how to design Hbase schema?
Secondary indexes would indeed be useful for many potential applications of HBase, and I believe the developers are in fact looking at it. Checkout http://www.mail-archive.com/hbase-dev@hadoop.apache.org/msg04801.html.
In the mean time though, if your application data storage can be modelled as a star schema (see http://en.wikipedia.org/wiki/Star_schema) you might like to checkout the solution that Hypertable proposes for secondary index-type needs http://markmail.org/message/rphm4q6cbar2ycgp
I recommend having two different flat tables: one for looking up attributes+values given entityID, and one for looking up the entityID given attributes+values.
Table 1 would look like this:
entityID1 { attribute1: value1; attribute2: value2; ...}and Table 2:
attribute1_value1 { entityID1;}attribute2_value2 { entityID1;}