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https://issues.apache.org/jira/browse/DRILL-7979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17391816#comment-17391816
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ASF GitHub Bot commented on DRILL-7979:
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cgivre commented on pull request #2283:
URL: https://github.com/apache/drill/pull/2283#issuecomment-891377214
> Yes, I think I did grok the unknown schema problem. The thought above,
which somehow escaped all the striking out I did to it after thinking a bit
more, was to take advantage of the fact that scalar string can be embedded into
a single element map. The tuple generating code would need to become aware when
it should do this.
>
> My second comment's comparison of the situation with a JSON property that
is first null, then an object, is also a bit dubious because empty XML elements
do not represent nulls (from I what read) so much as zero length strings.
>
> If there is an effort to make querying XML behave in a more similar way to
querying equivalent JSON, for some definition of equivalent, it should probably
wait for another PR.
I think you're right about that. From what I remember, there is an option
for Drill's JSON parser to treat `NaN` and something else as `null`. For XML
I don't know how you'd distinguish between an empty string and `null`.
This was also an issue with some data I was working on. The JSON version
used empty strings to denote `null` then subsequent rows would contain maps
which would cause SchemaChange exceptions. The only way to fix that was to use
the `UNION` data type.
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> Self-Closing XML Tags Cause Schema Change Exceptions
> ----------------------------------------------------
>
> Key: DRILL-7979
> URL: https://issues.apache.org/jira/browse/DRILL-7979
> Project: Apache Drill
> Issue Type: Bug
> Components: Storage - Other
> Affects Versions: 1.19.0
> Reporter: Charles Givre
> Assignee: Charles Givre
> Priority: Major
> Fix For: 1.20.0
>
>
> Self closing XML tags are dealt with strangely by java's streaming parser.
> If you have data where you have one row containing a self closing XML tag foo
> (<foo/>) but then in the next row `foo` contains a map or other nested field,
> Drill will throw a schema change exception.
> This proposed fix causes Drill to ignore self-closing tags unless they have
> attributes, which allows data like this to be successfully queried.
>
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