Document PSQL `btree_gist` extension requirement

Add documentation on how to enable `btree_gist`, required from 13.1 and
onwards

doc/administration/postgresql/replication_and_failover.md

@@ -305,6 +305,12 @@ Select one node as a primary node.
    CREATE EXTENSION pg_trgm;
    ```
 
+1. Enable the `btree_gist` extension:
+
+   ```shell
+   CREATE EXTENSION btree_gist;
+   ```
+
 1. Exit the database prompt by typing `\q` and Enter.
 
 1. Verify the cluster is initialized with one node:
@@ -736,9 +742,9 @@ consul['configuration'] = {
 
 After deploying the configuration follow these steps:
 
-1. On `10.6.0.31`, our primary database
+1. On `10.6.0.31`, our primary database:
 
-   Enable the `pg_trgm` extension
+   Enable the `pg_trgm` and `btree_gist` extensions:
 
    ```shell
    gitlab-psql -d gitlabhq_production
@@ -746,33 +752,34 @@ After deploying the configuration follow these steps:
 
    ```shell
    CREATE EXTENSION pg_trgm;
+   CREATE EXTENSION btree_gist;
    ```
 
-1. On `10.6.0.32`, our first standby database
+1. On `10.6.0.32`, our first standby database:
 
-   Make this node a standby of the primary
+   Make this node a standby of the primary:
 
    ```shell
    gitlab-ctl repmgr standby setup 10.6.0.21
    ```
 
-1. On `10.6.0.33`, our second standby database
+1. On `10.6.0.33`, our second standby database:
 
-   Make this node a standby of the primary
+   Make this node a standby of the primary:
 
    ```shell
    gitlab-ctl repmgr standby setup 10.6.0.21
    ```
 
-1. On `10.6.0.41`, our application server
+1. On `10.6.0.41`, our application server:
 
-   Set `gitlab-consul` user's PgBouncer password to `toomanysecrets`
+   Set `gitlab-consul` user's PgBouncer password to `toomanysecrets`:
 
    ```shell
    gitlab-ctl write-pgpass --host 127.0.0.1 --database pgbouncer --user pgbouncer --hostuser gitlab-consul
    ```
 
-   Run database migrations
+   Run database migrations:
 
    ```shell
    gitlab-rake gitlab:db:configure

doc/administration/troubleshooting/postgresql.md

@@ -57,6 +57,7 @@ This section is for links to information elsewhere in the GitLab documentation.
 - [GitLab database requirements](../../install/requirements.md#database) including
   - Support for MySQL was removed in GitLab 12.1; [migrate to PostgreSQL](../../update/mysql_to_postgresql.md)
   - required extension `pg_trgm`
+  - required extension `btree_gist`
   - required extension `postgres_fdw` for Geo
 
 - Errors like this in the `production/sidekiq` log; see: [Set default_transaction_isolation into read committed](https://docs.gitlab.com/omnibus/settings/database.html#set-default_transaction_isolation-into-read-committed):

doc/install/aws/index.md

@@ -473,9 +473,9 @@ Since we're adding our SSL certificate at the load balancer, we do not need GitL
    sudo gitlab-ctl reconfigure
    ```
 
-#### Install the `pg_trgm` extension for PostgreSQL
+#### Install the required extensions for PostgreSQL
 
-From your GitLab instance, connect to the RDS instance to verify access and to install the required `pg_trgm` extension.
+From your GitLab instance, connect to the RDS instance to verify access and to install the required `pg_trgm` and `btree_gist` extensions.
 
 To find the host or endpoint, navigate to **Amazon RDS > Databases** and click on the database you created earlier. Look for the endpoint under the **Connectivity & security** tab.
 
@@ -492,6 +492,7 @@ psql (10.9)
 Type "help" for help.
 
 gitlab=# CREATE EXTENSION pg_trgm;
+gitlab=# CREATE EXTENSION btree_gist;
 gitlab=# \q
 ```
 

doc/install/installation.md

@@ -339,6 +339,12 @@ Starting from GitLab 12.1, only PostgreSQL is supported. Since GitLab 13.0, we r
    sudo -u postgres psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS pg_trgm;"
    ```
 
+1. Create the `btree_gist` extension (required for GitLab 13.1+):
+
+   ```shell
+   sudo -u postgres psql -d template1 -c "CREATE EXTENSION IF NOT EXISTS btree_gist;"
+   ```
+
 1. Create the GitLab production database and grant all privileges on the database:
 
    ```shell
@@ -369,6 +375,24 @@ Starting from GitLab 12.1, only PostgreSQL is supported. Since GitLab 13.0, we r
    (1 row)
    ```
 
+1. Check if the `btree_gist` extension is enabled:
+
+   ```sql
+   SELECT true AS enabled
+   FROM pg_available_extensions
+   WHERE name = 'btree_gist'
+   AND installed_version IS NOT NULL;
+   ```
+
+   If the extension is enabled this will produce the following output:
+
+   ```plaintext
+   enabled
+   ---------
+    t
+   (1 row)
+   ```
+
 1. Quit the database session:
 
    ```shell

doc/install/requirements.md

@@ -140,8 +140,8 @@ GitLab version | Minimum PostgreSQL version
 12.10 | 11
 13.0 | 11
 
-You must also ensure the `pg_trgm` extension is loaded into every
-GitLab database. This extension [can be enabled](https://www.postgresql.org/docs/11/sql-createextension.html) using a PostgreSQL super user.
+You must also ensure the `pg_trgm` and `btree_gist` extensions are loaded into every
+GitLab database. These extensions [can be enabled](https://www.postgresql.org/docs/11/sql-createextension.html) using a PostgreSQL super user.
 
 On some systems you may need to install an additional package (for example,
 `postgresql-contrib`) for this extension to become available.