Anybody please help me understand why Cassandra is inserting null values in columns that was skipped? Isn't it supposed to skip the column? It should not insert any value (not even null) if I skip the column entirely while inserting data? I am bit confused because as per the following tutorial, data is stored by row key with the columns (the diagram in column family), if it is true then I should not get null for the column.
Or the whole concept I learned about the Cassandra column family is wrong?
http://www.tutorialspoint.com/cassandra/cassandra_data_model.htm
Here is the CQL script
create keyspace test WITH replication = {'class': 'SimpleStrategy', 'replication_factor': '1'};
create table users (firstname text,lastname text,age int, gender ascii, primary key(firstname))
insert into users(firstname,age,gender,lastname) values("Michael",30,"male","smith");
Here, I am skipping a column, but when I run select query, it shows null for that column. Why Cassandra is filling up null in that column?
insert into users(firstname,age,gender) values('Jane',23,'female');
select * from users;
Why don't you go to the most comprehensive source of documentation and learning for Cassandra : http://academy.datastax.com ? And it's free. The content and tutorialspoint.com is very old and not updated since ages (SuperColumn are deprecated since 2011 - 2012 ...)
Here, I am skipping a column, but when I run select query, it shows null for that column. Why Cassandra is filling up null in that column?
In CQL, null == value is not present or value has been deleted
Since you did not insert any value for column lastname Cassandra will return null (== not present in this case)
Related
I see an issue in Cassandra boolean datatype,
I have a table with one field as boolean
CREATE TABLE keyspace.issuetable (
"partitionId" text,
"name" text,
"field" text,
"testboolean" boolean,
PRIMARY KEY ("partitionId", "name"));
Now when I try to insert in table, I didn't add the boolean 'testboolean'
INSERT into keyspace.issuetable("partitionId", "name", "field")
VALUES ('testpartition', 'cluster1_name','testfiled');
Issue :
1) If the boolean entry (say testboolean entry) in INSERT query is not added so as per the data type it needs to be 'false' but it is added as null
SELECT * FROM issuetable ;
partitionId | name | field | testboolean
---------------+---------------+-----------+-------------
testpartition | cluster1_name | testfiled | null
Could you someone explain why? Also let me know the solution to solve this, I expect 'false' not 'null'
Cassandra is not like the traditional SQL databases. It does not store rows in tables. The best way to think about Cassandra's data model is to imagine a sortedMap<rowKey, map<columnKey, value>>.
This means that any particular row is not required to have the same fields/columns as any other one. In your example the inserted row simply does not have a property named testboolean.
To understand more I can recommend referring here.
And no, you cannot set a default value for a column (or rather you can do it only on application side).
I want to create a Cassandra collection with some list<int> field and insert an empty list;
CREATE TABLE test (
name text PRIMARY KEY,
scores list<int>,
);
INSERT INTO test (name, scores) VALUES ('John', []);
However, this returns null
SELECT * FROM test;
name |scores
------+------------
John | null
Does Cassandra not differentiate between null and empty list?
As always the recommendation goes with Cassandra don't insert NULL or try to insert EMPTY values. Its just saving yourselves from Tombstones, storage, I/O bandwidth.
The reason why Cassandra doesn't differentiate NULL Vs empty is because the way deletes are handled. There is no read before deleting any record in Cassandra. So it just marks as a tombstone and moves ahead.
So actually you get penalized to initialize the list as empty (essentially creating tombstone).
I have a very huge table in cassandra that consists of (caseid ,timestamp, activity)as columns with caseid and timestamp being the primary key.The values of caseid are getting repeated and I want to extract the 1st value of activity corresponding to a caseid and put it to another table(named initialActivity) that consists of only activity. Can someone please help me as to how I can acheive this using a cql query.Thanks.
Please try this
Insert into initialActivity() values
(select activity from preActivity where caseId = 111 LIMIT 1 );
Only first rows with column activity with caseId = 111 will get inserted into initialActivity table
Please refer this for more info
CQL
I am NoSQL n00b, and just trying things out. I have the following keyspace with a single table in cassandra 2.0.2
CREATE KEYSPACE PersonDB WITH replication = {
'class': 'SimpleStrategy',
'replication_factor': '1'
};
USE PersonDB;
CREATE TABLE Persons (
id int,
lastname text,
firstname text,
PRIMARY KEY (id)
)
I have close to 500 entries in the Persons table. I want to select any random row from the table. Is there an efficient way to do it in CQL? I am using groovy to invoke APIs exposed by datastax.
If want to get "any" row you can just use LIMIT.
select * from persons LIMIT 1;
You would get the row with the lower hash of the partition key (id).
It will not be random, it will depend on your partitioner, but you would get A row.
For Cassandra, do UPDATEs become an implied INSERT if the selected row does not exist? That is, if I say
UPDATE users SET name = "Raedwald" WHERE id = 545127
and id is the PRIMARY KEY of the users table, and the table has no row with a key of 545127, will that be equivalent to
INSERT INTO users (id, name) VALUES (545127, "Raedwald")
I know that the opposite is true: an INSERT for an id that already exists becomes an UPDATE of the row with that id. Older Cassandra documentation talked about inserts actually being "upserts" for that reason.
I'm interested in the case for CQL3, Cassandra version 1.2+.
Yes, for Cassandra UPDATE is synonymous with INSERT, as explained in the CQL documentation where it says the following about UPDATE:
Note that unlike in SQL, UPDATE does not check the prior existence of the row: the row is created if none existed before, and updated otherwise. Furthermore, there is no mean to know which of creation or update happened. In fact, the semantic of INSERT and UPDATE are identical.
For the semantics to be different, Cassandra would need to do a read to know if the row already exists. Cassandra is write optimized, so you can always assume it doesn't do a read before write on any write operation. The only exception is counters (unless replicate_on_write = false), in which case replication on increment involves a read.
Unfortunately the accepted answer is not 100% accurate. inserts are different than updates:
cqlsh> create table ks.t (pk int, ck int, v int, primary key (pk, ck));
cqlsh> update ks.t set v = null where pk = 0 and ck = 0;
cqlsh> select * from ks.t where pk = 0 and ck = 0;
pk | ck | v
----+----+---
(0 rows)
cqlsh> insert into ks.t (pk,ck,v) values (0,0,null);
cqlsh> select * from ks.t where pk = 0 and ck = 0;
pk | ck | v
----+----+------
0 | 0 | null
(1 rows)
Scylla does the same thing.
In Scylla and Cassandra rows are sequences of cells. Each column gets a corresponding cell (or a set of cells in the case of non-frozen collections or UDTs). But there is one additional, invisible cell - the row marker (in Scylla at least; I suspect Cassandra has something similar).
The row marker makes a difference for rows in which all other cells are dead: a row shows up in a query if and only if there's at least one alive cell. Thus, if the row marker is alive, the row will show up, even if all other columns were previously set to null using e.g. updates.
inserts create a live row marker, while updates don't touch the row marker, so clearly they are different. The example above illustrates that.
One could argue that row markers are "internal" to Cassandra/Scylla, but as you can see, their effects are visible. Row markers affect your life whether you like it or not, so it may be useful to remember about them.
It's sad that no documentation mentions row markers (well, I found this: https://docs.scylladb.com/architecture/sstable/sstable2/sstable-data-file/#cql-row-marker but it's in the context of explaining SSTable internals, which is probably dedicated to Scylla developers more than to users).
Bonus: a cell delete:
delete v from ks.t where pk = 0 and ck = 0
is the same as a null update:
update ks.t set v = null where pk = 0 and ck = 0
indeed, a cell delete also doesn't touch the row marker. It only sets the specified cell to null.
This is different from a row delete:
delete from ks.t where pk = 0 and ck = 0
because row deletes insert a row tombstone, which kills all cells in the row (including the row marker). You could say that row deletes are the opposite of an insert. Updates and cell deletes are somewhere in between.
What one can do is this however:
UPDATE table_name SET field = false WHERE key = 55 IF EXISTS;
This will ensure that your update is a true update and not an upsert.