On 4/22/15 1:51 PM, Kevin Grittner wrote:
> (1) WAL log the insert.
> (2) Write the tuple.
> (3) Hint and rewrite the tuple.
> (4) WAL log the freeze of the tuple.
> (5) Rewrite the frozen tuple.
> (6) WAL-log the delete.
> (7) Rewrite the deleted tuple.
> (8) Prune and rewrite the page.
> (9) Free line pointers and rewrite the page.
>
> If I was lucky some of the writes could be combined in cache
> because they happened close enough together. Also, one could hope
> that not too much of the WAL-logging involved full page writes to
> the WAL -- again, keeping steps close together in time helps with
> that.
This is why I like the idea of methods that tell us where we need to do
cleanup... they provide us with a rough ability to track what tuples are
in what part of their lifecycle. The VM helps with this a small amount,
but really it only applies after 1 and 6; it doesn't help us with any
other portions.
Having a way to track recently created tuples would allow us to be much
more efficient with 1-3, and with aggressive freezing, 1-5. A way to
track recently deleted tuples would help with 6-7, possibly 6-9 if no
indexes.
If we doubled the size of the VM, that would let us track 4 states for
each page:
- Page has newly inserted tuples
- Page has newly deleted tuples
- Page is all visible
- Page is frozen
though as discussed elsewhere, we could probably combine all visible and
frozen.
The win from doing this would be easily knowing what pages need hinting
(newly inserted) and pruning (newly deleted). Unfortunately we still
wouldn't know whether we could do real work without visiting the page
itself, but I suspect that for many workloads just having newly
inserted/deleted would be a serious win.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com