fix bug for missing where clause
fix several bugs
change query_fields default value to {}
fix bug when op query_fields is None
fix for getFeildValue bug
changed estimatenode behavior
fix bug when some distinct values are NoneType
add cardinality ratio to costmodel
fix bug for nonkey
change the host of orig_db to metadata host
ensure shard keys as index
a new shard key iterator
generated short compound keys first
convert back to generate long compound key first
should raise error when finished
use thread safe num_nodes
fix bug for comparison
output all digits for cost
fix bug for not passing num_nodes
use bulk write for dbmigrator