Clean up the mysql to postgres guide

This change refactors the pgloader instructions to have
less headings and be more straightforward.

doc/update/mysql_to_postgresql.md

 ---
-last_updated: 2018-02-07
+last_updated: 2019-03-27
 ---
 
 # Migrating from MySQL to PostgreSQL
 
-> **Note:** This guide assumes you have a working GitLab instance with
-> MySQL and want to migrate to bundled PostgreSQL database.
+This guide documents how to take a working GitLab instance that uses MySQL and
+migrate it to a PostgreSQL database.
 
-## Omnibus installation
+## Requirements
 
-### Prerequisites
+[pgloader](http://pgloader.io) 3.4.1+ is required.
 
-First, we'll need to enable the bundled PostgreSQL database with up-to-date
-schema. Next, we'll use [pgloader](http://pgloader.io) to migrate the data
-from the old MySQL database to the new PostgreSQL one.
+You can install it directly from your distribution, for example in
+Debian/Ubuntu:
 
-Here's what you'll need to have installed:
+1. Search for the version:
 
-- pgloader 3.4.1+
-- Omnibus GitLab
-- MySQL
+   ```bash
+   apt-cache madison pgloader
+   ```
+
+1. If the version is 3.4.1+, install it with:
+
+   ```bash
+   sudo apt-get install pgloader
+   ```
 
-### Enable bundled PostgreSQL database
+   If your distribution's version is too old, use PostgreSQL's repository:
+
+   ```bash
+   # Add repository
+   sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
+
+   # Add key
+   sudo apt-get install wget ca-certificates
+   wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
+
+   # Install package
+   sudo apt-get update
+   sudo apt-get install pgloader
+   ```
+
+For other distributions, follow the instructions in PostrgreSQL's
+[download page](https://www.postgresql.org/download/) to add their repository
+and then install `pgloader`.
+
+## Omnibus GitLab installations
+
+For [Omnibus GitLab packages](https://about.gitlab.com/install/), you'll first
+need to enable the bundled PostgreSQL:
 
 1. Stop GitLab:
 
-    ``` bash
+    ```bash
     sudo gitlab-ctl stop
     ```
 
@@ -40,39 +67,34 @@ Here's what you'll need to have installed:
    and alike. You could just comment all of them out so that we'll just use
    the defaults.
 
-1. [Reconfigure GitLab] for the changes to take effect:
-
-    ``` bash
-    sudo gitlab-ctl reconfigure
-    ```
-
+1. [Reconfigure GitLab](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure)
+   for the changes to take effect.
 1. Start Unicorn and PostgreSQL so that we can prepare the schema:
 
-    ``` bash
+    ```bash
     sudo gitlab-ctl start unicorn
     sudo gitlab-ctl start postgresql
     ```
 
 1. Run the following commands to prepare the schema:
 
-    ``` bash
+    ```bash
     sudo gitlab-rake db:create db:migrate
     ```
 
 1. Stop Unicorn to prevent other database access from interfering with the loading of data:
 
-    ``` bash
+    ```bash
     sudo gitlab-ctl stop unicorn
     ```
 
 After these steps, you'll have a fresh PostgreSQL database with up-to-date schema.
 
-### Migrate data from MySQL to PostgreSQL
-
-Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
+Next, we'll use `pgloader` to migrate the data from the old MySQL database to the
+new PostgreSQL one:
 
 1. Save the following snippet in a `commands.load` file, and edit with your
-   database `username`, `password` and `host`:
+   MySQL database `username`, `password` and `host`:
 
     ```
     LOAD DATABASE
@@ -90,7 +112,7 @@ Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
 
 1. Start the migration:
 
-    ``` bash
+    ```bash
     sudo -u gitlab-psql pgloader commands.load
     ```
 
@@ -117,106 +139,56 @@ Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
                                   Total import time       1894       1894          0         12.497s
     ```
 
-    If there is no output for more than 30 minutes, it's possible pgloader encountered an error. See
-    the [troubleshooting guide](#Troubleshooting) for more details.
+    If there is no output for more than 30 minutes, it's possible `pgloader` encountered an error. See
+    the [troubleshooting guide](#troubleshooting) for more details.
 
 1. Start GitLab:
 
-    ``` bash
+    ```bash
     sudo gitlab-ctl start
     ```
 
-Now, you can verify that everything worked by visiting GitLab.
+You can now verify that everything works as expected by visiting GitLab.
 
-### Troubleshooting
+## Source installations
 
-#### Permissions
+For installations from source that use MySQL, you'll first need to
+[install PostgreSQL and create a database](../install/installation.md#6-database).
 
-Note that the PostgreSQL user that you use for the above MUST have **superuser** privileges. Otherwise, you may see
-a similar message to the following:
-
-```
-debugger invoked on a CL-POSTGRES-ERROR:INSUFFICIENT-PRIVILEGE in thread
-    #<THREAD "lparallel" RUNNING {10078A3513}>:
-      Database error 42501: permission denied: "RI_ConstraintTrigger_a_20937" is a system trigger
-    QUERY: ALTER TABLE ci_builds DISABLE TRIGGER ALL;
-    2017-08-23T00:36:56.782000Z ERROR Database error 42501: permission denied: "RI_ConstraintTrigger_c_20864" is a system trigger
-    QUERY: ALTER TABLE approver_groups DISABLE TRIGGER ALL;
-```
-
-#### Experiencing 500 errors after the migration
-
-If you experience 500 errors after the migration, try to clear the cache:
-
-``` bash
-sudo gitlab-rake cache:clear
-```
-
-[reconfigure GitLab]: ../administration/restart_gitlab.md#omnibus-gitlab-reconfigure
-
-## Source installation
-
-### Prerequisites
-
-#### Install PostgreSQL and create database
-
-See [installation guide](../install/installation.md#6-database).
-
-#### Install [pgloader](http://pgloader.io) 3.4.1+
-
-Install directly from your distro:
-``` bash
-sudo apt-get install pgloader
-```
-
-If this version is too old, use PostgreSQL's repository:
-``` bash
-# add repository
-sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
-
-# add key
-sudo apt-get install wget ca-certificates
-wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
-
-# install package
-sudo apt-get update
-sudo apt-get install pgloader
-```
-
-### Enable bundled PostgreSQL database
+After the database is created, go on with the following steps:
 
 1. Stop GitLab:
 
-    ``` bash
+   ```bash
    sudo service gitlab stop
    ```
 
 1. Switch database from MySQL to PostgreSQL
 
-    ``` bash
+   ```bash
    cd /home/git/gitlab
    sudo -u git mv config/database.yml config/database.yml.bak
    sudo -u git cp config/database.yml.postgresql config/database.yml
    sudo -u git -H chmod o-rwx config/database.yml
    ```
+
 1. Install Gems related to Postgresql
 
-    ``` bash
+   ```bash
    sudo -u git -H rm .bundle/config
    sudo -u git -H bundle install --deployment --without development test mysql aws kerberos
    ```
 
 1. Run the following commands to prepare the schema:
 
-    ``` bash
+   ```bash
    sudo -u git -H bundle exec rake db:create db:migrate RAILS_ENV=production
    ```
 
 After these steps, you'll have a fresh PostgreSQL database with up-to-date schema.
 
-### Migrate data from MySQL to PostgreSQL
-
-Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
+Next, we'll use `pgloader` to migrate the data from the old MySQL database to the
+new PostgreSQL one:
 
 1. Save the following snippet in a `commands.load` file, and edit with your
    MySQL `username`, `password` and `host`:
@@ -237,7 +209,7 @@ Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
 
 1. Start the migration:
 
-    ``` bash
+   ```bash
    sudo -u postgres pgloader commands.load
    ```
 
@@ -264,23 +236,43 @@ Now, you can use pgloader to migrate the data from MySQL to PostgreSQL:
                                  Total import time       1894       1894          0         12.497s
    ```
 
-    If there is no output for more than 30 minutes, it's possible pgloader encountered an error. See
-    the [troubleshooting guide](#Troubleshooting) for more details.
+   If there is no output for more than 30 minutes, it's possible `pgloader` encountered an error. See
+   the [troubleshooting guide](#troubleshooting) for more details.
 
 1. Start GitLab:
 
-    ``` bash
+   ```bash
    sudo service gitlab start
    ```
 
-Now, you can verify that everything worked by visiting GitLab.
+You can now verify that everything works as expected by visiting GitLab.
 
-### Troubleshooting
+## Troubleshooting
 
-#### Experiencing 500 errors after the migration
+Sometimes, you might encounter some errors during or after the migration.
+
+### Database error permission denied
+
+The PostgreSQL user that you use for the migration MUST have **superuser** privileges.
+Otherwise, you may see a similar message to the following:
+
+```
+debugger invoked on a CL-POSTGRES-ERROR:INSUFFICIENT-PRIVILEGE in thread
+    #<THREAD "lparallel" RUNNING {10078A3513}>:
+      Database error 42501: permission denied: "RI_ConstraintTrigger_a_20937" is a system trigger
+    QUERY: ALTER TABLE ci_builds DISABLE TRIGGER ALL;
+    2017-08-23T00:36:56.782000Z ERROR Database error 42501: permission denied: "RI_ConstraintTrigger_c_20864" is a system trigger
+    QUERY: ALTER TABLE approver_groups DISABLE TRIGGER ALL;
+```
+
+### Experiencing 500 errors after the migration
 
 If you experience 500 errors after the migration, try to clear the cache:
 
-``` bash
+```bash
+# Omnibus GitLab
+sudo gitlab-rake cache:clear
+
+# Installations from source
 sudo -u git -H bundle exec rake cache:clear RAILS_ENV=production
 ```