Migrating from MySQL to PostgreSQL

This guide documents how to take a working GitLab instance that uses MySQL and migrate it to a PostgreSQL database.

Requirements

NOTE: Note: Support for MySQL was removed in GitLab 12.1. This procedure should be performed before installing GitLab 12.1.

pgloader 3.4.1+ is required, confirm with pgloader -V.

You can install it directly from your distribution, for example in Debian/Ubuntu:

Search for the version:
```
apt-cache madison pgloader
```

If the version is 3.4.1+, install it with:

sudo apt-get install pgloader

If your distribution's version is too old, use PostgreSQL's repository:

# 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 PostgreSQL's download page to add their repository and then install pgloader.

If you are migrating to a Docker based installation, you will need to install pgloader within the container as it is not included in the container image.

Start a shell session in the context of the running container:
```
docker exec -it gitlab bash
```

Install pgloader:

apt-get update
apt-get -y install pgloader

Omnibus GitLab installations

For Omnibus GitLab packages, you'll first need to enable the bundled PostgreSQL:

Stop GitLab:
```
sudo gitlab-ctl stop
```
Edit /etc/gitlab/gitlab.rb to enable bundled PostgreSQL:
```
postgresql['enable'] = true
```
Edit /etc/gitlab/gitlab.rb to use the bundled PostgreSQL. Please check all the settings beginning with db_, such as gitlab_rails['db_adapter'] and alike. You could just comment all of them out so that we'll just use the defaults.
Reconfigure GitLab for the changes to take effect.

Start Unicorn and PostgreSQL so that we can prepare the schema:

sudo gitlab-ctl start unicorn
sudo gitlab-ctl start postgresql

Run the following commands to prepare the schema:
```
sudo gitlab-rake db:create db:migrate
```
Stop Unicorn to prevent other database access from interfering with the loading of data:
```
sudo gitlab-ctl stop unicorn
```

After these steps, you'll have a fresh PostgreSQL database with up-to-date schema.

Next, we'll use pgloader to migrate the data from the old MySQL database to the new PostgreSQL one:

Save the following snippet in a commands.load file, and edit with your MySQL database username, password and host:

LOAD DATABASE
     FROM mysql://username:password@host/gitlabhq_production
     INTO postgresql://gitlab-psql@unix://var/opt/gitlab/postgresql:/gitlabhq_production

WITH include no drop, truncate, disable triggers, create no tables,
     create no indexes, preserve index names, no foreign keys,
     data only

SET MySQL PARAMETERS
net_read_timeout = '90',
net_write_timeout = '180'

ALTER SCHEMA 'gitlabhq_production' RENAME TO 'public'

;

Start the migration:

sudo -u gitlab-psql pgloader commands.load

Once the migration finishes, you should see a summary table that looks like the following:

                                 table name       read   imported     errors      total time
-----------------------------------------------  ---------  ---------  ---------  --------------
                                fetch meta data        119        119          0          0.388s
                                       Truncate        119        119          0          1.134s
-----------------------------------------------  ---------  ---------  ---------  --------------
                           public.abuse_reports          0          0          0          0.490s
                             public.appearances          0          0          0          0.488s
                                       .
                                       .
                                       .
                           public.web_hook_logs          0          0          0          1.080s
-----------------------------------------------  ---------  ---------  ---------  --------------
                        COPY Threads Completion          4          4          0          2.008s
                                Reset Sequences        113        113          0          0.304s
                               Install Comments          0          0          0          0.000s
-----------------------------------------------  ---------  ---------  ---------  --------------
                              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 for more details.

Start GitLab:
```
sudo gitlab-ctl start
```

You can now verify that everything works as expected by visiting GitLab.

Source installations

For installations from source that use MySQL, you'll first need to install PostgreSQL and create a database.

After the database is created, go on with the following steps:

Stop GitLab:
```
sudo service gitlab stop
```

Switch database from MySQL to PostgreSQL

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

Install Gems related to PostgreSQL

sudo -u git -H rm .bundle/config
sudo -u git -H bundle install --deployment --without development test mysql aws kerberos

Run the following commands to prepare the schema:

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.

Next, we'll use pgloader to migrate the data from the old MySQL database to the new PostgreSQL one:

Save the following snippet in a commands.load file, and edit with your MySQL username, password and host:

LOAD DATABASE
     FROM mysql://username:password@host/gitlabhq_production
     INTO postgresql://postgres@unix://var/run/postgresql:/gitlabhq_production

WITH include no drop, truncate, disable triggers, create no tables,
     create no indexes, preserve index names, no foreign keys,
     data only

SET MySQL PARAMETERS
net_read_timeout = '90',
net_write_timeout = '180'

ALTER SCHEMA 'gitlabhq_production' RENAME TO 'public'

;

Start the migration:
```
sudo -u postgres pgloader commands.load
```

Once the migration finishes, you should see a summary table that looks like the following:

                                 table name       read   imported     errors      total time
-----------------------------------------------  ---------  ---------  ---------  --------------
                                fetch meta data        119        119          0          0.388s
                                       Truncate        119        119          0          1.134s
-----------------------------------------------  ---------  ---------  ---------  --------------
                           public.abuse_reports          0          0          0          0.490s
                             public.appearances          0          0          0          0.488s
                                       .
                                       .
                                       .
                           public.web_hook_logs          0          0          0          1.080s
-----------------------------------------------  ---------  ---------  ---------  --------------
                        COPY Threads Completion          4          4          0          2.008s
                                Reset Sequences        113        113          0          0.304s
                               Install Comments          0          0          0          0.000s
-----------------------------------------------  ---------  ---------  ---------  --------------
                              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 for more details.

Start GitLab:
```
sudo service gitlab start
```

You can now verify that everything works as expected by visiting GitLab.

Troubleshooting

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:

# Omnibus GitLab
sudo gitlab-rake cache:clear

# Installations from source
sudo -u git -H bundle exec rake cache:clear RAILS_ENV=production