1) Comment Subscribe module is not compatible to PostgreSQL due to heavy usage of MySQL-specific SQL code
* ifnull function (coalesce() in postgress)
* IF statements (CASE in Postgress)
* Concat function ( '||' operator in Postgres)
* UPDATE t1,t2,t3 set t1.f=... join t2 .. (should be replaced with update t1 ... where t1.field in (select from t2,t3)
2) Also, module activation produces an error due to incorrect sql code generated if no comments exists on site (on MySQL too)
Both problems are fixed in attached patch. MySQL code is not touched, new SQL statements added under if $GLOBALS['db_type'] == 'pgsql'