On Mon, Jul 1, 2019 at 7:53 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> 3. Recognise UNDO_SHARED record set boundaries differently. Whereas
> undolog.c recognises transaction boundaries automatically for the
> other categories (UNDO_PERMANENT, UNDO_UNLOGGED, UNDO_TEMP), for
> UNDO_SHARED the
... set of records inserted in between calls to
BeginUndoRecordInsert() and FinishUndoRecordInsert() calls is
eventually discarded as a unit, and the rm_undo_status() callback for
the calling AM decides when that is allowed. In contrast, for the
other categories there may be records from any number undo-aware AMs
that are entirely unaware of each other and they must all be discarded
together if the transaction commits and becomes all visible, so
undolog.c automatically manages the boundaries to make that work when
inserting.
--
Thomas Munro
https://enterprisedb.com