Re: Free space management within heap page

Heikki Linnakangas <heikki(at)enterprisedb(dot)com> wrote:

> > * Consider shrinking expired tuples to just their headers.
>
> Yeah, same idea. You suggested in that thread that we should keep the
> headers because of line pointer bloat, but I don't see how that's
> better. You're still going to get some line pointer bloat, but not able
> to reclaim as much free space.

That is not an essential solution, as you are aware. I think it will be
better to combine tuple shrinking and other restrictions of LP area.

> > 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?

BLCKSZ is typically 8192 bytes and sizeof(ItemPointerData) is 4 bytes.
1/4 comes from 8192 / 4 = 2048. If we allow zero-size tuples, the line
pointers area can bloat up to the ratio. We have tuples no less than
32 bytes-size, so the area is restricted 256 bytes now.

> 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.

We can recycle unused line pointers, but we cannot shrink the area unless
the tail end of line pointers are removed. i.e, unusable free space will
remains at the middle of LP area.

[used lp][***unusable free space***][used lp] [free space] [heap tuples]

> Would it be enough cap the number of dead line pointers with a simple
> rule like "max 20% of line pointers can be dead"? I'd be happy with that.

Yeah, I think it is enough, too. It might be a signal of vacuum.

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