Re: tracking commit timestamps

On Sat, Nov 8, 2014 at 5:35 AM, Petr Jelinek  wrote:
> That's not what I said. I am actually ok with adding the LSN if people see
> it useful.
> I was just wondering if we can make the record smaller somehow - 24bytes per
> txid is around 96GB of data for whole txid range and won't work with pages
> smaller than ~4kBs unless we add 6 char support to SLRU (which is not hard
> and we could also not allow track_commit_timestamps to be turned on with
> smaller pagesize...).
>
> I remember somebody was worried about this already during the original patch
> submission and it can't be completely ignored in the discussion about adding
> more stuff into the record.

Fair point.  Sorry I misunderstood.

I think the key question here is the time for which the data needs to
be retained.  2^32 of anything is a lot, but why keep around that
number of records rather than more (after all, we have epochs to
distinguish one use of a given txid from another) or fewer?  Obvious
alternatives include:

- Keep the data for some period of time; discard the data when it's
older than some threshold.
- Keep a certain amount of total data; every time we create a new
file, discard the oldest one.
- Let consumers of the data say how much they need, and throw away
data when it's no longer needed by the oldest consumer.
- Some combination of the above.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company