solr的分组和统计功能
http://localhost:8983/solr/select?q=*:*&stats=true&stats.field=price&stats.field=popularity&rows=0&indent=true
http://localhost:8983/solr/collection1/select?q=*%3A*&wt=xml&indent=true&group=true&group.field=popularity&group.ngroups=true
可以实现 sum min等
http://wiki.apache.org/solr/FieldCollapsing
http://wiki.apache.org/solr/FunctionQuery
http://wiki.apache.org/solr/StatsComponent