Just in case this hits you too when you use CITEXT fields that were originally defined in a Postgres before version 9.1.

ProgrammingError: could not determine which collation to use for string comparison
HINT:  Use the COLLATE clause to set the collation explicitly.

This can happen if you use something like:

WHERE name='peter'


when field name is a case insensitive text field.

After some googling around and shooting in the dark I found the the only way to crack this is to run this command:

CREATE EXTENSION citext FROM unpackaged;

Hope that helps some poor schmuck with the same problem.

UPDATE

If you have problems applying this to new tables in Postgres 9.1 you might need to run this instead:

CREATE EXTENSION citext WITH SCHEMA public ;
Dallas G. - 03 January 2013 [«« Reply to this]
A very sincere THANKS from a very "poor schmuck".

P.S. Maybe its implied (I'm still a very green DBA), but the only thing I would add is a note stating that you'll need to create the extension on every database which contains citext values.


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