Unknown type varchar error on Nullable columns about clickhouse-sqlalchemy

[xzkostyan]

There was pull request in 20 version of ClickHouse server that saves type synonyms in table declarations and in data formats. VARCHAR is synonym for String. Earlier server versions have types canonization. So even you specify VARCHAR as source type you will get String as canonized type.

Many 3rd party tools are broken now. ClickHouse developers say that that PR is contentious and can be reverted or can lead to another changes.

I'd prefer to wait for server developers' final decision on types canonization.

[durchgedreht]

Many 3rd party tools are broken now.
You name it! Thanks so much for the explanation!

If anybody would be able to point me to the pull request as I have hard times finding it this would be awesome! I'd like to understand the behaviour and it's consequences in full.

I can still proactively decide how to roll forward and would like to make sure I take the right decision what to use where (table DDL and creation / driver / tool). Not all are 100% under my control, so this might get a bit tricky. Currently using VARCHAR in the DDL definition using JDBC to create table, sqlalchemy (native) to read (VARCHAR) which only works when calling toString() in the statement itself. Looks quite ugly and I'd like to streamline this mess! Would using String clean this up or will this cause problems when the Pull request is reverted? Unfortunately I cannot wait that much longer and take a decision now.

This is really annoying, especially as this went into the release and there's nothing mentioned in the backwards incompatibilities list (and other fixes needed to go into production):
https://github.com/ClickHouse/ClickHouse/blob/master/CHANGELOG.md

[xzkostyan]

I've found PR: ClickHouse/ClickHouse#8370

[durchgedreht]

Thanks very much, this is very useful! Highly appreciate your update. Will close issue here as this is more Clickhouse dependant than a driver issue!

[xzkostyan]

I don't know on which side this should be fixed, but it seems we can make workaround in driver.

[xzkostyan]

PR with discussion about canonical types: ClickHouse/ClickHouse#8429 (comment)

[xzkostyan]

Keep alias type in metadata was reverted. This fix is present in 20.3 version of server.