Difference between partition key composite key and clustering key in Cassandra

Question

I have been reading articles around the net to understand the differences between the following key types  But it just seems hard for me to grasp  Examples will definitely help  make understanding better   primary key  partition key   composite key  clustering key

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In brief sense   Partition Key is nothing but identification for a row  that identification most of the times is the single column  called Primary Key  sometimes a combination of multiple columns  called Composite Partition Key     Cluster key is nothing but Indexing  amp  Sorting  Cluster keys depend on few things             What columns you use in where clause except primary key columns  If you have very large records then on what concern I can divide the date for easy management  Example  I have data of 1million a county population records  So for easy management  I cluster data based on state and after pincode and so on

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Primary Key  Is composed of partition key s   and optional clustering keys or columns   Partition Key  The hash value of Partition key is used to determine the specific node in a cluster to store the data Clustering Key  Is used to sort the data in each of the partitions or responsible node and it s replicas   Compound Primary Key  As said above  the clustering keys are optional in a Primary Key  If they aren t mentioned  it s a simple primary key  If clustering keys are mentioned  it s a Compound primary key   Composite Partition Key  Using just one column as a partition key  might result in wide row issues  depends on use case data modeling   Hence the partition key is sometimes specified as a combination of more than one column   Regarding confusion of which one is mandatory  which one can be skipped etc  in a query  trying to imagine Cassandra as a giant HashMap helps  So in a HashMap  you can t retrieve the values without the Key  Here  the Partition keys play the role of that key  So each query needs to have them specified  Without which Cassandra won t know which node to search for  The clustering keys  columns  which are optional  help in further narrowing your query search after Cassandra finds out the specific node and it s replicas  responsible for that specific Partition key

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Worth to note  you will probably use those lots more than in similar concepts in relational world  composite keys    Example - suppose you have to find last N users who recently joined user group X  How would you do this efficiently given reads are predominant in this case  Like that  from offical Cassandra guide    CREATE TABLE group join dates       groupname text      joined timeuuid      join date text      username text      email text      age int      PRIMARY KEY   groupname  join date   joined    WITH CLUSTERING ORDER BY  joined DESC    Here  partitioning key is compound itself and the clustering key is a joined date  The reason why a clustering key is a join date is that results are already sorted  and stored  which makes lookups fast   But why do we use a compound key for partitioning key  Because we always want to read as few partitions as possible  How putting join date in there helps  Now users from the same group and the same join date will reside in a single partition  This means we will always read as few partitions as possible  first start with the newest  then move to older and so on  rather than jumping between them    In fact  in extreme cases you would also need to use the hash of a join date rather than a join date alone - so that if you query for last 3 days often those share the same hash and therefore are available from same partition

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In database design  a compound key is a set of superkeys that is not minimal   A composite key is a set that contains a compound key and at least one attribute that is not a superkey  Given table  EMPLOYEES  employee id  firstname  surname   Possible superkeys are    employee id   employee id  firstname   employee id  firstname  surname     employee id  is the only minimal superkey  which also makes it the only candidate key--given that  firstname  and  surname  do not guarantee uniqueness  Since a primary key is defined as a chosen candidate key  and only one candidate key exists in this example   employee id  is the minimal superkey  the only candidate key  and the only possible primary key   The exhaustive list of compound keys is    employee id  firstname   employee id  surname   employee id  firstname  surname    The only composite key is  employee id  firstname  surname  since that key contains a compound key   employee id firstname   and an attribute that is not a superkey   surname

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In cassandra   the difference between primary key partition key composite key  clustering key always makes some confusion   So I am going to explain below and co relate to each others  We use CQL  Cassandra Query Language  for Cassandra database access   Note - Answer is as per updated version of Cassandra    Primary Key  -   In cassandra there are 2 different way to use primary Key    CREATE TABLE Cass       id int PRIMARY KEY      name text         Create Table Cass      id int     name text     PRIMARY KEY id          In CQL  the order in which columns are defined for the PRIMARY KEY matters  The first column of the key is called the partition key having property that all the rows sharing the same partition key  even across table in fact  are stored on the same physical node  Also  insertion update deletion on rows sharing the same partition key for a given table are performed atomically and in isolation  Note that it is possible to have a composite partition key  i e  a partition key formed of multiple columns  using an extra set of parentheses to define which columns forms the partition key   Partitioning and Clustering The PRIMARY KEY definition is made up of two parts  the Partition Key and the Clustering Columns  The first part maps to the storage engine row key  while the second is used to group columns in a row   CREATE TABLE device check     device id   int    checked at  timestamp    is power    boolean    is locked   boolean    PRIMARY KEY  device id  checked at       Here device id is partition key and checked at is cluster key    We can have multiple cluster key as well as partition key too which depends on declaration

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The primary key in Cassandra usually consists of two parts - Partition key and Clustering columns   primary key  partition key   clustering col    Partition key - The first part of the primary key  The main aim of a partition key is to identify the node which stores the particular row    CREATE TABLE phone book   phone num int  name text   age int  city text  PRIMARY KEY   phone num  name   age    Here   phone num  name  is the partition key  While inserting the data  the hash value of the partition key is generated and this value decides which node the row should go into   Consider a 4 node cluster  each node has a range of hash values it can store   Write  INSERT INTO phone book VALUES  7826573732     Joey     25     New York       Now  the hash value of the partition key is calculated by Cassandra partitioner   say  hash value 7826573732     Joey       12   now  this row will be inserted in Node C    Read  SELECT   FROM phone book WHERE phone num 7826573732 and name    Joey      Now  again the hash value of the partition key  7826573732    Joey     is calculated  which is 12 in our case which resides in Node C  from which the read is done    Clustering columns - Second part of the primary key  The main purpose of having clustering columns is to store the data in a sorted order  By default  the order is ascending    There can be more than one partition key and clustering columns in a primary key depending on the query you are solving   primary key  pk1  pk2   col 1 col2

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Adding a summary answer as the accepted one is quite long  The terms  row  and  column  are used in the context of CQL  not how Cassandra is actually implemented    A primary key uniquely identifies a row  A composite key is a key formed from multiple columns  A partition key is the primary lookup to find a set of rows  i e  a partition  A clustering key is the part of the primary key that isn t the partition key  and defines the ordering within a partition     Examples    PRIMARY KEY  a   The partition key is a  PRIMARY KEY  a  b   The partition key is a  the clustering key is b  PRIMARY KEY   a  b    The composite partition key is  a  b   PRIMARY KEY  a  b  c   The partition key is a  the composite clustering key is  b  c   PRIMARY KEY   a  b   c   The composite partition key is  a  b   the clustering key is c  PRIMARY KEY   a  b   c  d   The composite partition key is  a  b   the composite clustering key is  c  d

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There is a lot of confusion around this  I will try to make it as simple as possible    The primary key is a general concept to indicate one or more columns used to retrieve data from a Table   The primary key may be SIMPLE and even declared inline    create table stackoverflow simple         key text PRIMARY KEY        data text              That means that it is made by a single column   But the primary key can also be COMPOSITE  aka COMPOUND   generated from more columns    create table stackoverflow composite         key part one text        key part two int        data text        PRIMARY KEY key part one  key part two               In a situation of COMPOSITE primary key  the  first part  of the key is called PARTITION KEY  in this example key part one is the partition key  and the second part of the key is the CLUSTERING KEY  in this example key part two   Please note that the both partition and clustering key can be made by more columns  here s how    create table stackoverflow multiple         k part one text        k part two int        k clust one text        k clust two int        k clust three uuid        data text        PRIMARY KEY  k part one  k part two   k clust one  k clust two  k clust three               Behind these names       The Partition Key is responsible for data distribution across your nodes  The Clustering Key is responsible for data sorting within the partition  The Primary Key is equivalent to the Partition Key in a single-field-key table  i e  Simple   The Composite Compound Key is just any multiple-column key   Further usage information  DATASTAX DOCUMENTATION   Small usage and content examples SIMPLE KEY   insert into stackoverflow simple  key  data  VALUES   han    solo    select   from stackoverflow simple where key  han     table content  key   data ---- ------ han   solo   COMPOSITE COMPOUND KEY can retrieve  wide rows   i e  you can query by just the partition key  even if you have clustering keys defined   insert into stackoverflow composite  key part one  key part two  data  VALUES   ronaldo   9   football player    insert into stackoverflow composite  key part one  key part two  data  VALUES   ronaldo   10   ex-football player    select   from stackoverflow composite where key part one    ronaldo     table content   key part one   key part two   data -------------- -------------- --------------------       ronaldo              9      football player       ronaldo             10   ex-football player   But you can query with all key  both partition and clustering       select   from stackoverflow composite     where key part one    ronaldo  and key part two    10    query output   key part one   key part two   data -------------- -------------- --------------------       ronaldo             10   ex-football player   Important note  the partition key is the minimum-specifier needed to perform a query using a where clause  If you have a composite partition key  like the following  eg  PRIMARY KEY  col1  col2   col10  col4    You can perform query only by passing at least both col1 and col2  these are the 2 columns that define the partition key  The  general  rule to make query is you have to pass at least all partition key columns  then you can add optionally each clustering key in the order they re set   so the valid queries are  excluding secondary indexes    col1 and col2 col1 and col2 and col10 col1 and col2 and col10 and col 4   Invalid    col1 and col2 and col4 anything that does not contain both col1 and col2   Hope this helps

[database] Difference between partition key, composite key and clustering key in Cassandra?

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