Current releases of PostgreSQL need to read every page in the database at least once every 2 billion write transactions (less, with default settings) to verify that there are no old transaction IDs on that page which require "freezing".
All of a sudden, when the number of transaction IDs that have been consumed crosses some threshold, autovacuum begins processing one or more tables, reading every page. This consumes much more I/O bandwidth, and exerts much more cache pressure on the system, than a standard vacuum, which reads only recently-modified page.
而作者送了 patch 改成只會讀還沒搞定的部份：
Instead of whole-table vacuums, we now have aggressive vacuums, which will read every page in the table that isn't already known to be entirely frozen.
要注意的是，agreesive vacuum 相較於 vacuum 會多吃很多資源，但可以打散掉 (有點像一次大 GC 導致 lag 與多次 minor GC 讓程式反應時間變得比較順暢的比較)：
An aggressive vacuum still figures to read more data than a regular vacuum, possibly a lot more. But at least it won't read the data that hasn't been touched since the last aggressive vacuum, and that's a big improvement.
這個功能預定在 PostgreSQL 9.6 出現，不知道會不會變 default...