On 14 February 2016 at 08:05, Robert Haas <robertmh...@gmail.com> wrote:
> First, the overall concept here is that processes can either be a > member of a lock group or a member of no lock group. The concept of a > lock group is formally separate from the concept of a parallel group > created by a ParallelContext, but it is not clear that there will ever > be any other context in which a lock group will be a good idea. It is > not impossible to imagine: for example, suppose you had a group of > backends that all had TCP client connections, and those processes all > wanted to ingest data and stuff it in a table but without allowing any > unrelated process to touch the table, say because it was going to be > inconsistent during the operation and until indexes were afterwards > rebuilt. The case that comes to mind for me is in logical decoding, for decoding prepared xacts. Being able to make the prepared xact a member of a "lock group" along with the decoding session's xact may provide a solution to the locking-related challenges there. I haven't looked closely at what's involved in the decoding prepared xact locking issues yet, just an idea. To do this it'd have to be possible to add an existing session/xact to a lock group (or make it the leader of a new lock group then join that group). Do you think that's practical with your design? > I don't have any plans to implement anything like that but I > felt it was better to keep the concept of a lock group - which is a > group of processes that cooperate so closely that their locks need not > conflict - from the concept of a parallel context - which is a leader > process that is most likely connected to a user plus a bunch of > ephemeral background workers that aren't. That way, if somebody later > wants to try to reuse the lock grouping stuff for something else, > nothing will get in the way of that; if not, no harm done, but keeping > the two things decoupled is at least easier to understand, IMHO. > Yeah, strong +1 -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
