columns/nothing.h is not installed

[iso01]

Hi,
Why include file clickhouse/columns/nothing.h is not installed?
Can class clickhouse::ColumnNothing be used to represent Nothing data type?
Best regards,

[slabko]

Good catch!
People probably do not use this column very often, which makes sense.

To answer your questions: yes, clickhouse::ColumnNothing can be used to represent the Nothing data type.
You can also use clickhouse::ColumnNothing or even clickhouse::ColumnNothingT<> for reading the data. In most cases, I think columns of type Nothing are simply skipped and never even accessed. (my mistake here, I meant to say clickhouse::ColumnNullable, but even that does not work)

Just wondering, what is your use case?

[iso01]

I am evaluating ClickHouse to be used as a timeseries database for our operational monitoring data.
For completeness of data types conversion supported by our data model I put void data type matching to clickhouse::ColumnNothing.

Thanks for follow up.

[slabko]

Actually, I have to take my words back. I decided to write a couple of tests for Nothing columns and realized that it does not work as I expected. Let me get back to you regarding your question.

Just to understand your case — do you insert your data using clickhouse::ColumnNothing? Would it be possible to share an example of your table definition (just the part related to the Nothing type) and what you are doing with it?

[iso01]

No, I am not inserting data using clickhouse::ColumnNothing.

I kept the Nothing matching to c++ void type in some template helper that looks like:

template <typename T> struct ch_type {};
template <> struct ch_type<bool> { using type = clickhouse::ColumnUInt8; };
template <> struct ch_type<int8_t> { using type = clickhouse::ColumnInt8; };
template <> struct ch_type<uint8_t> { using type = clickhouse::ColumnUInt8; };
template <> struct ch_type<int16_t> { using type = clickhouse::ColumnInt16; };
...
template <> struct ch_type<std::string> { using type = clickhouse::ColumnString; };
template <> struct ch_type<void> { using type = clickhouse::ColumnNothing; };

However when I tried to instantiate my database converter for void type, I got compilation error. I started to look around for cpp client examples using ColumnNothing and failed. That moment I decided do not spend too much time trying to instantiate converter for void type and implemented its use case differently. As you wrote, people do not need this column very often, so I avoided it as well.

I created this issue when realised that even this simple line:

template <> struct ch_type<void> { using type = clickhouse::ColumnNothing; };

does not compile when headers are taken from clickhouse-cpp client installation area, that seems to be installation bug.

However I am not using clickhouse::ColumnNothing for the moment.

[slabko]

I see. Yeah, I would also use clickhouse::ColumnNothing in this case.

I updated my original comment and also added nothing.h to the list of installed files: #435. I hope that helps in the future.