Add index for compliance merged MRs to events

This helps to improve query time on Compliance Dashboard where we query
for latest merged action events across projects. This change also
benefits count query which is later invoked on the tab headers.

changelogs/unreleased/improve-compliance-merge-request-query.yml

+---
+title: Add index for compliance merged MRs to events
+merge_request: 38885
+author:
+type: performance

db/migrate/20200807070820_add_index_for_compliance_merged_merge_request_to_events.rb

+# frozen_string_literal: true
+
+class AddIndexForComplianceMergedMergeRequestToEvents < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_events_on_project_id_and_id_desc_on_merged_action'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :events, [:project_id, :id],
+      order: { id: :desc },
+      where: "action = 7", name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :events, INDEX_NAME
+  end
+end

db/schema_migrations/20200807070820

+a1fb6b5d486eee27aaed5083aa6340073bf05abab8f201fd9097374dbd345b90
\ No newline at end of file

db/structure.sql

@@ -19572,6 +19572,8 @@ CREATE INDEX index_events_on_project_id_and_created_at ON public.events USING bt
 
 CREATE INDEX index_events_on_project_id_and_id ON public.events USING btree (project_id, id);
 
+CREATE INDEX index_events_on_project_id_and_id_desc_on_merged_action ON public.events USING btree (project_id, id DESC) WHERE (action = 7);
+
 CREATE INDEX index_events_on_target_type_and_target_id ON public.events USING btree (target_type, target_id);
 
 CREATE UNIQUE INDEX index_events_on_target_type_and_target_id_and_fingerprint ON public.events USING btree (target_type, target_id, fingerprint);