Optimise index on audit events for CSV export

To export large number of audit events record (> 10k) with filter, the
existing index is not efficient, especially with date range filter. This
change will add `created_at` field to the existing index and help
reducing the query time significantly.

changelogs/unreleased/1449-add-created-at-index-on-audit-events.yml

+---
+title: Optimise index on audit events for CSV export
+merge_request: 41266
+author:
+type: added

db/post_migrate/20200903064431_add_created_at_index_to_audit_events.rb

+# frozen_string_literal: true
+
+class AddCreatedAtIndexToAuditEvents < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'idx_audit_events_on_entity_id_desc_author_id_created_at'
+  OLD_INDEX_NAME = 'index_audit_events_on_entity_id_entity_type_id_desc_author_id'
+
+  def up
+    add_concurrent_index(:audit_events, [:entity_id, :entity_type, :id, :author_id, :created_at], order: { id: :desc }, name: INDEX_NAME)
+    remove_concurrent_index_by_name(:audit_events, OLD_INDEX_NAME)
+  end
+
+  def down
+    add_concurrent_index(:audit_events, [:entity_id, :entity_type, :id, :author_id], order: { id: :desc }, name: OLD_INDEX_NAME)
+    remove_concurrent_index_by_name(:audit_events, INDEX_NAME)
+  end
+end

db/schema_migrations/20200903064431

+5c065dc7905fd1292e270d2248810d71fa71d6b6996e9d60c463a7eb36042881
\ No newline at end of file

db/structure.sql

@@ -19075,6 +19075,8 @@ CREATE UNIQUE INDEX epic_user_mentions_on_epic_id_and_note_id_index ON public.ep
 
 CREATE UNIQUE INDEX epic_user_mentions_on_epic_id_index ON public.epic_user_mentions USING btree (epic_id) WHERE (note_id IS NULL);
 
+CREATE INDEX idx_audit_events_on_entity_id_desc_author_id_created_at ON public.audit_events USING btree (entity_id, entity_type, id DESC, author_id, created_at);
+
 CREATE INDEX idx_ci_pipelines_artifacts_locked ON public.ci_pipelines USING btree (ci_ref_id, id) WHERE (locked = 1);
 
 CREATE INDEX idx_container_scanning_findings ON public.vulnerability_occurrences USING btree (id) WHERE (report_type = 2);
@@ -19267,8 +19269,6 @@ CREATE INDEX index_approvers_on_user_id ON public.approvers USING btree (user_id
 
 CREATE UNIQUE INDEX index_atlassian_identities_on_extern_uid ON public.atlassian_identities USING btree (extern_uid);
 
-CREATE INDEX index_audit_events_on_entity_id_entity_type_id_desc_author_id ON public.audit_events USING btree (entity_id, entity_type, id DESC, author_id);
-
 CREATE INDEX index_award_emoji_on_awardable_type_and_awardable_id ON public.award_emoji USING btree (awardable_type, awardable_id);
 
 CREATE INDEX index_award_emoji_on_user_id_and_name ON public.award_emoji USING btree (user_id, name);