Hello.
PostgreSQL 9.6.2 on x86_64-pc-linux-gnu, compiled by gcc (Debian 4.9.2-10)
4.9.2, 64-bit
select *
from (select 'test'::text collate "C" id) c1
full join (select 'test'::text collate "en_US" id) c2 on
-- c1.id = c2.id ----< ok
c1 = c2
ERROR: could not determine which collation to use for string comparison
SQLSTATE: 42P22
Records comparison does not work if corresponding fields have different
collation.