Bro and port scan

I have another questions regarding bro(version 07a90).I'am testing the
of bro to detect port scanner.After , i have found others problem:

1) It don't detect Fin, Xmas and Null scans.
2) Bro don't detect two time the seme scan betweent the same hosts on
the same ports.
3) It consume a great quantity of memory.

To resolve the first i have added a script to detect Fin, Xmas and Null
scans usuing the weird event their produce.And all works good.
To resolve the 2° and the 3°, i have added to scan.bro a recursive
function (that use bro delete function and a table of support) to delete
the record of the table scan_triples(is deleted also the table of support).
While the first problem seems to be resolved, the second is gotten worse.
There is a way to easy delete a subset of a table? or another way to
reduce the consume of memory of the scan analyzer?

Pierfrancesco Porcu.