This is optional of course.

apt update
# apt remove --purge -y postgresql*   # if you want to delete all postgresql db & versions first. YMMV!
apt upgrade -y && apt autoremove -y && apt dist-upgrade -y
do-release-upgrade  # yes, reboot when asked

Login again on your server, and:

apt autoremove -y
apt install postgresql postgresql-client postgresql-client-common

You will see that it is more or less broken though

cd /root
mkdir mm_migration
cd mm_migration
systemctl stop mattermost
 
wget https://github.com/mattermost/migration-assist/releases/download/v0.3/migration-assist-Linux-x86_64.tar.gz
tar xvzf migration-assist-Linux-x86_64.tar.gz && rm migration-assist-Linux-x86_64.tar.gz

Set these two according to your setup in your sessions. They will be used more than once. Eg:

MYSQL_DSN="matteruser:mysqlpassword@tcp(127.0.0.1:3306)/mattermost"
POSTGRES_DSN="postgres://matteruser:postgresqlpassword@127.0.0.1:5432/mattermost"

su - postgres
dropdb -f mattermost
psql

DROP DATABASE mattermost;
CREATE DATABASE mattermost;
\CONNECT mattermost

CREATE USER matteruser WITH PASSWORD 'LJFHzzERLhf56';
ALTER ROLE "matteruser" WITH LOGIN;
GRANT ALL PRIVILEGES ON DATABASE mattermost TO matteruser;
ALTER DATABASE mattermost OWNER TO matteruser;
GRANT USAGE, CREATE ON SCHEMA PUBLIC TO matteruser;
ALTER SCHEMA public OWNER TO matteruser;
GRANT ALL ON SCHEMA public TO matteruser;
ALTER USER matteruser IN DATABASE mattermost SET search_path TO 'public';
\q

# Exit user postgres
[[ $USER = 'root' ]] || exit
 
vi /etc/postgresql/{version}/main/pg_hba.conf
# Add this line BEFORE the "local all all peer":
local   mattermost      matteruser                              md5
 
# restart postgresql
systemctl reload postgresql

You can test it with psql --dbname=mattermost --username=matteruser --password and:

\CONNECT mattermost
-- You are now connected to database "mattermost" as user "matteruser" --
\q

Then:

mysql -u root mattermost

  -- you may need this --
  -- ALTER USER 'matteruser'@'%' IDENTIFIED WITH mysql_native_password BY 'mysqlpassword'; --
 
  -- Fix also some data that does not fit in the pg target --
  UPDATE LinkMetadata SET URL = LEFT(URL, 2000) WHERE LENGTH(URL) > 2000;

# For this error:
#    ERROR mysql: Failed to connect to mysql at "127.0.0.1" (port 3306) as user "matteruser": Condition QMYND:MYSQL-UNSUPPORTED-AUTHENTICATION was signalled.
# We do this (once!)
echo -e "\n[mysqld]\ndefault-authentication-plugin=mysql_native_password\n" >> /etc/mysql/mysql.cnf
 
systemctl restart mysql

Build pgloader from source (other ways sometimes fails):

apt install libssl-dev freetds-dev
cd /root/mm_migration/
mkdir pgloader_from_source
cd pgloader_from_source
wget https://github.com/dimitri/pgloader/releases/download/v3.6.9/pgloader-bundle-3.6.9.tgz
tar xvzf pgloader-bundle-3.6.9.tgz && rm pgloader-bundle-3.6.9.tgz
cd pgloader-bundle-3.6.9/
apt install sbcl make
make DYNSIZE=32000 pgloader
mv bin/pgloader /root/mm_migration/
cd /root/mm_migration/
/root/mm_migration/pgloader --version   # pgloader version "3.6.9"
rm -r pgloader_from_source

cd ~/mm_migration
 
./migration-assist mysql "$MYSQL_DSN"
./migration-assist postgres "$POSTGRES_DSN" --run-migrations --mattermost-version="10.4.2"  # see "/opt/mattermost/bin/mattermost version"
./migration-assist pgloader --mysql="$MYSQL_DSN" --postgres="$POSTGRES_DSN" > migration.load
 
sed -i "s/_mem to '[^']*'/_mem to '12000MB'/" migration.load
sed -i "s/workers = [0-9]*/workers = 2/" migration.load
sed -i "s/prefetch rows = [0-9]*/prefetch rows = 2000/" migration.load
./pgloader migration.load | tee migration.log

Small fixes:

# This may fail because of "tsvector" maximum size:
#    ./migration-assist postgres post-migrate "$POSTGRES_DSN"
# So we do it manually: use 1000000 or shorter values (like 500000)
su - postgres
psql

</code sql> \connect mattermost CREATE INDEX IF NOT EXISTS idx_posts_message_txt ON public.posts USING gin(to_tsvector('english', substring(message,500000))); CREATE INDEX IF NOT EXISTS idx_fileinfo_content_txt ON public.fileinfo USING gin(to_tsvector('english', substring(content,500000))); \q </code>

[[ $USER = 'root' ]] || exit
./migration-assist postgres post-migrate "$POSTGRES_DSN"  # terminates the job
 
# Extra stuff to migrate:
for job in boards playbooks calls; do
  ./migration-assist pgloader $job --mysql="$MYSQL_DSN" --postgres="$POSTGRES_DSN" > $job.load
  ./pgloader $job.load | tee $job.log
done

cp /opt/mattermost/config/config.json /opt/mattermost/config/config.json-$(date +'%Y%m%d-%H%M%S')
sed -i 's/"DriverName": "mysql"/"DriverName": "postgres"/' /opt/mattermost/config/config.json
sed -i '/"DataSource"/r"DataSource": "'$POSTGRES_DSN'"?sslmode=disable&connect_timeout=10",' /opt/mattermost/config/config.json

And start the service:

  cd /opt/mattermost/
  sudo -u mattermost ./bin/mattermost

If all is OK, re-enable mattermost:

  systemctl start mattermost
  tail -F /opt/mattermost/logs/mattermost.log

rm -r /root/mm_migration/
# drop mysql database if and only when you are sure the migration is OK