The Layer Under SQL
Above the disk sits the storage engine, the component that turns rows and indexes into bytes on disk and back. The query planner decides what to fetch, but the storage engine actually moves the data. Examples include InnoDB for MySQL and the heap and btree access methods in PostgreSQL.
Pages Are The Unit
The engine never reads a single byte. It reads and writes fixed size blocks called pages, typically eight or sixteen kilobytes. A page holds many rows plus a header and a slot directory that points to each row inside it.
- The header records page type, free space, and checksums.
- The slot directory lets rows move within a page without changing their identifiers.
- Reading one row pulls its whole page into the buffer pool.
Why Pages
Aligning to a page size matches how disks and the operating system move data, so a row lookup costs one page read. It also makes caching, locking, and crash recovery operate on uniform units.
Key idea
The storage engine is the layer that maps logical rows to physical pages, and the page is the fixed size unit it reads, writes, caches, and recovers.