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Writing GenericUDAFs: A Tutorial

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User-Defined Aggregation Functions (UDAFs) are an excellent way to integrate advanced data-processing into Hive. Hive allows two varieties of UDAFs: simple and generic. Simple UDAFs, as the name implies, are rather simple to write, but incur performance penalties because of the use of Java Reflection, and do not allow features such as variable-length argument lists. Generic UDAFs allow all these features, but are perhaps not quite as intuitive to write as Simple UDAFs.

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Old interface org.apache.hadoop.hive.ql.udf.GenericUDAFResolver was deprecated as of the 0.6.0 release. The key difference between GenericUDAFResolver and GenericUDAFResolver2 interface is the fact that the latter allows the evaluator implementation to access extra information regarding the function invocation such as the presence of DISTINCT qualifier or the invocation with the wildcard syntax such as FUNCTION(*). UDAFs that implement the deprecated GenericUDAFResolver interface will not be able to tell the difference between an invocation such as FUNCTION() or FUNCTION (*) since the information regarding specification of the wildcard is not available. Similarly, these implementations will also not be able to tell the difference between FUNCTION(EXPR) vs FUNCTION(DISTINCT EXPR) since the information regarding the presence of the DISTINCT qualifier is also not available.

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