LowCardinality and Nullable

[waralex]

Implementation of LowCardinality and Nullable. When implementing it, I focused on the Python client. LowCardinality in C++ looks much more complex with a large number of options, but I haven't understood it in much detail yet.
While debugging LowCardinality, I made the table in ClickHouse non-consistent several times and a couple of times ClickHouse fell into the core. It seems that I have fixed everything, but I hope that you will also look at it carefully, since there is a danger of such unpleasant events

[athre0z]

Interesting -- I had previously assumed that LowCardinality(T) is just a storage modifier and doesn't affect how things are sent over the network. I'm actually very glad to learn that I was wrong because the way they do it is a lot more efficient and pretty elegant.

I actually have quite a lot of LowCardinality(String) columns in tables for my projects and did a few queries on those and all of them looked good.

Code LGTM, some minor things annotated below.

[athre0z]

Thanks, this LGTM now!

[athre0z]

Given that you now contributed like half the source code of this library, if this was still my private repo, I feel like it would only be appropriate to ask you whether you'd like to be become a proper maintainer / collaborator of this repo. However, I don't have permissions to do so and also don't really know if this is how things are done in the JuliaDatabases organization.

@quinnj it would be great if you could help us out here and provide some info on the management of this organization!

@waralex By the way, if you don't want to and rather continue just being an unassociated contributor, that is totally fine as well!

[waralex]

@athre0z Of course, I would be happy to become the so-maintainer / collaborator of this repo. However this does not relieve you of the obligation to review my changes :) Just as I am ready to review any changes made by other people

[athre0z]

However this does not relieve you of the obligation to review my changes :)

Heh, yes, obviously. Any non-trivial changes (trivial = typos, reformatting, ...) would still get reviewed.

[quinnj]

@athre0z, I've given you admin privileges (sorry that didn't happen w/ the migration to JuliaDatabases!)
@waralex I've given you maintainer rights.

Love to see all the activity going on here! I opened #12 and assigned myself and I'll try to implement that this week.