On Wed, Oct 14, 2015 at 6:28 PM, dinesh kumar <dineshkumar02@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.
