Postgres totalnum counter very slow

The postgres totalnum script takes a very long time to run because a self-join on an ontology table with a LIKE clause on c_fullname takes ages. The same query on MSSQL with the same tables and same indices takes <5s.

select distinct p1.c_fullname, p2.c_basecode
from dimCountOnt p1
inner join dimCountOnt p2
on p2.c_fullname like p1.c_fullname||'%' escape '#'
 where p2.c_fullname like '\i2b2\Diagnoses\%' escape '#'
and p1.c_fullname like '\\i2b2\\Diagnoses\\%'

Using specialized postgres indices did not help:

create index dimCountOntA on dimCountOnt using spgist (c_fullname);
CREATE INDEX dimCountOntB ON dimCountOnt(c_fullname text_pattern_ops);
CREATE INDEX dimCountOntBjk ON dimCountOnt USING btree(c_fullname varchar_pattern_ops);