Improve the performance of project/users API

What does this MR do?

This change improves the performance of the api/v4/projects/:id/users endpoint by changing the ORDER BY clause to benefit from an index. The API hits the master DB and it's the top 1 query: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/snippets/2137909#master-1022020102-16

The current query orders by users.id column, however the main filter is present on the project_authorizations JOIN query. Changing the sort to project_authorizations.user_id column will improve the query performance with the project_id, user_id index.

The API will return records in the same order, so no visible change to the end-user.

Note: the change is behind a feature flag (sort_by_project_users_by_project_authorizations_user_id).

Migration

The index creation takes <10 minutes on pg.ai.

up:

ReplaceProjectAuthorizationsProjectIdIndex: migrating =======
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_authorizations, [:project_id, :user_id], {:name=>"index_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0017s
-- add_index(:project_authorizations, [:project_id, :user_id], {:name=>"index_project_authorizations_on_project_id_user_id", :algorithm=>:concurrently})
   -> 0.0036s
-- transaction_open?()
   -> 0.0000s
-- indexes(:project_authorizations)
   -> 0.0015s
-- remove_index(:project_authorizations, {:algorithm=>:concurrently, :name=>"index_project_authorizations_on_project_id"})
   -> 0.0029s
== 20210621155328 ReplaceProjectAuthorizationsProjectIdIndex: migrated (0.0120s)

down:

== 20210621155328 ReplaceProjectAuthorizationsProjectIdIndex: reverting =======
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:project_authorizations, :project_id, {:name=>"index_project_authorizations_on_project_id", :algorithm=>:concurrently})
   -> 0.0026s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:project_authorizations, :project_id, {:name=>"index_project_authorizations_on_project_id", :algorithm=>:concurrently})
   -> 0.0144s
-- execute("RESET ALL")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:project_authorizations)
   -> 0.0016s
-- remove_index(:project_authorizations, {:algorithm=>:concurrently, :name=>"index_project_authorizations_on_project_id_user_id"})
   -> 0.0020s
== 20210621155328 ReplaceProjectAuthorizationsProjectIdIndex: reverted (0.0231s)

Query change:

see the query tab for the actual query

• Old: https://explain.depesz.com/s/EeU8
• New: https://explain.depesz.com/s/lXvy

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team