statistics the total number of documents return -1

We use oak1.10.2 inside a web application to store documents 

How can we program statistics such as : 

• the total number of documents 
• compute the number of documents per property values for a given property ?

we try these code,but it return -1

QueryManager qm = session.getWorkspace().getQueryManager(); 
Query q = qm.createQuery("SELECT * FROM [nt:file] WHERE LOCALNAME() LIKE '%.txt' and [\"jcr:createdBy\"] = 'anonymous'", Query.JCR_SQL2); 
QueryResult qr = q.execute(); 
long stat = qr.getRows().getSize();