PostgreSQL实现MVCC.

这意味着每次进行更新时，都会创建一个新的行副本并将旧的副本标记为已删除(但并未物理删除).

This means that each time you make an update, a new copy of row is created and the old one is marked as deleted (but is not physically deleted).

这会减慢查询速度.

你应该及时运行VACUUM.

PostgreSQL 8.4.4 运行 autovacuum 守护程序来执行此操作，但它可能会在您的安装中出现一些问题.

PostgreSQL 8.4.4 runs autovacuum daemon to do this, but it may have some problems on your installation.

当您手动运行 VACUUM 时情况会改善吗?

Does the situation improve when you run VACUUM manually?