Re: Changeset Extraction v7.1

On Sat, Jan 25, 2014 at 5:25 PM, Andres Freund <andres@2ndquadrant.com> wrote:
>> I'm also wondering about
>> whether we've got the right naming here.  AFAICT, it's not the case
>> that we're going to use the "catalog xmin" for catalogs and the "data
>> xmin" for non-catalogs.  Rather, the "catalog xmin" is going to always
>> be included in globalxmin calculations, so IOW it should just be
>> called "xmin".
>
> Well, not really. That's true for GetSnapshotData(), but not for
> GetOldestXmin() where we calculate an xmin *not* including the catalog
> xmin. And the data_xmin is always used, regardless of
> catalog/non_catalog, we peg the xmin further for catalog tables, based
> on that value.
> The reason for doing things this way is that it makes all current usages
> of RecentGlobalXmin safe, since that is the more conservative
> value. Only in inspected location we can use RecentGlobalDataXmin which
> *does* include data_xmin but *not* catalog_xmin.

Well, OK, so I guess I'm turned around.  But I guess my point is - if
one of data_xmin and catalog_xmin is really just xmin, then I think it
would be more clear to call that one "xmin".

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