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Drop an unused index for GitLab.com

Krasimir Angelov requested to merge 425974-drop-index_namespaces_on_created_at-for-gitlab-com into master

What does this MR do and why?

Drops the index index_namespaces_on_created_at which is not used on GitLab.com.

As the index may be used on self-managed instances that use the instance wide (admin) list of namespaces, we remove it only for GitLab.com to help with the LWLock saturation issues we are having.

See #425974 (closed).

Revert

In case we find we need to quickly restore this index for GitLab.com, we can do this with the following commands executed on the primary:

CREATE INDEX CONCURRENTLY index_namespaces_on_created_at ON namespaces USING btree (created_at);

This takes < 5 minutes on DBL - https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/23477/commands/75447.

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Related to #425974 (closed)

Edited by Krasimir Angelov

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