On Wed, May 15, 2013 at 08:12:34AM +0900, Michael Paquier wrote:
> The concept of clause parallelism for backend worker is close to the
> concept of clause shippability introduced in Postgres-XC. In the case of
> XC, the equivalent of the master backend is a backend located on a node
> called Coordinator that merges and organizes results fetched in parallel
> from remote nodes where data scans occur (on nodes called Datanodes). The
> backends used for tuple scans across Datanodes share the same data
> visibility as they use the same snapshot and transaction ID as the backend
> on Coordinator. This is different from the parallelism as there is no idea
> of snapshot import to worker backends.
Worker backends would indeed share snapshot and XID.
> However, the code in XC planner used for clause shippability evaluation is
> definitely worth looking at just considering the many similarities it
> shares with parallelism when evaluating if a given clause can be executed
> on a worker backend or not. It would be a waste to implement twice the same
> thing is there is code already available.
Agreed. Local parallel query is very similar to distributed query; the
specific IPC cost multipliers differ, but that's about it. I hope we can
benefit from XC's experience in this area.
--
Noah Misch
EnterpriseDB http://www.enterprisedb.com