haibow commented on issue #4774: Refactor virtualColumn URL: https://github.com/apache/incubator-pinot/pull/4774#issuecomment-553096395 > I don't fully understand what problem this PR is trying to solve. Is there a link to the big picture of the problem you want to solve? > As for the `SELECT *`, I think it make sense to only skip the column starts with '$' Detailed design/usage is in https://github.com/apache/incubator-pinot/issues/4225#issuecomment-548173120. Currently we have 3 builtin virtual columns. If you look at the current `Virtualcolumncontext`, hostname and segmentname are manually included, and we need separate column providers for hostname and segmentname which simply provide a different single value. This only works for predefined virtual columns. In the future I would like to add virtual columns dynamically (for any new column added to the consuming segment) with custom column name, default value and other specs like single/multi-value, data type, etc. The new `Virtualcolumncontext` will use fieldSpec which will contain all those specs. I've also merged `SegmentNameVirtualColumnProvider` and `HostNameVirtualColumnProvider` to `SingleStringVirtualColumnProvider` which will provide a single string for any single-value, string-type virtual column.
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