Fabian Hueske created FLINK-7217:
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Summary: Give hints about unsupported SQL / Table API features.
Key: FLINK-7217
URL: https://issues.apache.org/jira/browse/FLINK-7217
Project: Flink
Issue Type: Improvement
Components: Table API & SQL
Reporter: Fabian Hueske
The Table API and SQL do not support every type of query. While being valid
SQL, we cannot translate certain queries into execution plans. Especially, the
supported syntax of streaming queries is limited and will probably always be
limited.
Therefore it is important to explain a user why a query could not be
translated. The problem here is that we need to wait for the optimizer to
explore the full search space and apply all transformation rules before we can
tell the user that the query could not be translated. Hence, we cannot simply
throw an exception if a rule cannot be applied, because another rule might
succeed. If we throw an exception in the optimizer, we would kill the
exploration and miss a valid plan. However, after the optimizer failed, we are
missing the complete context and cannot provide valuable feedback beyond "query
uses an unsupported feature".
We should try to improve the situation and provide better feedback to users.
One approach would be that rules collect possible reasons why they could not be
applied and these reasons are returned to the user when the translation fails.
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