Struct PagePool 

pub struct PagePool { /* private fields */ }

The pages every reader of one database keeps, under one budget in bytes.

A reader lives as long as the database does, so the pages it holds are what the next query finds already in memory. They used to be four stripes a column, oldest out first, which on TPC-H SF1 meant every query read every page of lineitem off the file again and paid the system call for it. Keeping every page there costs 38 MB and took a third of the system time off the suite.

So the question is no longer how many stripes a column keeps but how many bytes the database does, and one budget answers it for every reader at once. A table nobody queries gives its pages up to one that is being queried, which a count per column cannot do.

Pages leave by the clock. Each has a bit a read sets, and when the pool is over budget it walks from the oldest: a page with the bit set loses the bit and goes round again, and a page without it goes. That keeps what is read over and over and lets a page one scan read once go first.

The old count is still a floor. A column never gives up a page while it holds four or fewer, because a scan whose workers evict each other’s pages reads a quarter of a megabyte for every part it takes, and a budget of zero is the cache as it was before the pool existed.

Implementations

impl PagePool

pub fn new(budget: usize) -> Self

A pool that keeps up to budget bytes of pages beyond each column’s floor.

pub fn bytes(&self) -> usize

The bytes of pages the pool is counting now.

Panics

If the pool’s lock is poisoned, which takes a panic while it was held.

Trait Implementations

impl Clone for PagePool

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for PagePool

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for PagePool

fn default() -> Self

Returns the “default value” for a type.