Re: Free space management within heap page

"Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com> wrote:

> > The overwhelming vast majoirty of tuples are going to be in one or more
> > indexes. Which means nearly all tuples are going to fall into this
> > category. So where's the benefit?
>
> The line pointers can not reused, but the space consumed by the tuple can be.
> So the benefit is in utilizing that space for newer tuples and thus reduce the
> bloat.

I think your idea is same as the following TODO Item, that I suggested before.

* Consider shrinking expired tuples to just their headers.
http://archives.postgresql.org/pgsql-patches/2006-03/msg00142.php
http://archives.postgresql.org/pgsql-patches/2006-03/msg00166.php

> One assumption I am making here is that its sufficient to mark the line pointer
> "unused" (reset LP_USED flag) even though there is an index entry pointing to
> the tuple. During index scan, we anyways check for ItemIdIsUsed() before
> proceeding further. I know it might break the ctid chain, but does that really
> matter ? I don't see any reason why somebody would need to follow ctid chain
> past a dead tuple.

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.
We need to work around the problem.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center