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https://issues.apache.org/jira/browse/DRILL-4679?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15289167#comment-15289167
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Aman Sinha commented on DRILL-4679:
-----------------------------------
With my proposed fix of generating an empty MapVector (by the
ProjectRecordBatch) the regression tests pass and I tried a few new tests that
also pass. Here's an example such query (this used to fail earlier due to
empty right side of the union-all):
{noformat}
0: jdbc:drill:zk=local> SELECT CONVERT_FROM('[{a : 100, b: 200}, {a:300, b:
400}]' ,'JSON') AS MYCOL1 FROM (VALUES(1)) union all SELECT CONVERT_FROM('[{a
: 100, b: 200}, {a:300, b: 400}]' ,'JSON') AS MYCOL1 FROM (VALUES(1)) where 1
= 0;
+----------------------------------------+
| MYCOL1 |
+----------------------------------------+
| [{"a":100,"b":200},{"a":300,"b":400}] |
+----------------------------------------+
{noformat}
[~jni], [~jaltekruse] (or anyone else).. if you have thoughts about this, let
me know. I will run more tests before creating a PR.
> CONVERT_FROM() json format fails if 0 rows are received from upstream
> operator
> -------------------------------------------------------------------------------
>
> Key: DRILL-4679
> URL: https://issues.apache.org/jira/browse/DRILL-4679
> Project: Apache Drill
> Issue Type: Bug
> Components: Execution - Relational Operators
> Affects Versions: 1.6.0
> Reporter: Aman Sinha
> Assignee: Aman Sinha
>
> CONVERT_FROM() json format fails as below if the underlying Filter produces 0
> rows:
> {noformat}
> 0: jdbc:drill:zk=local> select convert_from('{"abc":"xyz"}', 'json') as x
> from cp.`tpch/region.parquet` where r_regionkey = 9999;
> Error: SYSTEM ERROR: IllegalStateException: next() returned NONE without
> first returning OK_NEW_SCHEMA [#16, ProjectRecordBatch]
> Fragment 0:0
> {noformat}
> If the conversion is applied as UTF8 format, the same query succeeds:
> {noformat}
> 0: jdbc:drill:zk=local> select convert_from('{"abc":"xyz"}', 'utf8') as x
> from cp.`tpch/region.parquet` where r_regionkey = 9999;
> +----+
> | x |
> +----+
> +----+
> No rows selected (0.241 seconds)
> {noformat}
> The reason for this is the special handling in the ProjectRecordBatch for
> JSON. The output schema is not known for this until the run time and the
> ComplexWriter in the Project relies on seeing the input data to determine the
> output schema - this could be a MapVector or ListVector etc.
> If the input data has 0 rows due to a filter condition, we should at least
> produce a default output schema, e.g an empty MapVector ? Need to decide a
> good default. Note that the CONVERT_FROM(x, 'json') could occur on 2
> branches of a UNION-ALL and if one input is empty while the other side is
> not, it may still cause incompatibility.
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