Re: [pgsql-hackers] Daily digest v1.5568 (24 messages)

On Mon, 21 Nov 2005, Marc Munro wrote:

> I wonder if this idea might be taken a little further, to allow
> read-only tablespaces?
>
> This would allow old partitions in very large databases to be kept on
> read-only media, and would allow normal backups to ignore this
> unchanging set of data.

I guess you could do that, but it's really quite a different problem.

> It also allows for certain specific optimisations for this type of data,
> as the MVCC rules are now relaxed. Inclusion of a row in a read-only
> index is now enough to guarantee the visibility of that row to all
> backends, and fetches based solely on the index now become possible.

There's this TODO:

> Allow data to be pulled directly from indexes
>
> Currently indexes do not have enough tuple visibility information to
> allow data to be pulled from the index without also accessing the heap.
> One way to allow this is to set a bit to index tuples to indicate if a
> tuple is currently visible to all transactions when the first valid
> heap lookup happens. This bit would have to be cleared when a heap
> tuple is expired.

That method doesn't require read-only tablespaces.

- Heikki