在eXtremeDB中，Compound tree Index使用方式。
Compound Tree的定義：
tree<key1, key2,...,keyn>
Compound indexes can be used only for exact match searches. More precisely, it is possible using a compound index to execute the following query:
key1=val1 AND key2=val2 AND ... keyN-1=valN-1 AND keyN >valN.
(Note that only the last condition is “>”,all other are “=”.)