Add index on pages_deployment_id and deployed to metadata

it will be used by the pages migration in
only_on_legacy_storage scope

changelogs/unreleased/323175-database-timeout-in-pages-migration-task.yml

+---
+title: Add index for pages migration
+merge_request: 55757
+author:
+type: added

db/migrate/20210302103851_add_deployed_deployment_id_index_to_project_pages_metadata.rb

+# frozen_string_literal: true
+
+class AddDeployedDeploymentIdIndexToProjectPagesMetadata < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_on_pages_metadata_not_migrated'
+
+  def up
+    add_concurrent_index :project_pages_metadata, :project_id, where: "deployed = TRUE AND pages_deployment_id is NULL", name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :project_pages_metadata, INDEX_NAME
+  end
+end

db/schema_migrations/20210302103851

+b2dad27276941e17248f86764196525bd91b088eed78ad7aa6ae2e5a2c9e82bd
\ No newline at end of file

db/structure.sql

@@ -22881,6 +22881,8 @@ CREATE INDEX index_on_namespaces_lower_name ON namespaces USING btree (lower((na
 
 CREATE INDEX index_on_namespaces_lower_path ON namespaces USING btree (lower((path)::text));
 
+CREATE INDEX index_on_pages_metadata_not_migrated ON project_pages_metadata USING btree (project_id) WHERE ((deployed = true) AND (pages_deployment_id IS NULL));
+
 CREATE INDEX index_on_projects_lower_path ON projects USING btree (lower((path)::text));
 
 CREATE INDEX index_on_routes_lower_path ON routes USING btree (lower((path)::text));