Add partial index to improve mirrors update

Fixes slow UpdateAllMirrorsWorker SQL: 16-18s -> under 2s.

changelogs/unreleased/325503-fix-update-all-mirrors-db-performance.yml

+---
+title: Add partial index to improve mirrors update
+merge_request: 57353
+author:
+type: performance

db/migrate/20210324112439_add_index_mirror_data_on_retry_next_execution_where_status.rb

+# frozen_string_literal: true
+
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddIndexMirrorDataOnRetryNextExecutionWhereStatus < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  INDEX_NAME = 'index_mirror_data_non_scheduled_or_started'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :project_mirror_data,
+      [:next_execution_timestamp, :retry_count],
+      where: "(status)::text <> ALL ('{scheduled,started}'::text[])",
+      name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index :project_mirror_data,
+      [:next_execution_timestamp, :retry_count],
+      where: "(status)::text <> ALL ('{scheduled,started}'::text[])",
+      name: INDEX_NAME
+  end
+end

db/schema_migrations/20210324112439

+7e6dd4e247ad6b610ebebcf59b4212fd0d2258c8fff008d525b891da872613e5
\ No newline at end of file

db/structure.sql

@@ -23082,6 +23082,8 @@ CREATE INDEX index_milestones_on_title ON milestones USING btree (title);
 
 CREATE INDEX index_milestones_on_title_trigram ON milestones USING gin (title gin_trgm_ops);
 
+CREATE INDEX index_mirror_data_non_scheduled_or_started ON project_mirror_data USING btree (next_execution_timestamp, retry_count) WHERE ((status)::text <> ALL ('{scheduled,started}'::text[]));
+
 CREATE INDEX index_mirror_data_on_next_execution_and_retry_count ON project_mirror_data USING btree (next_execution_timestamp, retry_count);
 
 CREATE UNIQUE INDEX index_mr_blocks_on_blocking_and_blocked_mr_ids ON merge_request_blocks USING btree (blocking_merge_request_id, blocked_merge_request_id);