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Re: Lifecycle and Configuration of a hive UDF
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 Grover, Business Intelligence Analyst
OANDA Corporation

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----- Original Message -----
From: "Justin Coffey" <[EMAIL PROTECTED]>
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.
On Thu, Apr 19, 2012 at 1:21 AM, Ranjan Bagchi < [EMAIL PROTECTED] > wrote:

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?