Cassandra alter column type: which types are compatible?

Question

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:

Answer 1

The 2020 answer:

This functionality seems to have been completely removed. The version 6.8 documentation states:

Cannot change the data type of a column.

Answer 2

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.