Say I have an RDBMS table with a composite primary key e,g field1,field2,field3 which uniquely identify a record in the table. How can I model this on Dynamodb as it appears the primary on Dynamodb can only be made up of two fields (e.g a partition key and sort key)
You may need to somehow combine them into one value (such as concatenation with a field delimiter). For e.g. field1_field2_field3 as the partition key. If you happen to need sorting, you may also use sort key. You would also be able to search on bases for these fields for e.g. field1_ or field2 or _field3
Refrence: https://aws.amazon.com/blogs/database/choosing-the-right-dynamodb-partition-key/
Related
So I wanted to fetch the last item/row of my dynamodb table but i am not finding resources. My primary key is id having series of incremented numbers such as 1,2,3... for each row respectively.
This is my function.
async function readMessage(){
const params = {
TableName: table,
};
return dynamo.getItem(params).promise();
}
I am not sure as to what i should be adding in my params.
DynamoDB has two types of primary keys:
Partition key – A simple primary key, composed of one attribute known as the partition key.
Partition key and sort key – Referred to as a composite primary key, this type of key is composed of two attributes. The first attribute is the partition key, and the second attribute is the sort key.
When fetching an item by partition key, you need to specify the exact partition key. You cannot fetch the max/min partition key.
Instead, you may want to create a sort key with a timestamp (or the ID if it's a sequential number) and use the sort key to fetch the last item.
Check out the AWS docs on Choosing the Right Partition Key for more info.
The proper way to design a table in DynamoDB is based on its expected access patterns; if this is something you need perhaps you should consider using this id as Sort Key instead of Primary Key and then query the table in descending order while also limiting the amount of items to 1.
If instead you don't want to change the schema of your items and you don't care about making at least two operations to do this you have two, not optimal options:
If none of your items ever gets deleted, just make a count first and use that information to know what's the latest item that was written.
Alternatively, if you could consider keeping a "special" record in your DynamoDB table that is basically a count that gets always incremented/written when one of your "other" items gets written. Upon retrieval you first retrieve the value of this special record and use this info to retrieve the actual one.
The combination of the partition key and sort key, makes the primary key of your item in the dynamoDB, so their combination must be unique, otherwise the item will be overwritten.
In almost all my use-cases, I select the primary key as an object attribute, like the brand, an email or a class and then, for the sort key I select the TimeStamp. So in this way, you always know the partition key, we need it to retrieve the values and then you can query your dynamoDB by making some filters by the sort key. For more extensive examples using Python, check the AWS page: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GettingStarted.Python.04.html, where it shows, how you can query your DynamoDB items.
There is also other ways to define the keys in your Dynamo and for that I advise you to check https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/bp-sort-keys.html
Question
Having gone through verbose AWS documentations, I need some help to clarify the basic keywords and concept of DynamoDB.
Kindly assist to confirm if these are correct.
Hash key
The key which decides the partition of the item, so it is also called as the 'partition' key.
Primary key
A hash key, or (hash-key, range-key) pair that can identify only 1 item in the table. A (hash-key, range-key) pair is also called 'composite' key.
If the primary key has only hash-key, "hash-key" and "primary-key" can be used interchangeably(but doing so can cause confusions).
Local secondary index
In a simple term, "alternative range key" to be used with the hash-key of the primary key.
Besides the range key in the primary key (hash-key, range-key), we can have additional range keys that can be used with the hash-key of the primary key.
Global secondary index
Alternative (hash-key, range-key) pairs for Query.
KeyConditions
For a query on a table, you can have conditions only on the range key portion on the table/index primary key. The hash key condition must always be equal.
Expression attribute name
Dozens of words cannot be used as its attribute name in DynamoDB table, such as status. It is a way to get around this restriction to be able to use such word by prefixing with '#'. Perhaps a design error of DyanmoDB.
Key condition expression
SQL WHERE like part of Query which needs a hash-key of the primary key. It seems it identifies the one partition to get items, then additionally we can use a range-key to narrow down items.
KeyConditions
For a query on a table/index, you can have conditions only on the table/index primary key attributes. You must always provide the partition key name and value as an EQ condition. You can optionally provide a second condition, referring to the sort(aka range) key.
Filter expression
SQL WHERE like part that can be used for both with Query and Scan but only with non-key attributes.
Filter Expressions for Query
A filter expression cannot contain partition key or sort key attributes. You need to specify those attributes in the key condition expression, not the filter expression.
If used in Query in addition to the key expression, the unmatched items are thrown away.
select * from tableName where columnName="value";
How can I fetch a similar result in DynamoDB using java, without using primary key as my attribute (Need to group data based on a value for a particular column).
I have gone through articles regarding getbatchitems, QuerySpec but all these require me to pass the primary key.
Can someone give a lead here?
Short answer is you can't. Whenever you use the Query or GetItem operations in DynamoDB you must always supply the table or index primary key.
You have two options:
Perform a Scan operation on the table and filter by columnName="value". However this requires DynamoDB to look at every item in the table so it is likely to be slow and expensive.
Add a Global Secondary Index to your table. This will require you to define a primary key for the index that contains the columnName you want to query
Hi, I have created a dynamodb table but this has errors when i try to perform table.GetItem with only username (image attached)
Found this is poorly designed table, so thought of recreating a new table, my question is how to set attributes, local secondary index and global secondary index for a table with one primary key and 3 search columns.
or
Is it possible to have 3 more search columns(User_email, Username,Usertype) apart from partition key column(user_ID) in dynamodb?
The GetItem API requires both partition key and sort key. However, you can use Query API with only partition key attribute value. Sort key is not mandatory for Query API.
Get Item Rule:-
For the primary key, you must provide all of the attributes. For
example, with a simple primary key, you only need to provide a value
for the partition key. For a composite primary key, you must provide
values for both the partition key and the sort key.
You can define a maximum of 5 local secondary indexes and 5 global secondary indexes per table.
An LSI is attached to a specific partition key value, whereas a GSI spans all partition key values. Since items having the same partition key value share the same partition in DynamoDB, the "Local" Secondary Index only covers items that are stored together (on the same partition). Thus, the purpose of the LSI is to query items that have the same partition key value but different sort key values. For example, consider a DynamoDB table that tracks Orders for customers, where CustomerId is the partition key.
With a local secondary index, there is a limit on item collection
sizes: For every distinct partition key value, the total sizes of all
table and index items cannot exceed 10 GB. This might constrain the
number of sort keys per partition key value.
I only use primary key integer ID for it's "auto-increment function".
What if I don't need an "auto-increment"? Do I still need primary key if I don't care the uniqueness of record?
Example: Lets compare this table:
create table if not exists `table1`
(
name text primary key,
tel text,
address text
);
with this:
create table if not exists `table2`
(
name text,
tel text,
address text
);
table1 applies primary key and table2 don't. Is there any bad thing happen to table2?
I don't need the record to be unique.
SQLite is a relational database system. So it's all about relations. You build relations between tables on keys.
You can have tables without a primary key; it is not necessary for a table to have a primary key. But you will almost always want a primary key to show what makes a record unique in that table and to build relations.
In your example, what would it mean to have two identical records? They would mean the same person, no? Then how would you count how many persons named Anna are in the database? If you count five, how many of them are unique, how many are mere duplicates? Such queries can be done properly, but get overly complicated because of the lacking primary key. And how would you build relations, say the cars a person drives? You would have a car table and then how to link it to the persons table in your example?
There are cases when you want a table without a primary key. These are usually log tables and the like. They are rare. Whenever you are creating a table without a primary key, ask yourself why this is the case. Maybe you are about to build something messy ;-)
You get auto-incrementing primary keys only when a column is declared as INTEGER PRIMARY KEY; other data types result in plain primary keys.
You are not required to declare a PRIMARY KEY.
But even if you do not do this, there will be some column(s) used to identify and look up records.
The PRIMARY KEY declaration helps to document this, enforces uniqueness, and optimizes lookups through the implicit index.