Paul A Jungwirth <pj@illuminatedcomputing.com> writes:
> The docs for cmax say:[0]
>> The command identifier within the deleting transaction, or zero.
> This was true once upon a time, I think. But nowadays cmax and cmin
> are the same physical field, and the user-facing system columns don't
> seem to be trying to interpret it.
Yeah, this is a mess. Nobody ever updated this text when we decided we
could pack those fields into one. I think it would be better to do
what you suggest:
> ... And maybe we should be more drastic: combine cmin &
> cmax into one entry, and explain that they are two names for the same
> value, which might signify the insert cid, the delete cid, or a
> combocid.
I'm not sure about good wording, but maybe like
cmin, cmax:
Originally, cmin and cmax were separate fields. cmin was the
inserting command's command identifier within the inserting
transaction, while cmax was the updating or deleting command's
command identifier within the updating/deleting transaction, or
zero if no update or delete attempt had occurred yet. Nowadays
these system columns refer to the same field and will always read
as the same value. That might be the inserting command's command
identifier, or the deleting command's command identifier, or a
"combocid" that reflects both actions when those happened in the
same transaction.
I don't know if we want to go into any more detail than that.
regards, tom lane
