_to_mr_metrics_target_project_id.rb

+# frozen_string_literal: true
+
+class AddIndexToMrMetricsTargetProjectId < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_merge_request_metrics_on_target_project_id_merged_at'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_request_metrics, [:target_project_id, :merged_at], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name(:merge_request_metrics, INDEX_NAME)
+  end
+end

db/schema_migrations/20200812112204

+4fb7fda59db193250ca393cae5561fa52d61e025bb93b10829f114ac7fef4cfa
\ No newline at end of file

db/structure.sql

@@ -19990,6 +19990,8 @@ CREATE INDEX index_merge_request_metrics_on_pipeline_id ON public.merge_request_
 
 CREATE INDEX index_merge_request_metrics_on_target_project_id ON public.merge_request_metrics USING btree (target_project_id);
 
+CREATE INDEX index_merge_request_metrics_on_target_project_id_merged_at ON public.merge_request_metrics USING btree (target_project_id, merged_at);
+
 CREATE UNIQUE INDEX index_merge_request_user_mentions_on_note_id ON public.merge_request_user_mentions USING btree (note_id) WHERE (note_id IS NOT NULL);
 
 CREATE INDEX index_merge_requests_closing_issues_on_issue_id ON public.merge_requests_closing_issues USING btree (issue_id);