Sahil Takiar created IMPALA-8819:
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Summary: BufferedPlanRootSink should handle non-default fetch sizes
Key: IMPALA-8819
URL: https://issues.apache.org/jira/browse/IMPALA-8819
Project: IMPALA
Issue Type: Sub-task
Components: Backend
Reporter: Sahil Takiar
Assignee: Sahil Takiar
As of IMPALA-8780, the {{BufferedPlanRootSink}} returns an error whenever a
client sets the fetch size to a value lower than the {{BATCH_SIZE}}. The issue
is that when reading from a {{RowBatch}} from the queue, the batch might
contain more rows than the number requested by the client. So the
{{BufferedPlanRootSink}} needs to be able to partially read a {{RowBatch}} and
remember the index of the rows it read. Furthermore, {{num_results}} in
{{BufferedPlanRootSink::GetNext}} could be lower than {{BATCH_SIZE}} if the
query results cache in {{ClientRequestState}} has a cache hit (only happens if
the client cursor is reset).
Another issue is that the {{BufferedPlanRootSink}} can only read up to a single
{{RowBatch}} at a time. So if a fetch size larger than {{BATCH_SIZE}} is
specified, only {{BATCH_SIZE}} rows will be written to the given
{{QueryResultSet}}. This is consistent with the legacy behavior of
{{PlanRootSink}} (now {{BlockingPlanRootSink}}), but is not ideal because that
means clients can only read {{BATCH_SIZE}} rows at a time. A higher fetch size
would potentially reduce the number of round-trips necessary between the client
and the coordinator, which could improve fetch performance (but only if the
{{BlockingPlanRootSink}} is capable of filling all the requested rows).
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