``` create table test_source( __test_metadata__ varchar METADATA, f0 varchar, f1 varchar, f2 bigint, ts as CURRENT_TIMESTAMP ) with( 'connector'='test', ... ) ```
If we not pre define `__test_metadata__` as meta keys by implementing listReadableMetadata, run the above sql, it will cause exception like this: org.apache.flink.table.api.ValidationException: Invalid metadata key '__test_metadata__' in column '__test_metadata__' of table 'default_catalog.default_database.test_source'. The DynamicTableSource class 'com.alipay.flink.connectors.test.source.TestDynamicTableSource' supports the following metadata keys for reading: xxx, yyy at org.apache.flink.table.planner.connectors.DynamicSourceUtils.lambda$validateAndApplyMetadata$5(DynamicSourceUtils.java:409) Because the current flink metadata column must exist in results returned by `listReadableMetadata`. But when a certain connector adds some metadatas, we can not use it directly unless we modify this connector code and support it. In some situations, It can be intolerable. Can we support 'DYNAMIC MetadataColumn'? Its basic mechanism is not to check a column with existing metadatas and users can define it dynamically. If a certain connector without this metadata, the column value will return null otherwise return it's concrete value. It has great benefits in some scenarios. Looking forward to your opinions. -- Best Regards, Ran Tao https://github.com/chucheng92
