Writing user-defined aggregate functions (UDAFs)

A UDAF must maintain a state value across subsequent calls, so that it can accumulate a result across a set of calls, rather than derive it purely from one set of arguments. For that reason, a UDAF is represented by multiple underlying functions:

• An initialization function that sets any counters to zero, creates empty buffers, and does any other one-time setup for a query.
• An update function that processes the arguments for each row in the query result set and accumulates an intermediate result for each node. For example, this function might increment a counter, append to a string buffer, or set flags.
• A merge function that combines the intermediate results from two different nodes.
• A serialize function that flattens any intermediate values containing pointers, and frees any memory allocated during the init, update, and merge phases.
• A finalize function that either passes through the combined result unchanged, or does one final transformation.

In the SQL syntax, you create a UDAF by using the statement CREATE AGGREGATE FUNCTION. You specify the entry points of the underlying C++ functions using the clauses INIT_FN, UPDATE_FN, MERGE_FN, SERIALIZE_FN, and FINALIZE_FN.

For convenience, you can use a naming convention for the underlying functions and Impala automatically recognizes those entry points. Specify the UPDATE_FN clause, using an entry point name containing the string update or Update. When you omit the other _FN clauses from the SQL statement, Impala looks for entry points with names formed by substituting the update or Update portion of the specified name.

uda-sample.h:

See this file online at: uda-sample.h

uda-sample.cc:

See this file online at: uda-sample.cc

A user-defined aggregate function might produce and combine intermediate results during some phases of processing, using a different data type than the final return value. For example, if you implement a function similar to the built-in AVG() function, it must keep track of two values, the number of values counted and the sum of those values. Or, you might accumulate a string value over the course of a UDA, then in the end return a numeric or Boolean result.

In such a case, specify the data type of the intermediate results using the optional INTERMEDIATE type_name clause of the CREATE AGGREGATE FUNCTION statement. If the intermediate data is a typeless byte array (for example, to represent a C++ struct or array), specify the type name as CHAR(n), with n representing the number of bytes in the intermediate result buffer.

For an example of this technique, see the trunc_sum() aggregate function, which accumulates intermediate results of type DOUBLE and returns BIGINT at the end. View the appropriate CREATE FUNCTION statement and the implementation of the underlying TruncSum*() functions on Github.