[jira] [Created] (IMPALA-8786) BufferedPlanRootSink should directly write to a QueryResultSet if one is available

["Sahil Takiar (JIRA)" <ji...@apache.org>]

Sahil Takiar created IMPALA-8786:
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             Summary: BufferedPlanRootSink should directly write to a QueryResultSet if one is available
                 Key: IMPALA-8786
                 URL: https://issues.apache.org/jira/browse/IMPALA-8786
             Project: IMPALA
          Issue Type: Sub-task
          Components: Backend
            Reporter: Sahil Takiar
            Assignee: Sahil Takiar


{{BufferedPlanRootSink}} uses a {{RowBatchQueue}} to buffer {{RowBatch}}-es and then the consumer thread reads them and writes them to a given {{QueryResultSet}}. Implementations of {{RowBatchQueue}} might end up copying the buffered {{RowBatch}}-es (e.g. if the queue is backed by a {{BufferedTupleStream}}). An optimization would be for the producer thread to directly write to the consumer {{QueryResultSet}}. This optimization would only be triggered if (1) the queue is empty, and (2) the consumer thread has a {{QueryResultSet}} available for writing.

This "fast path" is useful in a few different scenarios:
 * If the consumer is faster than at reading rows than the producer is at sending them; in this case, the overhead of buffering rows in a {{RowBatchQueue}} can be completely avoided
 * For queries that return under 1024 its likely that the consumer will produce a {{QueryResultSet}} before the first {{RowBatch}} is returned (except perhaps for very trivial queries)



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