Virtual Schemas: Data types not compatible error in union all query

Detail name	Value
Changelog Number	4367
Type	Bug
Status	Open
Affected Versions	EXASOL 6.0.0

This error only occurs in special circumstances.
It is necessary that a query uses a virtual schema inside UNION ALL.
Additionally, the query needs a surrounding SELECT COUNT( * ) that makes
the column types inside the UNION ALL irrelevant. If a query uses
this combination, Exasol creates a pushdown request that contains
SELECT NULL. This causes datatype errors for some databases.

Reproducibility:

SELECT COUNT(*) FROM
(SELECT c1 FROM VIRTUAL_SCHEMA.T1 UNION ALL SELECT c2 FROM SCHEMA2.T2)

Error:

[0A000] Feature not supported: datatypes are not compatible for Union

The PUSHDOWN_SQL (displayed with EXPLAIN VIRTUAL) contains:

SELECT NULL FROM VIRTUAL_SCHEMA.T1

Workaround:

Set the Exasol parameter -disableUnionAllOptimization=1 .