Re: Remove xmin and cmin from frozen tuples

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Considering the cost/benefits, rather than doing some optimization for
> long-lived tuples, I would like to see us merge the existing
> xmin/xmax/cmin/cmax values back into three storage fields like we had in
> 7.4 and had to expand to a full four in 8.0 to support subtransactions.

There is another reason for trying to do that rather than the frozen-row
optimization, which is that to get it down to two visibility-related
fields, we'd have to find another representation for tuples that are
Datums in memory.  The current Datum representation overlays three int32
fields where the visibility fields are for a tuple on-disk.  This works
fine now, and would still work fine if we could revert to the 7.4
approach, but it doesn't play nicely with a scheme to remove 2 of the 4
fields.
        regards, tom lane