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https://issues.apache.org/jira/browse/DRILL-7305?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16879740#comment-16879740
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Paul Rogers commented on DRILL-7305:
------------------------------------
As it turns out, the [Union
All|https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/union/UnionAllRecordBatch.java#L92]
operator generates an empty schema-only batch as its first step.
This puts us in an awkward position: one the one hand, the new scan operator
will back out the schema-only batch feature because some operators do not
support an empty batch. But, meanwhile, the Union All operator creates such
batches.
It may be that working around the empty-batch issues is, at best, a short-term
hack. Work must be done to test each operator to ensure it handles empty
batches so that the Union All operator output is properly handled.
> Multiple operators do not handle empty batches
> ----------------------------------------------
>
> Key: DRILL-7305
> URL: https://issues.apache.org/jira/browse/DRILL-7305
> Project: Apache Drill
> Issue Type: Bug
> Affects Versions: 1.16.0
> Reporter: Paul Rogers
> Priority: Major
>
> While testing the new "EVF" framework, it was found that multiple operators
> incorrectly handle empty batches. The EVF framework is set up to return a
> "fast schema" empty batch with only schema as its first batch. It turns out
> that many operators fail with problems such as:
> * Failure to set the value counts in the output container
> * Fail to initialize the offset vector position 0 to 0 for variable-width or
> repeated vectors
> And so on.
> Partial fixes are in the JSON reader PR.
> For now, the easiest work-around is to disable the "fast schema" path in the
> EVF: DRILL-7306.
> To discover the remaining issues, enable the
> {{ScanOrchestratorBuilder.enableSchemaBatch}} option and run unit tests. You
> can use the {{VectorChecker}} and {{VectorAccessorUtilities.verify()}}
> methods to check state. Insert a call to {{verify()}} in each "next" method:
> verify the incoming and outgoing batches. The checker only verifies a few
> vector types; but these are enough to show many problems.
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