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Thread: Re: [HACKERS] varchar/char size

Re: [HACKERS] varchar/char size

From
darrenk@insightdist.com (Darren King)
Date:
>
> A related question: Is it possible to store tuples over more than one
> block? Would it be possible to split a big TEXT into multiple blocks?
>

Possible, but would cut the access speed to (1 / # blocks), no?

There is a var in the tuple header, t_chain, 6.2.1 that has since been
removed for 6.3.  I think its original purpose was with time-travel,
_but_, if we go with a ROWID instead of an oid in the future, this could
be put back in the header and would be the actual address of the next
block in the chain.

Oracle has this concept of chained rows.  It is how they implement all
of their LONG* types and also handle rows of normal types that are
larger than the block size.

darrenk

Re: [HACKERS] Storing rows bigger than one block

From
Mattias Kregert
Date:
Darren King wrote:
> > A related question: Is it possible to store tuples over more than one
> > block? Would it be possible to split a big TEXT into multiple blocks?
> Possible, but would cut the access speed to (1 / # blocks), no?

For "big" (multile blocks) rows, maybe. Consecutive blocks should be
buffered by the disk or the os, so I don't think the difference would
be big, or even noticeable.

> There is a var in the tuple header, t_chain, 6.2.1 that has since been
> removed for 6.3.  I think its original purpose was with time-travel,
> _but_, if we go with a ROWID instead of an oid in the future, this could
> be put back in the header and would be the actual address of the next
> block in the chain.
>
> Oracle has this concept of chained rows.  It is how they implement all
> of their LONG* types and also handle rows of normal types that are
> larger than the block size.

Yes! I can't see why PostgreSQL should not be able to store rows bigger
than one block? I have seen people referring to this limitation every
now and then, but I don't understand why it has to be that way?
Is this something fundamental to PostgreSQL?

/* m */

Re: [HACKERS] Storing rows bigger than one block

From
"Vadim B. Mikheev"
Date:
Mattias Kregert wrote:
>
> Darren King wrote:
>
> > There is a var in the tuple header, t_chain, 6.2.1 that has since been
> > removed for 6.3.  I think its original purpose was with time-travel,
> > _but_, if we go with a ROWID instead of an oid in the future, this could
> > be put back in the header and would be the actual address of the next
> > block in the chain.

No, this is not for time-travel. Look at implementation guide.

> >
> > Oracle has this concept of chained rows.  It is how they implement all
> > of their LONG* types and also handle rows of normal types that are
> > larger than the block size.
>
> Yes! I can't see why PostgreSQL should not be able to store rows bigger
> than one block? I have seen people referring to this limitation every
> now and then, but I don't understand why it has to be that way?
> Is this something fundamental to PostgreSQL?
                    ^^^^^^^^^^^
It seems that answeer is "No". Just - not implemented feature.
Personally, I would like multi-representation feature more than that.
And easy to implement.

Vadim