Re: auto truncate/vacuum full

Alvaro Herrera  writes:
> Now 40 mins walking those pages to figure out that they need to be
> truncated, I concede that it's too much.  Maybe we shouldn't be doing a
> backwards scan; perhaps this breaks the OS readahead and make it even
> slower.

That's very possible, since a backwards scan is guaranteed to destroy
any rotational positioning the OS has done --- you'll have to wait
at least one disk rotation for each page, whereas a forward scan could
hope to do better than that.

Maybe we could do this along with my idea of incremental truncation.
Scan the last K pages of the relation *forwards*, truncate as
appropriate, repeat, until finding a nonempty page.  Choosing the max
value of K might be a bit tricky.

			regards, tom lane