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[SPARK-21589][SQL][DOC] Add documents about Hive UDF/UDTF/UDAF #18792

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[SPARK-21589][SQL][DOC] Add documents about Hive UDF/UDTF/UDAF #18792

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Add documents about Hive UDFs
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17 changes: 17 additions & 0 deletions docs/sql-programming-guide.md
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Expand Up @@ -1903,6 +1903,23 @@ releases of Spark SQL.
Hive can optionally merge the small files into fewer large files to avoid overflowing the HDFS
metadata. Spark SQL does not support that.

**Hive UDF/UDTF/UDAF**

Not all the APIs of the Hive UDF/UDTF/UDAF are supported by Spark SQL. Below are the unsupported APIs:

* `getRequiredJars` and `getRequiredFiles` (`UDF` and `GenericUDF`) are functions to automatically
include additional resources required by this UDF.
* `initialize(StructObjectInspector)` in `GenericUDTF` is not supported yet. Spark SQL currently uses
a deprecated interface `initialize(ObjectInspector[])` only.
* `configure` (`GenericUDF`, `GenericUDTF`, and `GenericUDAFEvaluator`) is a function to initialize
functions with `MapredContext`, which is inapplicable to Spark.
* `close` (`GenericUDF` and `GenericUDAFEvaluator`) is a function to release associated resources.
Spark SQL does not call this function when tasks finish.
* `reset` (`GenericUDAFEvaluator`) is a function to re-initialize aggregation for reusing the same aggregation.
Spark SQL currently does not support the reuse of aggregation.
* `getWindowingEvaluator` (`GenericUDAFEvaluator`) is a function to optimize aggregation by evaluating
an aggregate over a fixed window.

# Reference

## Data Types
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