i just noticed Sort for Dataset has a global flag. and Dataset also has sortWithinPartitions.
how about: repartition + sortWithinPartitions + mapPartitions? the plan looks ok, but it is not clear to me if the sort is done as part of the shuffle (which is the important optimization). scala> val df = Seq((1, "1"), (2, "2"), (1, "1"), (2, "2")).toDF("key", "value") scala> df.repartition(2, col("key")).sortWithinPartitions("value").as[(String, String)].mapPartitions{ (x: Iterator[(String, String)]) => x }.explain == Physical Plan == *SerializeFromObject [staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, assertnotnull(input[0, scala.Tuple2, true], top level non-flat input object)._1, true) AS _1#39, staticinvoke(class org.apache.spark.unsafe.types.UTF8String, StringType, fromString, assertnotnull(input[0, scala.Tuple2, true], top level non-flat input object)._2, true) AS _2#40] +- MapPartitions <function1>, obj#38: scala.Tuple2 +- DeserializeToObject newInstance(class scala.Tuple2), obj#37: scala.Tuple2 +- *Sort [value#6 ASC], false, 0 +- Exchange hashpartitioning(key#5, 2) +- LocalTableScan [key#5, value#6] On Fri, Nov 4, 2016 at 9:18 AM, Koert Kuipers <ko...@tresata.com> wrote: > sure, but then my values are not sorted per key, right? > > so a group by key with values sorted according to to some ordering is an > operation that can be done efficiently in a single shuffle without first > figuring out range boundaries. and it is needed for quite a few algos, > including Window and lots of timeseries stuff. but it seems there is no way > to express i want to do this yet (at least not in an efficient way). > > which makes me wonder, what does Window do? > > > On Fri, Nov 4, 2016 at 12:59 AM, Michael Armbrust <mich...@databricks.com> > wrote: > >> Thinking out loud is good :) >> >> You are right in that anytime you ask for a global ordering from Spark >> you will pay the cost of figuring out the range boundaries for partitions. >> If you say orderBy, though, we aren't sure that you aren't expecting a >> global order. >> >> If you only want to make sure that items are colocated, it is cheaper to >> do a groupByKey followed by a flatMapGroups >> <https://databricks-prod-cloudfront.cloud.databricks.com/public/4027ec902e239c93eaaa8714f173bcfc/1023043053387187/1828840559545742/2840265927289860/latest.html> >> . >> >> >> >> On Thu, Nov 3, 2016 at 7:31 PM, Koert Kuipers <ko...@tresata.com> wrote: >> >>> i guess i could sort by (hashcode(key), key, secondarySortColumn) and >>> then do mapPartitions? >>> >>> sorry thinking out loud a bit here. ok i think that could work. thanks >>> >>> On Thu, Nov 3, 2016 at 10:25 PM, Koert Kuipers <ko...@tresata.com> >>> wrote: >>> >>>> thats an interesting thought about orderBy and mapPartitions. i guess i >>>> could emulate a groupBy with secondary sort using those two. however isn't >>>> using an orderBy expensive since it is a total sort? i mean a groupBy with >>>> secondary sort is also a total sort under the hood, but its on >>>> (hashCode(key), secondarySortColumn) which is easier to distribute and >>>> therefore can be implemented more efficiently. >>>> >>>> >>>> >>>> >>>> >>>> On Thu, Nov 3, 2016 at 8:59 PM, Michael Armbrust < >>>> mich...@databricks.com> wrote: >>>> >>>>> It is still unclear to me why we should remember all these tricks (or >>>>>> add lots of extra little functions) when this elegantly can be expressed >>>>>> in >>>>>> a reduce operation with a simple one line lamba function. >>>>>> >>>>> I think you can do that too. KeyValueGroupedDataset has a >>>>> reduceGroups function. This probably won't be as fast though because you >>>>> end up creating objects where as the version I gave will get codgened to >>>>> operate on binary data the whole way though. >>>>> >>>>>> The same applies to these Window functions. I had to read it 3 times >>>>>> to understand what it all means. Maybe it makes sense for someone who has >>>>>> been forced to use such limited tools in sql for many years but that's >>>>>> not >>>>>> necessary what we should aim for. Why can I not just have the sortBy and >>>>>> then an Iterator[X] => Iterator[Y] to express what I want to do? >>>>>> >>>>> We also have orderBy and mapPartitions. >>>>> >>>>>> All these functions (rank etc.) can be trivially expressed in this, >>>>>> plus I can add other operations if needed, instead of being locked in >>>>>> like >>>>>> this Window framework. >>>>>> >>>>> I agree that window functions would probably not be my first choice >>>>> for many problems, but for people coming from SQL it was a very popular >>>>> feature. My real goal is to give as many paradigms as possible in a >>>>> single >>>>> unified framework. Let people pick the right mode of expression for any >>>>> given job :) >>>>> >>>> >>>> >>> >> >