Merge branch 'patch-28' into 'master'

Update database requirements documentation with more MySQL limitations

See merge request gitlab-org/gitlab-ce!21686

doc/install/requirements.md

@@ -27,7 +27,7 @@ Please see the [installation from source guide](installation.md) and the [instal
 
 ### Non-Unix operating systems such as Windows
 
-GitLab is developed for Unix operating systems. 
+GitLab is developed for Unix operating systems.
 It does **not** run on Windows, and we have no plans to support it in the near future. For the latest development status view this [issue](https://gitlab.com/gitlab-org/gitlab-ce/issues/46567).
 Please consider using a virtual machine to run GitLab.
 
@@ -103,19 +103,21 @@ features of GitLab work with MySQL/MariaDB:
 
 1. MySQL support for subgroups was [dropped with GitLab 9.3][post].
    See [issue #30472][30472] for more information.
-1. GitLab Geo does [not support MySQL](https://docs.gitlab.com/ee/gitlab-geo/database.html#mysql-replication).
-1. [Zero downtime migrations][zero] do not work with MySQL
+1. Geo **[STARTER ONLY]** does [not support MySQL](https://docs.gitlab.com/ee/gitlab-geo/database.html#mysql-replication). This means no supported Disaster Recovery solution if using MySQL.
+1. [Zero downtime migrations][../update/README.md#upgrading-without-downtime] do not work with MySQL.
 1. GitLab [optimizes the loading of dashboard events](https://gitlab.com/gitlab-org/gitlab-ce/issues/31806) using [PostgreSQL LATERAL JOINs](https://blog.heapanalytics.com/postgresqls-powerful-new-join-type-lateral/).
 1. In general, SQL optimized for PostgreSQL may run much slower in MySQL due to
    differences in query planners. For example, subqueries that work well in PostgreSQL
-   may not be [performant in MySQL](https://dev.mysql.com/doc/refman/5.7/en/optimizing-subqueries.html)
+   may not be [performant in MySQL](https://dev.mysql.com/doc/refman/5.7/en/optimizing-subqueries.html).
+1. Binary column index length is limited to 20 bytes. This is accomplished with [a hack](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/mysql_set_length_for_binary_indexes.rb).
+1. MySQL requires a variety of hacks to increase limits on various columns, [for example](https://gitlab.com/gitlab-org/gitlab-ce/issues/49583).
+1. [The milestone filter runs slower queries on MySQL](https://gitlab.com/gitlab-org/gitlab-ce/issues/51173#note_99391731).
 1. We expect this list to grow over time.
 
 Existing users using GitLab with MySQL/MariaDB are advised to
 [migrate to PostgreSQL](../update/mysql_to_postgresql.md) instead.
 
 [30472]: https://gitlab.com/gitlab-org/gitlab-ce/issues/30472
-[zero]: ../update/README.md#upgrading-without-downtime
 [post]: https://about.gitlab.com/2017/06/22/gitlab-9-3-released/#dropping-support-for-subgroups-in-mysql
 
 ### PostgreSQL Requirements