On Wed, Oct 14, 2015 at 4:06 PM, Robert Haas <robertmh...@gmail.com> wrote:
> On Wed, Oct 14, 2015 at 6:28 PM, dinesh kumar <dineshkuma...@gmail.com> > wrote: > > I see this feature as an add on to do the parallel DML operations. > > There won't be any problem, if operations are mutually exclusive. > > I mean, each session operates on unique set of tuples. > > > > In the above case, we don't even need of SKIP LOCKED wait policy. > > > > But, when it comes to mutually depend operations, isn't it nice to > provide, > > how much were locked by the other sessions. OR atlest a HINT to the other > > session like, > > > > GET DIAGNOSTICS var = DID_I_MISS_ANYTHING_FROM_OTHER_SESSIONS; > > > > I agree that, adding counter will take a performance hit. > > Rather going to my actual proposal on providing the counter value, > > isn't it good to provide a boolean type HINT, if we miss atleast a single > > tuple. > > Suppose there are 5 locked rows and 5 unlocked rows in the heap and you do > this: > > select * from t1 for share skip locked limit 5 > > The Boolean you propose will be false if the first 5 rows in physical > order are locked, and otherwise it will be false. But there's no > difference between those two scenarios from the perspective of the > application. Here's another example: > > with foo as (select * from t1 for share skip locked) select * from foo > where a = 2; > > If foo contains any locked rows at all, this will return true, > regardless of whether a = 2. > > It's true that, for a lot of normal-ish queries, LockRows is applied > late enough that your proposed Boolean would return the intended > answer. But there are a bunch of exceptions, like the ones shown > above, and there might be more in the future. > > Hi Robert, As usual, a great guidance from you. Thanks :-) But I'm still trying to see, is there a way we can implement this for all use cases. Will update this thread with my findings. > -- > Robert Haas > EnterpriseDB: http://www.enterprisedb.com > The Enterprise PostgreSQL Company > -- Regards, Dinesh manojadinesh.blogspot.com
