I have a UDT type in Cassandra. I want to ALTER this type to remove the country field. I don't find any delete or remove field for ALTER TYPE documentation. https://docs.datastax.com/en/cql/3.3/cql/cql_reference/cqlAlterType.html
create type bank_payment (
account_number text,
name text,
city text,
country text,
key text
);
Please help sharing ALTER command to remove one field from above UDT.
Cassandra UDT doesn't store any actual data and its the table where this UDT referred to has actual data. So go ahead and drop the UDT & recreate it with the correct definition that you want. Remember the serialization of old sstables that had this UDT (with additional column) will be different from the new one.
So if possible add a new version name to this UDT as you recreate, say
bank_payment_v2
I have recently started learning No-SQL and Cassandra through this article. The author explains the data model through this diagram:
The author also gives the below column family example:
Book {
key: 9352130677{ name: “Hadoop The Definitive Guide”, author:” Tom White”, publisher:”Oreilly”, priceInr;650, category: “hadoop”, edition:4},
key: 8177228137{ name”” Hadoop in Action”, author: “Chuck Lam”, publisher:”manning”, priceInr;590, category: “hadoop”},
key: 8177228137{ name:” Cassandra: The Definitive Guide”, author: “Eben Hewitt”, publisher:” Oreilly”, priceInr:600, category: “cassandra”},
}
But in that tutorial and every other tutorial I have gone through, then end up creating regular tables in cassandra. I am unable to connect the Cassandar model with what I am creating.
For example, I created a column family called Employee as below:
create columnfamily Employee(empid int primary key,empName text,age int);
Now I inserted some data and my column family looks as this:
For me this looks like a regular relational table and not like the data model the author has explained. How do I create a Employee column family where each row represents an employee with different attributes? Something like:
Employee{
101:{name:Emp1,age:20}
102:{name:Emp2,salary:1000}
102:{manager_name:Emp3,age:45}
}
}
You need to understand that in the representation using cql, is may look like regular relational table, but the internal structure of the rows in Cassandra is completely different. It is saving different set of attributes for each employee, and the nulls you can see while querying with cql is just a representation of empty/nonexistent cells.
What you trying to achieve, is unstructured data model. Cassandra started with this model, and all was working as described in the tutorial you've read, but there is an opinion that unstructured data design is unhealthy to development and makes more problems than it solves. So, after sometime, Cassandra moved to the "structured" data structure (and from thrift to cql). It doesn't mean that you have to store all attributes for all keys/rows, it doesn't mean that all the rows are have same number of attributes, it just means that you have to declare attributes before you use them.
You can achieve some kind of unstructured data modeling using Map, List, Set, etc. data types, UDT (User defined types) or just saving your data as json string and parsing it on the application side.
What you have understood is correct. Just believe it. Internally cassandra stores columns exactly like the image in your question.
Now, what you are expecting is to insert a column which is not defined while creating the Employee table. For dynamic columns, you can always use Map data types .
For example
create table Employee(
empid int primary key,
empName text,
age int,
attributes Map<text,text>);
To add new attributes you can use below queries.
UPDATE Employee SET attributes = { manager_name : Emp3, age:45 } WHERE empid = 102;
Update -
another way to to create a dynamic column model is as below
create table Employee(
empid int primary key,
empName text,
attribute text,
attributevalue text,
primary key (empid,empName,attribute)
);
Lets take few inserts -
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','age','25') ;
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','manager','emp2') ;
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','department','hr') ;
this data structure will create a wide row, and behaves like dynamic column. you can see primary key empid and name is common for all three rows, only attribute and value will change.
Hope this will help
Cassandra uses a special primary key called compositie key. This is the representation of the partitions. This is also one reason why cassandra scales well. The composite key is used to determine the nodes on which the rows are stored.
The result in your console may be a result set of rows, but the intern organization of cassandra is differnt from that. Have you ever tried to query a table without an primary key? You will quickly see that you can't query that flexible (because of the partitioning).
After that you will understand why we have to use a query-first design aproach for cassandra. This is completely different from RDBBS.
I have created a table my_table with a column phone, which has been declared as of type varint. After entering some data, I realized that it would have been better if I had declared this column as list<int>.
I tried to:
ALTER TABLE my_table
ALTER phone TYPE list<int>
but unfortunately I am not allowed to do so. Hopefully, there is a way to make this change.
UPDATE: Assume that I make a new column phonelist of type list<int>. Is there any efficient way to move the data in the phone column into the phonelist column?
You cannot change the type of an existing column to a map or collection.
The table shows the allowed alterations for data types
I am not good in English!
There is a table in Cassandra 3.5 which all columns of a row don't come at same time. Unique of table is some columns that are unique in a row together, but some of them are null at first. I can not set them the primary key because of null value. I have identify a column with name id and type uuid in Cassandra.
How can I have a unique key with that columns together in Cassandra?
Is my data model true?
How can I solve this problem?
You can't. It's not a relational DB. Use clustering and/or partitioning keys to add an unique constraint.
See this answer
To store unique values, create a separate table having your unique value as a key. Check if it exists by requesting this table before inserting a row. But beware, even doing this, you cannot ensure it will be unique in your final table if you have two concurrent inserts.
Basically, I would recommend using Cassandra as it really is: A data store. And find a way to implement your business logic where it belongs: in your code.
I'm having problems with using the time_uuid type as a key in my columnfamily. I want to store my records, and have them ordered by when they were inserted, and then I figured that the time_uuid is a good way to go. This is how I've set up my column family:
sys.create_column_family("keyspace", "records", comparator_type=TIME_UUID_TYPE)
When I try to insert, I do this:
q=pycassa.ColumnFamily(pycassa.connect("keyspace"), "records")
myKey=pycassa.util.convert_time_to_uuid(datetime.datetime.utcnow())
q.insert(myKey,{'somedata':'comevalue'})
However, when I insert data, I always get an error:
Argument for a v1 UUID column name or value was neither a UUID, a datetime, or a number.
If I change the comparator_type to UTF8_TYPE, it works, but the order of the items when returned are not as they should be. What am I doing wrong?
The problem is that in your data model, you are using the time as a row key. Although this is possible, you won't get a meaningful ordering unless you also use the ByteOrderedPartitioner.
For this reason, most people insert time-ordered data using the time as a column name, not a row key. In this model, your insert statement would look like:
q.insert(someKey, {datetime.datetime.utcnow(): 'somevalue'})
where someKey is a key that relates to the entire time series that you're inserting (for example, a username). (Note that you don't have to convert the time to UUID, pycassa does it for you.) To store something more than a single value, use a supercolumn or a composite key.
If you really want to store the time in your row keys, then you need to specify key_validation_class, not comparator_type. comparator_type sets the type of the column names, while key_validation_class sets the type of the row keys.
sys.create_column_family("keyspace", "records", key_validation_class=TIME_UUID_TYPE)
Remember the rows will not be sorted unless you also use the ByteOrderedPartitioner.
The comparator for a column family is used for ordering the columns within each row. You are seeing that error because 'somedata' is valid utf-8 but not a valid uuid.
The ordering of the rows stored in cassandra is determined by the partitioner. Most likely you are using RandomPartitioner which distributes load evenly across your cluster but does not allow for meaningful range queries (the rows will be returned in a random order.)
http://wiki.apache.org/cassandra/FAQ#range_rp