add specific clause for postgresql to purge ofPubsubItem

http://issues.igniterealtime.org/browse/OF-756

src/java/org/jivesoftware/openfire/pubsub/PubSubPersistenceManager.java

@@ -72,6 +72,14 @@ public class PubSubPersistenceManager {
 			"ORDER BY creationDate DESC LIMIT ?) AS noDelete " +
 			"ON ofPubsubItem.id = noDelete.id WHERE noDelete.id IS NULL AND " +
 			"ofPubsubItem.serviceID = ? AND nodeID = ?";
+    
+    private static final String PURGE_FOR_SIZE_POSTGRESQL = 
+    		"DELETE from ofPubsubItem where id in " +
+    		"(select ofPubsubItem.id FROM ofPubsubItem LEFT JOIN " +
+    		"(SELECT id FROM ofPubsubItem WHERE serviceID=? AND nodeID=? " +
+    		"ORDER BY creationDate DESC LIMIT ?) AS noDelete " +
+    		"ON ofPubsubItem.id = noDelete.id WHERE noDelete.id IS NULL " +
+    		"AND ofPubsubItem.serviceID = ? AND nodeID = ?)";
 
 	private static final String PURGE_FOR_SIZE_HSQLDB = "DELETE FROM ofPubsubItem WHERE serviceID=? AND nodeID=? AND id NOT IN "
 			+ "(SELECT id FROM ofPubsubItem WHERE serviceID=? AND nodeID=? ORDER BY creationDate DESC LIMIT ?)";
@@ -1917,6 +1925,8 @@ public class PubSubPersistenceManager {
 	{
 		switch (type)
 		{
+		case postgresql:
+			return PURGE_FOR_SIZE_POSTGRESQL;
 		case hsqldb:
 			return PURGE_FOR_SIZE_HSQLDB;