How to read AggregateFunction(groupBitmap, UInt32) column directly

[xichenAiguang]

Hi all, I have a job ingest data into Clickhouse, which has a RoaringBitmap field, its type is AggregateFunction(groupBitmap, UInt32) in Clickhouse. I encoutered this exception " Unknown type: AggregateFunction(groupBitmap, UInt32)" when calling ExecuteReader(). I understand it's due to lack of support of this type, I'd like to extend TypeConverter to support it, but seems like it's impossible because TypeConverter is a internal class. My questions is how to read this column, note that I don't want to convert this column to String or Bit or other datatypes with Clickhouse built-in function like ToString(), bitmapToArray, etc. Thanks in advance for any suggestions

[DarkWanderer]

Hi. As with all AggregateFunction fields, when selecting the data, you need to use appropriate *Merge function - see docs. For groupBitmap, the correct function would be groupBitmapMerge:

SELECT groupBitmapMerge(myColumn) FROM myTable

[xichenAiguang]

Thank you for prompt reply. The function groupBitmapMerge returns the cardinality of that Bitmap, not Bitmap itself. I'd like to query the Bitmap from my service, and deserialize to C# RoaringBitmap instance. To achieve that, we need to call groupBitmapMergeState instead, unfortunately hit the exception " Unknown type: AggregateFunction(groupBitmap, UInt32)" again.

[xichenAiguang]

Attach the image for easier understanding. My purpose is that get ClickhouseRoaringBitmap value

[DarkWanderer]

I don't yet understand the purpose of selecting raw AggregateFunction state, but if you would really like to read it, there is a function ExecuteRawResultAsync which allows to read raw column values directly - you just need to specify a FORMAT you can parse

ClickHouse support channel may be a good help in finding a better way - https://telegram.me/clickhouse_en

[xichenAiguang]

Got it, thank you for pointing out. Wondering do you have any plan support this type?

[DarkWanderer]

No, as the internals of aggregation function state are not supposed to be used directly, and are not part of ClickHouse interface I can use

[DarkWanderer]

Since this seems to be a common question, I'll add a helper function which gives a more appropriate exception text in next release