A SlackBuild for building and installing PostgreSQL can be found at SlackBuilds.org; follow the instructions there, and install PostgreSQL. After installing PostgreSQL perform the following steps:

1. Initialize the PostgreSQL database:

   su postgres -c "initdb -D /var/lib/pgsql/10.2/data --locale=en_US.UTF-8 -A md5 -W"

2. Start the PostgreSQL database:

   /etc/rc.d/rc.postgresql start

3. Switch to the postgres Linux user account that was created during the installation.

   su -l postgres

4. Start the PostgreSQL interactive terminal.

   psql

5. Create the Mattermost database.

   CREATE DATABASE mattermostdb;

6. Create mattermost user and give it the rights on the database:

   CREATE USER mattermost;

   GRANT ALL PRIVILEGES ON DATABASE mattermostdb to mattermost;

7. Exit out of psql with \q.
8. Setup the Unix socket by editing file: /var/lib/pgsql/10.2/data/pg_hba.conf, replace

   # "local" is for Unix domain socket connections only

   local all all md5

   with

   local mattermostdb mattermost peer

9. Add the following to “/var/lib/pgsql/10.2/data/pg_ident.conf”

   echo "mattermap mattermostdb mattermost" >> /var/lib/pgsql/10.2/data/pg_ident.conf

10. Restart PostgreSQL

    exit

    /etc/rc.d/rc.postgresql restart

11. Add the mattermost system user:

    useradd --system --user-group mattermost

12. Verify things are working thus far

    sudo -u mattermost psql --dbname=mattermostdb --username=mattermost