Optimize database query for last deployment

This commit optimizes the database query for
last deployment by introducing a new database index.

changelogs/unreleased/optimize-query-for-last-deployment.yml

+---
+title: Optimize database query for last deployment
+merge_request: 57979
+author:
+type: performance

db/migrate/20210331145548_add_index_for_last_deployment.rb

+# frozen_string_literal: true
+
+class AddIndexForLastDeployment < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_deployments_on_environment_id_status_and_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :deployments, [:environment_id, :status, :id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :deployments, INDEX_NAME
+  end
+end

db/schema_migrations/20210331145548

+f27446d1950acaf45f623b2cec7733cd7ba4b82eefddfa2203acbbaf77d59e18
\ No newline at end of file

db/structure.sql

@@ -22518,6 +22518,8 @@ CREATE INDEX index_deployments_on_environment_id_and_iid_and_project_id ON deplo
 
 CREATE INDEX index_deployments_on_environment_id_status_and_finished_at ON deployments USING btree (environment_id, status, finished_at);
 
+CREATE INDEX index_deployments_on_environment_id_status_and_id ON deployments USING btree (environment_id, status, id);
+
 CREATE INDEX index_deployments_on_environment_status_sha ON deployments USING btree (environment_id, status, sha);
 
 CREATE INDEX index_deployments_on_id_and_status_and_created_at ON deployments USING btree (id, status, created_at);