索引
创建索引
create index employees_index
on table employees(country)
as 'bitmap' #使用bitmap函数建立索引
with deferred rebuild
idxproperties('creator'='dirk','created_at'='some_time')
in table employees_index_table
partitioned by (country,name)
comment 'employees indexed by country and name'
显示表的索引 #formatted是可选
show formatted index on employees
show formatted indexed on employees
删除索引表（不确定）
drop index if exists employees_index on table employees;
索引表的handler，会初始化，定义，验证索引表的schema
在索引建立时，会使用rebuilding process，读取表为索引表并且写到索引数据库中，所有的存储都会
索引删除时，会删除明显的索引数据库
handler会参与优化查询
