Add virtual columns support.

[vchimishuk]

Hi,
We use Druidry in our company and faced lack of virtual columns (http:https://druid.io/docs/latest/querying/virtual-columns.html). Here is a simple patch which adds virtual columns support.

Usage example:

        DruidGroupByQuery.DruidGroupByQueryBuilder query = DruidGroupByQuery
                .builder()
                .dataSource("druidDatasource")
                .granularity(new SimpleGranularity(PredefinedGranularity.ALL))
                .dimensions(Arrays.asList(new SimpleDimension("size")))
                .intervals(Collections.singletonList(interval))
                .virtualColumns(Collections.singletonList(new ExpressionVirtualColumn("size",
                        "concat(width, ':', height)", OutputType.STRING)));

Result:

{version=v1, timestamp=2019-02-15T00:00:00.000Z, event={size=160:600}}
{version=v1, timestamp=2019-02-15T00:00:00.000Z, event={size=80:80}}

[abhi-zapr]

Is there any list of what all druid query supports virtual columns ? Or all the druid query supports virtual columns ?
In later case, if possible, can you also apply virtual column patch in other queries ?

And thanks for contributing.

[vchimishuk]

@abhi-zapr, added one more commit with requested changes. Also added virtual columns support for other query types.
Sorry, for delay.

[vchimishuk]

Hi, @abhi-zapr. Is there any plans to include it into new release any time soon?

[abhi-zapr]

Hey @vchimishuk
We will include you patch in next release, hopefully by the end of the next week.
Thanks.

[vchimishuk]

Thank you for the update, @abhi-zapr.

[vchimishuk]

Hi, guys.
Any progress on releasing this PR?

[vchimishuk]

@abhi-zapr, @GG-Zapr, any update on the issue?

[abhi-zapr]

Hey @vchimishuk
Sorry for the delay, we are having Tech Conference this week at our organization and we are really busy in that. We haven't released any new version yet, otherwise would have included your patch. We will try to merge and release next week.

[vchimishuk]

@abhi-zapr, thank you.