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https://issues.apache.org/jira/browse/DRILL-6824?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16670963#comment-16670963
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ASF GitHub Bot commented on DRILL-6824:
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amansinha100 commented on issue #1518: DRILL-6824: Handle schema changes in
MapRDBJsonRecordReader
URL: https://github.com/apache/drill/pull/1518#issuecomment-434894435
Based on the description in the JIRA, the changes lgtm. +1
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> Drill Query on MapRDB JSON table failing on schema SchemaChangeException, the
> only distinct Values are NULL and Text
> --------------------------------------------------------------------------------------------------------------------
>
> Key: DRILL-6824
> URL: https://issues.apache.org/jira/browse/DRILL-6824
> Project: Apache Drill
> Issue Type: Bug
> Affects Versions: 1.14.0, 1.15.0
> Reporter: Gautam Parai
> Assignee: Gautam Parai
> Priority: Major
> Fix For: 1.15.0
>
> Original Estimate: 24h
> Remaining Estimate: 24h
>
> Drill Query on MapR DB JSON Table or it View fails with a
> SchemaChangeException. The only distinct values is NULL and some Text.
> The issue is that when Drill sees a NULL or does not see any values for a
> column in the first batch it assumes the type as NULLABLE INT. Subsequently,
> when the data shows up and it is different from NULLABLE INT there is a
> schema change. Operators e.g. aggregators etc. cannot handle such a Schema
> Change and throw a SchemaChangeException.
>
> One of the short-term solution implemented in this fix: Add a CAST expression
> which will cast null values to the target type. Hence, we would never see a
> SchemaChange due to NULLs. However, the MapRDB Reader code was written
> differently than other reader and was hitting a SchemaChangeException. The
> code was changed to make it similar to other scans i.e. emit a new batch
> whenever a schema change is encountered.
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