Bug introduced in 11.0.0 and fixed in 11.0.1
The following executes without issue in 10.4.1 and returns immediately. However in 11.0 when the JoinAcross line is executed all available system memory is immediately consumed and after a long while the kernel crashes.
dates = Transpose[{ConstantArray[First@# , Length@Last@#], Last@#,
RandomInteger[{20, 100}, Length@Last@#]}] &@
{#, DateRange[#, {2016, 6}, "Quarter"]} & /@
DateRange[{1994, 3}, {2016, 6}, "Quarter"] //
Flatten[#, 1] &;
dsDetail =
AssociationThread[{"InitialDate", "AccountingDate", "Value"}, #] & /@
dates // Dataset;
dsSummary = AssociationThread[{"AccountingDate", "Value"}, #] & /@
With[{d = DateRange[{1994, 3}, {2016, 6}, "Quarter"]},
Transpose@{d, RandomInteger[{100, 200}, Length@d]}] //
Dataset;
This next line will run fine in 10.4.1 but will consume all available system memory and crash the kernel in 11.0. The OS is very unresponsive while this is executing in 11.0 so I would advise you have all your Mathematica work saved and closed as a precaution.
JoinAcross[dsDetail, dsSummary, "AccountingDate",
KeyCollisionFunction -> ({"Detail" <> #, "Summary" <> #} &)]
It is a bug, correct? Version 11.0 Win 7 Ent SP1
Bug: [CASE:3694291]

dsDetail//AtomQanddsSummary//AtomQboth yieldTrue. On Mathematica 11, they yieldFalse, so the finalJoinAcrosscommand doesn't work. Actually, on my computer (v11)Datasetdoesn't even create theDatasetobject... something is wrong. – QuantumDot Aug 22 '16 at 12:56Normalon theDatasets inJoinAcrossstill results in the same behavior. After your comment I would have thought that this change would work since it drops theDatasetinto its list ofAssociations. However there was no joy to be found. – Edmund Aug 22 '16 at 14:28MemoryConstrainedif you don't actually want to see it crash. – mikado Aug 22 '16 at 21:24