Re: Free space management within heap page

ITAGAKI Takahiro wrote:

> Keeping only line pointers itself is not a problem, but it might lead
> bloating of line pointers. If a particular tuple in a page is replaced
> repeatedly, the line pointers area bloats up to 1/4 of the page.

Where does the 1/4 figure come from?

> We need to work around the problem.

If a row is updated many times until vacuum comes along, what currently
happens is that we end up with a bunch of pages full of dead tuples.
With the truncation scheme, we could fit way more dead tuples on each
page, reducing the need to vacuum. If a row is for example 40 bytes
long, including header (a quite narrow one), you could fit 10 line
pointers to the space of one row, which means that you could ideally
multiply your vacuum interval by a factor of 10x. That's a huge benefit,
though indexes would still bloat unless selects marking index pointers
as dead keep the bloat in control.

The problem is that if a tuple is updated say hundreds of times before
vacuum, but then it's not updated anymore, you'll have a page full of
useless line pointers that are not reclaimed. Clearly we should start
reclaiming line pointers, but we can only do that for unused line
pointers after the last used one.