How to find the log file for a PostgreSQL database

Look in the file $PGDATA/postgresql.conf:

#------------------------------------------------------------------------------
# REPORTING AND LOGGING
#------------------------------------------------------------------------------

# - Where to Log -

log_destination = stderr # Valid values are combinations of
                                        # stderr, csvlog, syslog, and eventlog,
                                        # depending on platform.  csvlog
                                        # requires logging_collector to be on.

# This is used when logging to stderr:
logging_collector = on # Enable capturing of stderr and csvlog
                                        # into log files. Required to be on for
                                        # csvlogs.
                                        # (change requires restart)

# These are only used if logging_collector is on:
log_directory = log # directory where log files are written,
                                        # can be absolute or relative to PGDATA

log_filename = 'postgresql-%a.log' # log file name pattern,
                                        # can include strftime() escapes

log_truncate_on_rotation = on # If on, an existing log file with the
                                        # same name as the new log file will be
                                        # truncated rather than appended to.
                                        # But such truncation only occurs on
                                        # time-driven rotation, not on restarts
                                        # or size-driven rotation.  Default is
                                        # off, meaning append to existing files
                                        # in all cases.
log_rotation_age = 1d # Automatic rotation of logfiles will
                                        # happen after that time.  0 disables.

log_rotation_size = 0 # Automatic rotation of logfiles will
                                        # happen after that much log output.
                                        # 0 disables.

So in my case, the directory $PGDATA/log should contain the database server's log files.

The directory listing shows:

[postgres@myserver /pgdata/data/log]$ ls -latrh
total 5.6M
drwx------  2 postgres postgres 4.0K Dec 23 00:00 .
-rw-------  1 postgres postgres 867K Jan 24 23:59 postgresql-Thu.log
-rw-------  1 postgres postgres 869K Jan 25 23:59 postgresql-Fri.log
-rw-------  1 postgres postgres 873K Jan 26 23:59 postgresql-Sat.log
-rw-------  1 postgres postgres 873K Jan 27 23:59 postgresql-Sun.log
-rw-------  1 postgres postgres 873K Jan 28 23:59 postgresql-Mon.log
-rw-------  1 postgres postgres 873K Jan 29 23:59 postgresql-Tue.log
drwx------ 21 postgres postgres 4.0K Jan 30 10:47 ..
-rw-------  1 postgres postgres 407K Jan 30 11:01 postgresql-Wed.log

Since I am writing this post on a Wednesday, the log currently being written to is postgresql-Wed.log, and putting a tail on that will be somewhat like tailing the alert log of an oracle database.

Source: The online documentation