How do I resolve Postgresql error, "could not determine which collation to use for string comparison"?
Found the error in the Release Notes for 9.1.2


  Make contrib/citext's upgrade script fix collations of citext columns
  and indexes (Tom Lane)
  
  Existing citext columns and indexes aren't correctly marked as being
  of a collatable data type during pg_upgrade from a pre-9.1 server.
  That leads to operations on them failing with errors such as "could
  not determine which collation to use for string comparison". This
  change allows them to be fixed by the same script that upgrades the
  citext module into a proper 9.1 extension during CREATE EXTENSION
  citext FROM unpackaged.
  
  If you have a previously-upgraded database that is suffering from this
  problem, and you already ran the CREATE EXTENSION command, you can
  manually run (as superuser) the UPDATE commands found at the end of
  SHAREDIR/extension/citext--unpackaged--1.0.sql. (Run pg_config
  --sharedir if you're uncertain where SHAREDIR is.)


So I looked in that file for the UPDATE commands and found these:

UPDATE pg_catalog.pg_type SET typcollation = 100
WHERE oid = 'citext'::pg_catalog.regtype;

UPDATE pg_catalog.pg_attribute SET attcollation = 100
WHERE atttypid = 'citext'::pg_catalog.regtype;

UPDATE pg_catalog.pg_index SET indcollation[0] = 100
WHERE indclass[0] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[1] = 100
WHERE indclass[1] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[2] = 100
WHERE indclass[2] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[3] = 100
WHERE indclass[3] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[4] = 100
WHERE indclass[4] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[5] = 100
WHERE indclass[5] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[6] = 100
WHERE indclass[6] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);

UPDATE pg_catalog.pg_index SET indcollation[7] = 100
WHERE indclass[7] IN (SELECT oid FROM pg_catalog.pg_opclass
                      WHERE opcintype = 'citext'::pg_catalog.regtype);