Changes have been made in Git for this ticket:

Fix casting errors when using SQL shares and groups on PostgreSQL (Bug #8899).

http://git.horde.org/diff.php/framework/Share/Share/sql.php?rt=horde-git&r1=9f529bff639f1cafde6bae97fa3d6c7ee0112bce&r2=6c1bfbb2741e6788db1c8acdfe41e42d0830f212
http://git.horde.org/diff.php/framework/Share/Share/sql_hierarchical.php?rt=horde-git&r1=a9e8ae51befc5a9fc97366e1f623e3a822060afa&r2=6c1bfbb2741e6788db1c8acdfe41e42d0830f212
http://git.horde.org/diff.php/horde/docs/CHANGES?rt=horde-git&r1=0af0faa7c463545483f3fae542f518ed8d0d2fcd&r2=6c1bfbb2741e6788db1c8acdfe41e42d0830f212

The horde_groups table is the *only* table that uses integers. The 
reason that we are not using integers everywhere is simply because 
some group drivers don't have integer keys.

Changes have been made in CVS for this ticket:

Fix casting errors when using SQL shares and groups on PostgreSQL (Bug #8899).
http://cvs.horde.org/diff.php/framework/Share/Share/sql.php?rt=horde&r1=1.1.2.57&r2=1.1.2.58&ty=u
http://cvs.horde.org/diff.php/framework/Share/Share/sql_hierarchical.php?rt=horde&r1=1.17.2.27&r2=1.17.2.28&ty=u
http://cvs.horde.org/diff.php/horde/docs/CHANGES?rt=horde&r1=1.515.2.598&r2=1.515.2.599&ty=u

The "patch" was also requires in sql_hierarchical.php on line 230.

Many Horde and associated projects have a group_uid column. In some 
tables this is an INT (eg. horde_groups) in others a VARCHAR(255) (eg. 
ansel_shares_groups). Some code (eg horde/lib/Horde/Share/sql.php) 
ends up generating SQL fragments such as "group_uid in ( 1, 2 )". In 
PostgreSQL, this causes an error when group_uid has a datatype on 
VARCHAR(255).

A simple patch in horde/lib/Horde/Share/sql.php (1.1.2.57 2009-08-12) 
on line 691 from
    $group_ids[] = $this->_db->quote($id);
to
    $group_ids[] = $this->_db->quote((string)$id);
fixes this because PostgreSQL can cast automatically from string to 
int, but not vice versa (don't know why since one might things its the 
easier cast)

However, I suspect this is not the best approach.

Is there any reason why the column has different datatypes in 
different tables?

I have toyed with the ideas of creating a database update script which 
forces all the group_uid columns to be INT, but without a lot of code 
analyzing, I'm not sure whether this would work.