[jira] [Created] (FLINK-7217) Give hints about unsupported SQL / Table API features.

["Fabian Hueske (JIRA)" <ji...@apache.org>]

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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