Gautam Das Dimitrios Gunopulos Nick Koudas Nikos Sarkas Ad-hoc Top-k Query Answering for Data Streams. 183-194 2007 conf/vldb/2007 VLDB http://www.vldb.org/conf/2007/papers/research/p183-das.pdf db/conf/vldb/vldb2007.html#DasGKS07