Hi Kenn, If we consider the following two *Unbounded* PCollections: - PCollection1 => [*Non-Global* Window with Default Trigger] - PCollection2 => [Global Window with *Non-Default* Trigger] :) coincidentally turned out to be the opposite
Joining these two PCollections in BeamSql currently is not possible because of https://jira.apache.org/jira/browse/BEAM-3345(WindowFn Mismatch) But in this case, PCollection1 can be joined with PCollection2 using SideInputJoin ( https://github.com/apache/beam/blob/c7911043510a266078a3dc8faef7a1dbe1f598c5/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/rel/BeamJoinRel.java#L456), which is being done for Joining an Unbounded PCollection with Bounded PCollection. I am thinking that Beam can guarantee it joins all input elements once per window for this case. The result of the join might be fuzzy for the window when the Trigger for PCollection2 fires and sideinput gets loaded into Memory. PCollection2 can be considered as Slowly Changing Lookup Cache and BeamSql can support Pattern: https://beam.apache.org/documentation/patterns/side-input-patterns/#slowly-updating-global-window-side-inputs, which is currently not possible. I am working on https://jira.apache.org/jira/browse/BEAM-7758 for BeamSql to natively support PCollectionView so that BeamSql supports "Slowly Updating Global Window Sideinput Pattern" using SqlTransform's TableProvider. If we can support this, User will be able to do: PCollection mainStream = ...; PCollection lookupStream = ...; PCollectionTuple tuple = PCollectionTuple.of(new TupleTag("MainTable"), new TupleTag("LookupTable")); tuple.apply(SqlTransform.query("MainTable JOIN LookupTable")); Can we implement SideInputJoin for this case? I might be wrong in my understanding. Please let me know your thoughts. Thanks, Rahul On Thu, Jul 25, 2019 at 9:28 AM Kenneth Knowles <k...@apache.org> wrote: > I think the best way to approach this is probably to have an example SQL > statement and to discuss what the relational semantics should be. > > Windowing is not really part of SQL (yet) and in a way it just needs very > minimal extensions. See https://arxiv.org/abs/1905.12133. In this > proposal for SQL, windowed aggregation is explicitly be part of the GROUP > BY operation, where you GROUP BY window columns that were added. So it is > more explicit than in Beam. Relations do not have a WindowFn so there is no > problem of them being incompatible. > > With Beam SQL there are basically two ways of windowing that work totally > differently: > > 1. SQL style windowing where you GROUP BY windows. This does not use the > input PCollection windowfn > 2. PCollection windowing where the SQL does not do any windowing - this > should apply the SQL expression to each window independently > > In order to support a hybrid of these, it might be: > > 3. SQL style windowing, where when a PCollection has window assigned, the > window columns are added before the SQL is applied. It is a bit strange but > might enable your use. > > Kenn > > On Mon, Jul 22, 2019 at 10:39 AM rahul patwari <rahulpatwari8...@gmail.com> > wrote: > >> Hi, >> >> Beam currently doesn't support Join of Unbounded PCollections of >> different WindowFns ( >> https://beam.apache.org/documentation/programming-guide/#groupbykey-and-unbounded-pcollections >> ). >> >> BeamSql performs [Unbounded PCollection] JOIN [Bounded PCollection], by >> performing 'SideInputJoin' with Bounded PCollection as a SideInput. >> >> Can we support [Unbounded PCollection] JOIN [Unbounded PCollection], when >> one of the Unbounded PCollection has [GlobalWindows Applied with >> Non-Default Trigger(probably a slow-changing lookup cache >> https://beam.apache.org/documentation/patterns/side-input-patterns/#slowly-updating-global-window-side-inputs)] >> by performing 'SideInputJoin'? >> >> Regards, >> Rahul >> >
