Cassandra alter column type: which types are compatible?

There's some patchy information on the interwebs about some examples when column type can't be changed. For example, on the DataStax site there's a mention:

Changing the type of a clustering column.
Changing columns on which an index is defined.

Or, for example, here is mentioned that you can't convert uuid to timeuuid. And from my personal experience, I can't change text to timestamp (we store dates in ISO8601 format as text, an unfortunate decision early in the project timeline).

However, I can't find a full description of which types can be converted to which, or at least which types can't be converted to which. Does anyone know?

Solution

Great question! I have to admit that I was a bit surprised at the lack of thorough documentation on this one, as well. Your question inspired me to do a little investigation and blog about it as well.

Essentially, here is a comprehensive list of Cassandra compatible CQL types (using Cassandra 2.2.0):

ascii -> blob, text, varchar
bigint -> blob, timestamp, varint
int -> blob, varint
text -> blob, varchar
timestamp -> bigint, blob, varint
timeuuid -> blob, UUID
varchar -> blob, text

Notes:

blob is in there a lot, because anything can be converted to a blob.
cqlsh allows you to convert a varint to the new date type, but this is a bug (CASSANDRA-10027). Don't actually try that.