Re: Heap WARM Tuples - Design Draft

On Thu, Aug 4, 2016 at 10:49 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:

> On Thu, Aug 4, 2016 at 06:16:02PM +0100, Simon Riggs wrote:
> > On 4 August 2016 at 18:05, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> >
> > >> Approach 2 seems more reasonable and simple.
> > >>
> > >> There are only 2 bits for lp_flags and all combinations are already
> used. But
> > >> for LP_REDIRECT root line pointer, we could use the lp_len field to
> store this
> > >> special flag, which is not used for LP_REDIRECT line pointers. So we
> are able
> > >> to mark the root line pointer.
> > >
> > > Uh, as I understand it, we only use LP_REDIRECT when we have _removed_
> > > the tuple that the ctid was pointing to, but it seems you would need to
> > > set HEAP_RECHECK_REQUIRED earlier than that.
> >
> > Hmm. Mostly there will be one, so this is just for the first update
> > after any VACUUM.
> >
> > Adding a new linepointer just to hold this seems kludgy and could mean
> > we run out of linepointers.
>
> Ah, so in cases where there isn't an existing LP_REDIRECT for the chain,
> you create one and use the lp_len to identify it as a WARM chain? Hmm.
>
>
If the root tuple still exists, we store the WARM flag (or
HEAP_RECHECK_REQUIRED as used in the original post) in the tuple header
itself. When the root tuple becomes dead and HOT prune decides to replace
it with a LP_REDIRECT line pointer, the information is moved to lp_len
(which is currently set to 0 for LP_REDIRECT items). Does that answer your
question?

> You can't update the indexes pointing to the existing ctid, so what you
> would really have to do is to write over the existing ctid with
> LP_REDIRECT plus WARM marker, and move the old ctid to a new ctid slot?
>
>
Not really. I hope the above answers this, but please let me know if you
mean something else.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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