slow read for cassandra time series

[michael]

I have a slow query that is making me think I don't understand the data model 
for
time series: 
select asset, returns from marketData where date >= 20130101 and date <= 
20130110
allow filtering;

create table marketData {
asset varchar,
returns double,
date timestamp,
PRIMARY KEY(asset, date) }

cassandra blows up with out of memory exception when one's day of data
is only about 1 mb in size.