Radim Marek takes us through the structure of a page:
If you read previous post about buffers, you already know PostgreSQL might not necessarily care about your rows. You might be inserting a user profile, or retrieving payment details, but all that Postgres works with are blocks of data. 8KB blocks, to be precise. You want to retrieve one tiny row? PostgreSQL hauls an entire 8,192-byte page off the disk just to give it to you. You update a single boolean flag? Same thing. The 8KB page is THE atomic unit of I/O.
But knowing those pages exist isn’t enough. To understand why the database behaves the way it does, you need to understand how it works. Every time you execute
INSERT, PostgreSQL needs to figure out how to fit it into one of those 8,192-byte pages.
It is a little wild how three of the largest relational database systems use 8KB pages. I know that, on the SQL Server side, they’ve experimented with different page sizes internally and have repeatedly said that, even recently (the last time I heard this was maybe about 3 years ago at a SQL Saturday), there just isn’t a benefit from moving away from 8KB. But what’s in those 8KB differ, and Radim goes into details on what’s in PostgreSQL.