How to debug systemd script?

[Oliver Elphick <olly@lfix.co.uk>]

I have installed Debian alongside Linux Mint and wish to have
PostgreSQL use the same database.  So I have copied the system
configuration files in /etc/postgresql from the Linux Mint partition.
They point to the PostgreSQL data files in /lvhome/postgresql.  

This works in Linux Mint but not in Debian; PostgreSQL does not start.
There is nothing in the logs of either system or PostgreSQL. It works
if I call pg_ctl or if I run postgres directly.

I put set -x in the init script. When it is run, we see:

# /etc/init.d/postgresql start
...
+ echo -n Starting postgresql (via systemctl): postgresql.service
Starting postgresql (via systemctl): postgresql.service+
log_daemon_msg_post Starting postgresql (via systemctl)
postgresql.service
+ :
+ /bin/systemctl start postgresql.service
+ rc=0
...


There is a long pause at /bin/systemctl, but no error is indicated.
However, the server is not started.

How can I find out what is going on to stop the normal init script from
working? Is there some way to force systemctl to log what it is doing?
If it does log, where does it put the log?

Thanks
Oliver Elphick