I have a table with a composite primary key. name,description, ID
PRIMARY KEY (id, name, description)
whenever searching Cassandra I need to provide the three keys, but now I have a use case where I want to delete, update, and get just based on ID.
So I created a materialized view against this table, and reordered the keys to have ID first so I can search just based on ID.
But how do I delete or update record with just an ID ?
It's not clear if you are using a partition key with 3 columns, or if you are using a composite primary key.
If you are using a partition key with 3 columns:
CREATE TABLE tbl (
id uuid,
name text,
description text,
...
PRIMARY KEY ((id, name, description))
);
notice the double parenthesis you need all 3 components to identify your data. So when you query your data by ID from the materialized view you need to retrieve also both name and description fields, and then issue one delete per tuple <id, name, description>.
Instead, if you use a composite primary key with ID being the only PARTITION KEY:
CREATE TABLE tbl (
id uuid,
name text,
description text,
...
PRIMARY KEY (id, name, description)
);
notice the single parenthesis, then you can simply issue one delete because you already know the partition and don't need anything else.
Check this SO post for a clear explanation on primary key types.
Another thing you should be aware of is that the materialized view will populate a table under the hood for you, and the same rules/ideas about data modeling should also apply for materialized views.
Related
In cassandra joins are not supported. So I want to show 20 Videos with comments.
I saw this example from a data modelling:
CREATE TABLE videos (
id number(12),
userid number(12) NOT NULL,
name nvarchar2(255),
description nvarchar2(500),
location nvarchar2(255),
location_type int,
added_date timestamp,
CONSTRAINT users_userid_fk FOREIGN KEY (userid) REFERENCES users (Id) ON DELETE CASCADE,
PRIMARY KEY (id)
);
CREATE TABLE comments (
id number(12),
userId number(12),
videoId number(12),
comment_text nvarchar2(500),
comment_time timestamp(6),
PRIMARY KEY (id),
CONSTRAINT user_comment_fk FOREIGN KEY (userid) REFERENCES users (Id) ON DELETE CASCADE,
CONSTRAINT video_comment_fk FOREIGN KEY (videoId) REFERENCES videos (Id) ON DELETE CASCADE
);
So how can I get now all videos with comments ? Because joins are not supported.
Can anyone help me ?
You are right that Cassandra does not support joins. And reason for that joins may become slow for big web scale data and Cassandra was designed to solve that particular problem.
Now coming back to your problem, to solve this in Cassandra it it recommended to create a table (call it a join table) which can answer your query. So if you want to see list of videos with comments, you can group videos and comments in a single partition using user_id as partition key. The table can look like this
CREATE TABLE KEYSPACE.videos_comments_by_user_id (\
user_id int,\
video_id int,\
comments list\<text>,
PRIMARY KEY((user_id), video_id);
While designing a table in Cassandra one should always keep one thing in mode, What query does this table shall serve? Do not think about joining the two tables, that will never work with Cassandra. Instead design to serve a query.
Below is the table design which will serve your query-
Table
comments_by_user_videos
Columns
Userid
Videoid
CommentId
CommentTimeStam
Videoname static
VideoDescription static
Location static
Location type static
Comment_text
Constraints-
Primary Key ((userid,videoid), CommentId).
The static columns are shared for a row kye. In this design , user id and video id define one row so for a user id and video id the value of static columns remains same. Static column can used to represent one to many relationship.
Query- to get all the comments for a video of a user.
Select * from comments_by_user_videos where userid=? and videoid=?
Edit:
If you want to display few number of vides, you should design another table.
Table: user_videos
userid
timestamp
videoid
videoname
videoLocation
locationtype
Primary key (userid, videoid, timestamp) ordered by timestamp DESC
Query:
select * from users_by_id_name where userid=? limit 10;
The query should return latest 10 videos by user.
Insert data in to both tables, never hesitate to duplicate data.
I have a table in cassandra with following schema:
CREATE TABLE user_album_entity (
userId text,
albumId text,
updateDateTimestamp timestamp,
albumName text,
description text,
PRIMARY KEY ((userId), updateDateTimestamp)
);
The query required to get data would have a where userId = xxx order by updateTimestamp. Hence the schema had updateDateTimestamp.
Problem comes in updating the column of table.The query is: Update the album information for user where user id = xxx. But as per specs,for update query I would need the exact value of updateDateTimestamp which in normal world scenario, an application would never send.
What should be the answer to such problems since I believe this a very common use case where select query requires ordering on timestamp. Any help is much appreciated.
The problem is that your table structure allows the same album to have multiple records with the only difference being the timestamp (the clustering key).
Three possible solutions:
Remove the clustering key and sort your data at application level.
Remove the clustering key and add a Secondary Index to the timestamp field.
Remove the clustering key and create a Materialized View to perform the query.
If your usecase is such that each partition will contain exactly one row,
then you can model your table like:
CREATE TABLE user_album_entity (
userId text,
albumId text static,
updateDateTimestamp timestamp,
albumName text static,
description text static,
PRIMARY KEY ((userId), updateDateTimestamp)
);
modelling the table this way enables Update query to be done in following way:
UPDATE user_album_entity SET albumId = 'updatedAlbumId' WHERE userId = 'xyz'
Hope this helps.
I have table with node_id, node_name and data. My requirement is to getByID and getByName. So I have made id and name as the primary keys. But I also need to sometimes update the name as well.
I know Cassandra does not allow updating primary keys and having non primary key in the WHERE clause.
How can I achieve this?
I did consider deleting the record first, and then inserting again with the same id and new name. But I read that this would create tombstones and affect the performance.
Use only node_id as the primary key. To implement getByName create a materialized view. materialized views in cassandra
create table users_by_id_name(
id int,
createdOn bigint, -- timestamp in millisec
name text,
age int,
primary key (id,name,createdOn)
)WITH CLUSTERING ORDER BY ( name DESC, createdOn DESC);
Use above table definition to insert users.
Insert query --
insert into users_by_id_name (id,createdOn,name,age) values (1,100,'darthvedar',28);
to update the user, insert the row again with same user id and updated name and createdOn value.
insert into users_by_id_name (id,createdOn,name,age) values (1,200,'obi-wan-kenobi',28);
while selecting the user use below query --
select by user id -
select * from users_by_id_name where id=1 limit 1;
Select user by name -
select * from users_by_id_name where name='obi-wan-kenobi' ALLOW FILTERING;
Other way is to use secondary index on user name. Think, user name is not going to change too frequently, so secondary index is also one good option.
Edit after comments -
If you have very frequent updates on user name, it would be better to use two different tables.
create table users_by_id(
id int,
name text,
age int,
primary key (id)
);
create table users_by_name(
id int,
name text,
age int,
primary key (name)
);
While inserting , insert in both the tables using batch statement.
Hope this will help.
What is the best approach to update table with duplicate data?
I have a table
table users (
id text PRIMARY KEY,
email text,
description,
salary
)
I will delete, update, insert etc to this table. But I also have a requirement to be able to search by email, and description. If I create new table with new composite keys for email, and description,
when I update my base table I do
insert into users (id, salary) values (1, 500);
I do not have the required data to also update my secondary table since all the client has is id and salary. How is the second table updated.
Other workarounds and shortcomings
I could have created a materialized view, but since the base table has only one primary key I can only add one more column. my search requirement requires more than one column.
Create secondary indexes on the columns that will be searched on. But the performance for this would be bad since the columns I will be searching on would have high cardinality. i.e. description, email, etc
So, the "correct" way of doing this is to create 3 tables. salary_by_id, salary_by_email and salary_by_description.
table salary_by_id (
id text PRIMARY KEY,
salary int
)
table salary_by_email (
email text PRIMARY KEY,
salary int
)
table salary_by_description (
description text,
id int,
salary int,
primary key (description, id)
)
The reason i added id to salary_by_description is that, from my own guessing, description won't be globally uniq, so it has to have something else in it's primary key.
Depending on the size of these tables the last one might need something extra added to it's partitioning key. And if needed you can add id, email and description to the other tables.
Now, when inserting or deleting values you need so do it in all 3 tables. If you use a driver, like in java, that supports asynchronous calls, then this doesn't cost very much extra.
Suppose I have table with the following structure
create table tasks (
user_id uuid,
name text,
task_id uuid,
description text,
primary key ((user_id), name, task_id)
);
It allows me to get all tasks for user sorted by name ascending. Also I added task_id to primary key to avoid upserts. The following query holds
select * from tasks where user_id = ?
as well as
select * from tasks where user_id = ? and name > ?
However, I cannot get task with specific task_id. For example, following query crashes
select * from tasks where user_id = ? and task_id = ?
with this error
PRIMARY KEY column "task_id" cannot be restricted as preceding column "name" is not restricted
It requires name column to be specified, but at the moment I have only task_id (from url, for example) and user_id (from session).
How should I create this table to perform both queries? Or I need create separate table for second case? What is the common pattern in this situation?
You could simply add one more redundant column taskId with same value as task_id and create a secondary index on taskId.
Then you can query user_id=? and tsakId=?
PRIMARY KEY column "task_id" cannot be restricted as preceding
column "name" is not restricted
You are seeing this error because CQL does not permit queries to skip primary key components.
How should I create this table to perform both queries? Or I need create separate table for second case? What is the common pattern in this situation?
As you suspect, the typical way that problems like this are solved with Cassandra is that an additional table is created for each query. In this case, recreating the table with a PRIMARY KEY designed to match your additional query pattern would simply look like this:
create table tasks_by_user_and_task (
user_id uuid,
name text,
task_id uuid,
description text,
primary key ((user_id), task_id)
);
You could simply add one more redundant column taskId with same value as task_id and create a secondary index on taskId.
While I am usually not a fan of using secondary indexes, in this case it may perform ok. Reason being, is that you would still be restricting your query by partition key, which would eliminate the need to examine additional nodes. The drawback (as Undefined_variable pointed out) is that you cannot create a secondary index on a primary key component, so you would need to duplicate that column (and apply the index to the non-primary key column) to get that solution to work.
It might be a good idea to model and test both solutions for performance.
If you have the extra disk space, the best method would be to replicate the data in a second table. You should avoid using secondary indexes in production. Your application would, of course, need to write to both these tables. But Cassandra is darn good at making that efficient.
create table tasks_by_name (
user_id uuid,
name text,
task_id uuid,
description text,
primary key ((user_id), name, task_id)
);
create table tasks_by_id (
user_id uuid,
name text,
task_id uuid,
description text,
primary key ((user_id), task_id)
);