diff --git a/lib/galaxy/model/util.py b/lib/galaxy/model/util.py
index 76128609496..7295589ab44 100644
--- a/lib/galaxy/model/util.py
+++ b/lib/galaxy/model/util.py
@@ -10,25 +10,24 @@ def pgcalc( sa_session, id, dryrun=False ):
     TODO: Check against the recently updated versions of sqlalchemy if this
     'special' postgresql version is even necessary.
     """
-    sql = """
-           UPDATE galaxy_user
-              SET disk_usage = (SELECT COALESCE(SUM(total_size), 0)
-                                  FROM (  SELECT DISTINCT ON (d.id) d.total_size, d.id
-                                            FROM history_dataset_association hda
-                                                 JOIN history h ON h.id = hda.history_id
-                                                 JOIN dataset d ON hda.dataset_id = d.id
-                                           WHERE h.user_id = :id
-                                                 AND h.purged = false
-                                                 AND hda.purged = false
-                                                 AND d.purged = false
-                                                 AND d.id NOT IN (SELECT dataset_id
-                                                                    FROM library_dataset_dataset_association)
-                                         ) sizes)
-            WHERE id = :id
-        RETURNING disk_usage;
-    """
-    r = sa_session.execute(sql, {'id': id})
-    new = r.fetchone()[0]
+    sql_calc = """SELECT COALESCE(SUM(total_size), 0)
+                  FROM (  SELECT DISTINCT ON (d.id) d.total_size, d.id
+                          FROM history_dataset_association hda
+                               JOIN history h ON h.id = hda.history_id
+                               JOIN dataset d ON hda.dataset_id = d.id
+                          WHERE h.user_id = :id
+                                AND h.purged = false
+                                AND hda.purged = false
+                                AND d.purged = false
+                                AND d.id NOT IN (SELECT dataset_id
+                                                 FROM library_dataset_dataset_association)
+                  ) sizes"""
+    sql_update = """UPDATE galaxy_user
+                    SET disk_usage = (%s)
+                    WHERE id = :id
+                    RETURNING disk_usage;""" % sql_calc
     if dryrun:
-        sa_session.rollback()
-    return new
+        r = sa_session.execute(sql_calc, {'id': id})
+    else:
+        r = sa_session.execute(sql_update, {'id': id})
+    return r.fetchone()[0]