What is Hash and Range Primary Key

Question

I am not able to understand what Range   primary key is here in the docs on Working with Tables and Data in DynamoDB How does it work  What do they mean by  quot unordered hash index on the hash attribute and a sorted range index on the range attribute quot

User · Accepted Answer

Hash and Range Primary Key  means that a single row in DynamoDB has a unique primary key made up of both the hash and the range key  For example with a hash key of X and range key of Y  your primary key is effectively XY  You can also have multiple range keys for the same hash key but the combination must be unique  like XZ and XA   Let s use their examples for each type of table      Hash Primary Key     The primary key is made of one attribute  a hash   attribute  For example  a ProductCatalog table can have ProductID as   its primary key  DynamoDB builds an unordered hash index on this   primary key attribute    This means that every row is keyed off of this value  Every row in DynamoDB will have a required  unique value for this attribute  Unordered hash index means what is says - the data is not ordered and you are not given any guarantees into how the data is stored  You won t be able to make queries on an unordered index such as Get me all rows that have a ProductID greater than X  You write and fetch items based on the hash key  For example  Get me the row from that table that has ProductID X  You are making a query against an unordered index so your gets against it are basically key-value lookups  are very fast  and use very little throughput        Hash and Range Primary Key     The primary key is made of two   attributes  The first attribute is the hash attribute and the second   attribute is the range attribute  For example  the forum Thread table   can have ForumName and Subject as its primary key  where ForumName is   the hash attribute and Subject is the range attribute  DynamoDB builds   an unordered hash index on the hash attribute and a sorted range index   on the range attribute    This means that every row s primary key is the combination of the hash and range key  You can make direct gets on single rows if you have both the hash and range key  or you can make a query against the sorted range index  For example  get Get me all rows from the table with Hash key X that have range keys greater than Y  or other queries to that affect  They have better performance and less capacity usage compared to Scans and Queries against fields that are not indexed  From their documentation      Query results are always sorted by the range key  If the data type of   the range key is Number  the results are returned in numeric order    otherwise  the results are returned in order of ASCII character code   values  By default  the sort order is ascending  To reverse the order    set the ScanIndexForward parameter to false   I probably missed some things as I typed this out and I only scratched the surface  There are a lot more aspects to take into consideration when working with DynamoDB tables  throughput  consistency  capacity  other indices  key distribution  etc    You should take a look at the sample tables and data page for examples

User · Answer

As the whole thing is mixing up let s look at it function and code to simulate what it means consicely  The only way to get a row is via primary key  getRow pk  PrimaryKey   Row  Primary key data structure can be this      If you decide your primary key is just the partition key  class PrimaryKey partitionKey  String      and in thids case getRow somePartitionKey   Row   However you can decide your primary key is partition key   sort key in this case      if you decide your primary key is partition key   sort key class PrimaryKey partitionKey  String  sortKey  String   getRow partitionKey  sortKey   Row getMultipleRows partitionKey   Row        So the bottom line          Decided that your primary key is partition key only  get single row by partition key    Decided that your primary key is partition key   sort key    2 1 Get single row by  partition key  sort key  or get range of rows by  partition key       In either way you get a single row by primary key the only question is if you defined that primary key to be partition key only or partition key   sort key  Building blocks are    Table Item KV Attribute    Think of Item as a row and of KV Attribute as cells in that row    You can get an item  a row  by primary key  You can get multiple items  multiple rows  by specifying  HashKey  RangeKeyQuery     You can do  2  only if you decided that your PK is composed of  HashKey  SortKey    More visually as its complex  the way I see it    ----------------------------------------------------------------------------------   Table                                                                                 ------------------------------------------------------------------------------       Item                                                                                  -----------   -----------   -----------   -----------                                primaryKey    kv attr       kv attr       kv attr                                    -----------   -----------   -----------   -----------                               ------------------------------------------------------------------------------       ------------------------------------------------------------------------------       Item                                                                                  -----------   -----------   -----------   -----------   -----------                  primaryKey    kv attr       kv attr       kv attr       kv attr                      -----------   -----------   -----------   -----------   -----------                 ------------------------------------------------------------------------------                                                                                           ----------------------------------------------------------------------------------    ----------------------------------------------------------------------------------   1  Always get item by PrimaryKey                                                     2  PK is  Hash RangeKey   great get MULTIPLE Items by Hash  filter sort by range        3  PK is HashKey  just get a SINGLE ITEM by hashKey                                                                                         --------------------------                                      ---------------      getByPK   gt  getBy 1                              -----------    gt   HashKey Range  --- gt  hashKey   gt   lt  or startWith                    - gt  Composite   -   ---------------      of rangeKeys                                    -----------                          --------------------------     -----------                                                                          PrimaryKey  -                                                                        -----------                                           --------------------------                      -----------     ---------------      getByPK   gt  get by specific                   - gt  HashType    -- gt  get one item    --- gt  hashKey                                         -----------     ---------------                                                                                           --------------------------    ----------------------------------------------------------------------------------    So what is happening above  Notice the following observations  As we said our data belongs to  Table  Item  KVAttribute   Then Every Item has a primary key  Now the way you compose that primary key is meaningful into how you can access the data   If you decide that your PrimaryKey is simply a hash key then great you can get a single item out of it  If you decide however that your primary key is hashKey   SortKey then you could also do a range query on your primary key because you will get your items by  HashKey   SomeRangeFunction on range key    So you can get multiple items with your primary key query   Note  I did not refer to secondary indexes

User · Answer

A well-explained answer is already given by  mkobit  but I will add a big picture of the range key and hash key   In a simple words range   hash key   composite primary key CoreComponents of Dynamodb      A primary key is consists of a hash key and an optional range key    Hash key is used to select the DynamoDB partition  Partitions are   parts of the table data  Range keys are used to sort the items in the   partition  if they exist    So both have a different purpose and together help to do complex query  In the above example hashkey1 can have multiple n-range  Another example of range and hashkey is game  userA hashkey  can play Ngame range        The Music table described in Tables  Items  and Attributes is an   example of a table with a composite primary key  Artist and   SongTitle   You can access any item in the Music table directly  if   you provide the Artist and SongTitle values for that item       A composite primary key gives you additional flexibility when querying   data  For example  if you provide only the value for Artist  DynamoDB   retrieves all of the songs by that artist  To retrieve only a subset   of songs by a particular artist  you can provide a value for Artist   along with a range of values for SongTitle      https   www slideshare net InfoQ amazon-dynamodb-design-patterns-best-practices https   www slideshare net AmazonWebServices awsome-day-2016-module-4-databases-amazon-dynamodb-and-amazon-rds https   ceyhunozgun blogspot com 2017 04 implementing-object-persistence-with-dynamodb html

User · Answer

vnr you can retrieve all the sort keys associated with a partition key by just using the query using partion key  No need of scan  The point here is partition key is compulsory in a query   Sort key are used only to get range of data

[hash] What is Hash and Range Primary Key?

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