Aman Sinha created DRILL-4679:
---------------------------------
Summary: 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
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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