PostgreSQL: ERROR: cannot execute CREATE DATABASE in a read-only transaction
You could always double check your problem by issuing a command such as 

/usr/bin/pgsql/bin/createdb test

You can frequently get better/clearer error messages and/or other information from the command line than from GUI tools such as pgAdmin III.

But, in order to change a system parameter, you have to reload the postgresql.conf file after making changes. You can do this by either


rebooting the server or
reloading the .conf file or
change on the client side


From here, some parameters always need a reboot and some can be dynamically changed by simply reloading.

I tried to find a clear list of those which required reboot and which required reload, but failed (grateful for input on this!).

For a system reboot, 

pgctl restart

To reload, you could also try these commands (from here - or check the documentation page given above).

Option 1: From the command-line shell

su - postgres /path/to/pg_ctl reload


Option 2: Using SQL

SELECT pg_reload_conf();


You can verify that the setting has "taken" from the table pg_settings. Also of interest is this page.

For the default_transaction_read_only parameter, you can also set it directly from the client (see here). Check the links here for how to set all parameters - reboot, reload or client.

test=# SELECT name, setting FROM pg_settings WHERE name = 'default_transaction_read_only';
             name              | setting 
-------------------------------+---------
 default_transaction_read_only | on
(1 row)

test=# 
test=# `SET default_transaction_read_only = OFF;`
SET
test=# 
test=# SELECT name, setting FROM pg_settings WHERE name = 'default_transaction_read_only';
             name              | setting 
-------------------------------+---------
 default_transaction_read_only | off
(1 row)

test=# 
test=# SET default_transaction_read_only = ON;
SET