Calls to Merge Requests API fail due to database timeout

Summary

A Premium SaaS customer with ~8,000 seats is unable to retrieve merge requests via the /groups/:id/merge_requests REST endpoint.

Steps to reproduce & Example Project

https://gitlab.zendesk.com/agent/tickets/225458 (preceded by https://support.gitlab.com/hc/en-us/requests/209147 this past April) has details (Internal gitlab only).

Relevant logs and/or screenshots

• https://log.gprd.gitlab.net/goto/5bc1156a32fbcfce24fc4ac504374a9a
• https://sentry.gitlab.net/gitlab/gitlabcom/?query=url%3A%22https%3A%2F%2Fgitlab.com%2Fapi%2Fv4%2Fgroups%2F4249178%2Fmerge_requests%22

Possible fixes