[jira] [Created] (FLINK-20059) Outdated SQL docs on aggregate functions' merge

[Shang Yuanchun (Jira)]

Nico Kruber created FLINK-20059:
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             Summary: Outdated SQL docs on aggregate functions' merge
                 Key: FLINK-20059
                 URL: https://issues.apache.org/jira/browse/FLINK-20059
             Project: Flink
          Issue Type: Bug
          Components: Documentation, Table SQL / API
    Affects Versions: 1.11.2, 1.12.0
            Reporter: Nico Kruber


In the java docs as well as the user docs, the {{merge}} method of an aggregation UDF is described as optional, e.g.
{quote}Merges a group of accumulator instances into one accumulator instance. This function must be implemented for data stream session window grouping aggregates and data set grouping aggregates.{quote}

However, it seems that nowadays this method is required in more cases (I stumbled on this for a HOP window in streaming):
{code}
StreamExecGlobalGroupAggregate.scala
      .needMerge(mergedAccOffset, mergedAccOnHeap, mergedAccExternalTypes)
StreamExecGroupWindowAggregateBase.scala
      generator.needMerge(mergedAccOffset = 0, mergedAccOnHeap = false)
StreamExecIncrementalGroupAggregate.scala
      .needMerge(mergedAccOffset, mergedAccOnHeap = true, mergedAccExternalTypes)
StreamExecLocalGroupAggregate.scala
      .needMerge(mergedAccOffset = 0, mergedAccOnHeap = true)
{code}



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