Re: Getting rid of cmin and cmax

Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
>
>> I'm thinking of removing cmin and cmax, and keeping that information in
>> backend-private memory instead.
>>
>
> I don't believe you can remove *both*. What's been discussed is
> removing one of them, by letting the field represent a lookup key for an
> in-memory structure in the infrequent case that xmin and xmax are both
> the current xact. You solve the table size problem by only having one
> entry for each unique cmin/cmax pair in use.
>

That's another possibility, but removing both cmin and cmax has also
been discussed. It's also mentioned in the TODO item:

> One possible solution is to create a phantom cid which represents a
cmin/cmax pair and is stored in local memory. *Another idea is to store
both cmin and cmax only in local memory.*

Saving 4 bytes per tuple with the phantom cid is nice, but saving 8
bytes (assuming we get rid of xvac in the future, or overlay it with
xmin for example) is even better.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com