What happens when adding a field in UDT in Cassandra? - cassandra

Question

For example, suppose I have a basic_info type:
CREATE TYPE basic_info (first_name text, last_name text, nationality text)
And table like this:
CREATE TABLE student_stats (id int PRIMARY KEY, grade text, basics FROZEN<basic_info>)
And I have millions of record in the table.
If I add a field in the basic_info like this:
ALTER TYPE basic_info ADD address text;
I want to ask what happens in Cassandra when you add a new field in UDT type (it's currently a column in a table)? The reason for this question is I afraid that some side effects will happen when the table contains a lot of data (millions of record). It's best if you can explain things that will happen from the start to the end.

Answer 1

fields of UDT are described in table system_schema.types. When you add a new field, the entry for that type is updated inside Cassandra, but no changes in data on disk will happen (SSTables are immutable). Instead, when Cassandra read data, it checks if field is present or not, and if not (because it wasn't set, or it's a new field of UDT), then it will return null for that value, but not modify data on disk.
For example, if I have following type and table that uses it:
CREATE TYPE test.udt (
id int,
t1 int
);
CREATE TABLE test.u2 (
id int PRIMARY KEY,
u udt
)
And I have some data in the table, so I get:
cqlsh> select * from test.u2; id | u ----+---------------- 5 | {id: 1, t1: 3}
If I add a field to UDT with alter type test.udt add t2 int;, I immediately see the null as a value for a new UDT field:
cqlsh> select * from test.u2;
id | u
----+--------------------------
5 | {id: 1, t1: 3, t2: null}
And if I do sstabledump on the SSTable, I can see that it contains only old data:
[
{
"partition" : {
"key" : [ "5" ],
"position" : 0
},
"rows" : [
{
"type" : "row",
"position" : 46,
"liveness_info" : { "tstamp" : "2019-07-28T09:33:12.019Z" },
"cells" : [
{ "name" : "u", "path" : [ "id" ], "value" : 1 },
{ "name" : "u", "path" : [ "t1" ], "value" : 3 }
]
}
]
}
]
See also my answer about adding/removing columns