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:

• 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?

Spadiceous

I found this link from DataStax for version 3. – Spunk

Answer 1

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.

Washtub

I am surprised to know that one can't change any type to collection of that type. For e.q changing text to List<text>. – Teodorateodorico

bigint -> timestamp causes an error "Altering of types is not allowed" in CQLSH 5 and Cassandra 3.11 – Modulate

Answer 2

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.

Titanesque

It's removed started with Cassandra 3.0 (DSE 5.0) – Cleotildeclepe