MySQL specific SQL doesn't work with PostgreSQL

There are many MySQL specific SQL statements in the xmlsitemap_node module. I.e. the function xmlsitemap_node_enable uses a MySQL specific UPDATE statement:

UPDATE {xmlsitemap_node} xn, {node} n
SET xn.previously_changed = xn.last_changed, xn.last_changed = n.changed
WHERE xn.nid = n.nid AND xn.last_changed <> n.changed

for PostgreSQL this should be:

UPDATE {xmlsitemap_node}
SET previously_changed = last_changed, last_changed = n.changed
FROM {node} n
WHERE nid = n.nid AND last_changed <> n.changed

Many statements of that kind can be found in the code and should be reworked. If I find the time I'll provide patches for PostgreSQL.