Re: Freeze avoidance of very large table.

On 04/23/2015 05:52 PM, Jim Nasby wrote:
> On 4/23/15 2:42 AM, Heikki Linnakangas wrote:
>> On 04/22/2015 09:24 PM, Robert Haas wrote:
>>> Yeah. We have a serious need to reduce the size of our on-disk
>>> format. On a TPC-C-like workload Jan Wieck recently tested, our data
>>> set was 34% larger than another database at the beginning of the test,
>>> and 80% larger by the end of the test. And we did twice the disk
>>> writes. See "The Elephants in the Room.pdf" at
>>> https://sites.google.com/site/robertmhaas/presentations
>>
>> Meh. Adding an 8-byte header to every 8k block would add 0.1% to the
>> disk size. No doubt it would be nice to reduce our disk footprint, but
>> the page header is not the elephant in the room.
>
> I've often wondered if there was some way we could consolidate XMIN/XMAX
> from multiple tuples at the page level; that could be a big win for OLAP
> environments where most of your tuples belong to a pretty small range of
> XIDs. In many workloads you could have 80%+ of the tuples in a table
> having a single inserting XID.

It would be doable for xmin - IIRC someone even posted a patch for that
years ago - but xmax (and ctid) is difficult. When a tuple is inserted,
Xmax is basically just a reservation for the value that will be put
there later. You have no idea what that value is, and you can't
influence it, and when it's time to delete/update the row, you *must*
have the space for that xmax. So we can't opportunistically use the
space for anything else, or compress them or anything like that.

- Heikki