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https://issues.apache.org/jira/browse/DRILL-1781?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Steven Phillips resolved DRILL-1781.
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Resolution: Fixed
Fixed by 3581a32
> For complex functions, don't return until schema is known
> ---------------------------------------------------------
>
> Key: DRILL-1781
> URL: https://issues.apache.org/jira/browse/DRILL-1781
> Project: Apache Drill
> Issue Type: Bug
> Reporter: Steven Phillips
> Priority: Blocker
> Fix For: 0.7.0
>
> Attachments: DRILL-1781.patch, DRILL-1781.patch
>
>
> In the case of complex output functions, it is impossible to determine the
> output schema until the actual data is consumed. For example, with
> convert_form(VARCHAR, 'json'), unlike most other functions, it is not
> sufficient to know that the incoming data type is VARCHAR, we actually need
> to decode the contents of the record before we can determine what the output
> type is, whether it be map, list, or primitive type.
> For fast schema return, we worked around this problem by simply assuming the
> type was Map, and if it happened to be different, there would be a schema
> change. This solution is not satisfactory, as it ends up breaking other
> functions, like flatten.
> The solution is to continue returning a schema whenever possible, but when it
> is not possible, drill will wait until it is.
> For non-blocking operators, drill will immediately consume the incoming
> batch, and thus will not return empty schema batches if there is data to
> consume. Blocking operators will return an empty schema batch. If a flattten
> function occurs downstream from a blocking operator, it will not be able to
> return a schema, and thus fast schema return will not happen in this case.
> In the cases where the complex function is not downstream from a blocking
> operator, fast schema return should continue to work.
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