Have a read of the thread "Lag function in Hive", linked from: http://mail-archives.apache.org/mod_mbox/hive-user/201204.mbox/thread
There's an example of how to force a function to run reduce-side. I've written a UDF which replicates RANK () OVER (...), but it requires the syntactic sugar given in the thread. I'd like to make changes to the hive query planner at some point, so that you can annotate a UDF with a "run on reducer" hint, and after that I'd happily open source everything. If you want more details of how to implement your own partitionedRowNumber() UDF then I'd be happy to elaborate. Cheers, Phil. On 20 April 2012 18:35, Mark Grover <mgro...@oanda.com> wrote: > Hi Rajan and Justin, > > As per my understanding, the scope of a UDF is only one row of data at a > time. Therefore, it can be done all map side without the need for the reducer > being involved. Now, depending on where you are storing the result of the > query, your query may have reducers that do something. > > A simple query like Rajan mentioned > select MyUDF(field1,field2) from table; > > should have the UDF execute() being called in the map phase. > > > Now to Justin's question, > rank function > (http://msdn.microsoft.com/en-us/library/ms176102%28v=sql.110%29.aspx) > seems to have a sytax like: > RANK ( ) OVER ( [ partition_by_clause ] order_by_clause ) > > Rank function works on a collection of rows (distributed by the some column - > the same one you would use in your partition_by_clause in MS SQL). > You can accomplish that using UDAF (read more about them at > https://cwiki.apache.org/Hive/genericudafcasestudy.html) or by writing a > custom reducer (read about that at > https://cwiki.apache.org/confluence/display/Hive/LanguageManual+Transform). > > I don't think rank can be done using a UDF. > > Good luck! > > Mark > > Mark Grover, Business Intelligence Analyst > OANDA Corporation > > www: oanda.com www: fxtrade.com > > "Best Trading Platform" - World Finance's Forex Awards 2009. > "The One to Watch" - Treasury Today's Adam Smith Awards 2009. > > > ----- Original Message ----- > From: "Justin Coffey" <jqcof...@gmail.com> > To: user@hive.apache.org > Sent: Thursday, April 19, 2012 10:29:11 AM > Subject: Re: Lifecycle and Configuration of a hive UDF > > Hello All, > I second this question. I have a MS SQL "rank" function which I would like to > run, the results it gives appears to suggest it is executed Mapper side as > opposed to reducer side, even when run with "cluster by" constraints. > > > -Justin > > > On Thu, Apr 19, 2012 at 1:21 AM, Ranjan Bagchi < ran...@powerreviews.com > > wrote: > > > Hi, > > What's the lifecycle of a hive udf. If I call > > select MyUDF(field1,field2) from table; > > Then MyUDF is instantiated once per mapper, and within each mapper > execute(field1, field2) is called for each reducer? I hope this is the case, > but I can't find anything about this in the documentation. > > So I'd like to have some run-time configuration of my UDF: I'm curious how > people do this. Is there a way I can send it a value or have it access a > file, etc? How about performing a query against the hive store? > > Thanks, > > Ranjan > > > > > > -- > jqcof...@gmail.com > -----
