Vacuum processing is a maintenance process that facilitates the persistent operation of PostgreSQL. Its two main tasks are removing dead tuples and the freezing transaction ids
To remove dead tuples, vacuum processing provides two modes, namely Concurrent VACUUM and Full VACUUM. Concurrent VACUUM, often simply called VACUUM, removes dead tuples for each page of the table file, and other transactions can read the table while this process is running. In contrast, Full VACUUM removes dead tuples and defragments live tuples in the whole file, and other transactions cannot access tables while Full VACUUM is running.
Despite the fact that vacuum processing is essential for PostgreSQL, improving its functionality has been slow compared to other functions. For example, until version 8.0, this process had to be executed manually (with the psql utility or using the cron daemon). It was automated in 2005 when the autovacuum daemon was implemented.
Since vacuum processing involves scanning whole tables, it is a costly process. In version 8.4 (2009), the Visibility Map (VM) was introduced to improve the efficiency of removing dead tuples. In version 9.6 (2016), the freeze process was improved by enhancing the VM.
真空处理是一种维护过程，旨在支持PostgreSQL的持续运行。其主要任务是移除死元组（dead tuples）和冻结事务ID（transaction ids）。
为了移除死元组，真空处理提供了两种模式，即并发真空（Concurrent VACUUM）和完整真空（Full VACUUM）。并发真空通常简称为VACUUM，它逐页移除表文件中的死元组，而其他事务在该过程运行时仍可读取表。相比之下，完整真空移除死元组并整理整个文件中的存活元组，其他事务在完整真空运行期间无法访问表。
尽管真空处理对PostgreSQL至关重要，但其功能的改进速度与其他功能相比较为缓慢。例如，直到8.0版本（2005年），该过程仍需手动执行（通过psql工具或使用cron守护进程）。2005年，随着自动真空（autovacuum）守护进程的实现，这一过程才得以自动化。
由于真空处理涉及扫描整个表，它是一个开销较大的过程。在8.4版本（2009年），引入了可见性映射（Visibility Map, VM）来提高移除死元组的效率。在9.6版本（2016年），通过优化VM进一步改进了冻结过程。