Re: vacuum, performance, and MVCC

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Yeah, you should be able to find the older version easily enough, if you
> arrived at the newer version and realized you needed to visit the older
> version. But this fails in scenarios where you are searching on a
> column that's been updated --- the index entry for the newer version
> will not lead you to visit it at all. I've not seen any documentation
> about what Oracle does with cases like that.

That's a good question. I had the impression rollback was handled at a lower
level so index pages were stored in rollback segments just like table pages.
If you saw that the page of the index had been updated you went and fetched
the index page of the appropriate age from the rollback segment, giving you
pointers to the right records or at least the right places to start from.

Now that I think about it though I'm not too sure. It seems like the
performance of that would be pretty awful.

--
greg