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https://issues.apache.org/jira/browse/IMPALA-8925?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sahil Takiar updated IMPALA-8925:
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Component/s: Clients
> Consider replacing ClientRequestState ResultCache with result spooling
> ----------------------------------------------------------------------
>
> Key: IMPALA-8925
> URL: https://issues.apache.org/jira/browse/IMPALA-8925
> Project: IMPALA
> Issue Type: Improvement
> Components: Backend, Clients
> Reporter: Sahil Takiar
> Priority: Major
>
> The {{ClientRequestState}} maintains an internal results cache (which is
> really just a {{QueryResultSet}}) in order to provide support for the
> {{TFetchOrientation.FETCH_FIRST}} fetch orientation (used by Hue - see
> [https://github.com/apache/impala/commit/6b769d011d2016a73483f63b311e108d17d9a083]).
> The cache itself has some limitations:
> * It caches all results in a {{QueryResultSet}} with limited admission
> control integration
> * It has a max size, if the size is exceeded the cache is emptied
> * It cannot spill to disk
> Result spooling could potentially replace the query result cache and provide
> a few benefits; it should be able to fit more rows since it can spill to
> disk. The memory is better tracked as well since it integrates with both
> admitted and reserved memory. Hue currently sets the max result set fetch
> size to
> [https://github.com/cloudera/hue/blob/master/apps/impala/src/impala/impala_flags.py#L61],
> would be good to check how well that value works for Hue users so we can
> decide if replacing the current result cache with result spooling makes sense.
> This would require some changes to result spooling as well, currently it
> discards rows whenever it reads them from the underlying
> {{BufferedTupleStream}}. It would need the ability to reset the read cursor,
> which would require some changes to the {{PlanRootSink}} interface as well.
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