How to combine output of several journals by journalctl?

I need to see all postgresql logs. I try:

$ journalctl -f -u postgresql.service SYSLOG_IDENTIFIER=postgres
-- Logs begin at Ср 2013-03-06 00:57:40 KRAT. --
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  database system was interrupted; last known up at 2013-08-11 20:12:28 KRAT
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  database system was not properly shut down; automatic recovery in progress
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  record with zero length at 5/DACF208
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  redo is not required
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  autovacuum launcher started
авг 11 23:41:07 nuzzflonk postgres[2432]: LOG:  database system is ready to accept connections
-- Reboot --
авг 19 20:23:22 nuzzflonk postgres[2361]: FATAL:  the database system is starting up
авг 19 20:23:22 nuzzflonk postgres[2361]: LOG:  database system was shut down at 2013-08-19 20:21:25 KRAT
авг 19 20:23:22 nuzzflonk postgres[2361]: LOG:  autovacuum launcher started
авг 19 20:23:22 nuzzflonk postgres[2361]: LOG:  database system is ready to accept connections

But:

$ journalctl -f SYSLOG_IDENTIFIER=postgres 
-- Logs begin at Ср 2013-03-06 00:57:40 KRAT. --
авг 24 10:50:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 11:10:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 11:20:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 11:30:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 11:40:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 11:50:08 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 12:00:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 12:30:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 12:40:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction
авг 24 12:50:07 nuzzflonk postgres[2361]: LOG:  unexpected EOF on client connection with an open transaction

How to combine two journals?

UPD: Solved. $ journalctl -f SYSLOG_IDENTIFIER=postgres show all journals.

To combine the output of multiple journals you just need to add them to your command using the -u option as follows:

journalctl -u nginx.service -u mysql.service -f

No need to use SYSLOG_IDENTIFIER as that will likely not be what you want in most cases.