The type parameter is of type DBTYPE, and must be set to one of DB_BTREE, DB_HASH,
DB_HEAP, DB_QUEUE, DB_RECNO, or DB_UNKNOWN. If type is DB_UNKNOWN, the database must
already exist and Db::open() will automatically determine its type. The Db::get_type() (page
64) method may be used to determine the underlying type of databases opened using
DB_UNKNOWN.
It is an error to specify the incorrect type for a database that already exi
原文：https://www.cnblogs.com/hshy/p/13237308.html