I know we can do update by two operations, first get the primary key by querying the db, and then update it by put operation. But does DynamoDB support update by one operation as the relational db (such as mysql)? Since two operations will cost more time in network transferring.
My situation is as:
I have a table A with fields ID, Name, Location, Value.
And name+location can uniquely define a row.
So now I want to update the field "Value" when Name and Location satisfied some condition, but I don't know the ID. So if I use mysql, then I can update it by "Update A set value = XXX where name = "abc" and location="123"".
But when I use dynamoDB, I have to first get the primary key ID.
Then use the Key to update the item. So my question is that does DynamoDB also support similar update operation as mysql does.
Thanks!
Chen hit it on the nose. Joey, the situation you described (Get followed by a Put) is equivalent to 2 mysql functions
SELECT *
FROM TABLE
WHERE key = x
UPDATE TABLE
SET var = param
WHERE key = x
Do you see how the Select/PutItem aren't part of the update process? As long as you have the keys, you don't need to perform a query. I'm assuming you're performing the GetItem before the PutItem request because the PutItem replaces the entire item/row (i.e. deletes all attributes not specified in the Put request).
So if the original item looked like: < key-id=1, first-name=John, last-name=Doe, age=22>
and you perform a PutItem of: < key-id=1,location=NY>
The final item looks like: < key-id=1,location=NY>
If you perform an UpdateItem in place of PutItem then you would instead get:
< key-id=1, first-name=John, last-name=Doe, age=22, location=NY>
Here's a link for using the UpdateItem with Java. There also examples using .net and php
UpdateItem for Java
Correct me if I am wrong but Update Item will consume 1 operation only it will get hash key value and update it if exists else will create new Item (up-to 1 kb item)
here is the link for reference : http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/WorkingWithTables.html#CapacityUnitCalculations
Hope that helps
You don't need to get the primary key first. If you know the primary key, you don't need to get anything and you can simply use the UpdateItem API call to update your item.
If that still isn't clear, please edit your question and add some code samples of what you are trying to do.
Related
First of all, I have table structure like this,
Users:{
UserId
Name
Email
SubTable1:[{
Column-111
Column-112
},
{
Column-121
Column-122
}]
SubTable2:[{
Column-211
Column-212
},
{
Column-221
Column-222
}]
}
As I am new to DynamoDB, so I have couple of questions regarding this as follows:
1. Can I create structure like this?
2. Can we set primary key for subtables?
3. Luckily, I found DynamoDB helper class to do some operations into my DB.
https://www.gopiportal.in/2018/12/aws-dynamodb-helper-class-c-and-net-core.html
But, don't know how to fetch only perticular subtable
4. Can we fetch only specific columns from my main table? Also need suggestion for subtables
Note: I am using .net core c# language to communicate with DynamoDB.
Can I create structure like this?
Yes
Can we set primary key for subtables?
No, hash key can be set on top level scalar attributes only (String, Number etc.)
Luckily, I found DynamoDB helper class to do some operations into my DB.
https://www.gopiportal.in/2018/12/aws-dynamodb-helper-class-c-and-net-core.html
But, don't know how to fetch only perticular subtable
When you say subtables, I assume that you are referring to Array datatype in the above sample table. In order to fetch the data from DynamoDB table, you need hash key to use Query API. If you don't have hash key, you can use Scan API which scans the entire table. The Scan API is a costly operation.
GSI (Global Secondary Index) can be created to avoid scan operation. However, it can be created on scalar attributes only. GSI can't be created on Array attribute.
Other option is to redesign the table accordingly to match your Query Access Pattern.
Can we fetch only specific columns from my main table? Also need suggestion for subtables
Yes, you can fetch specific columns using ProjectionExpression. This way you get only the required attributes in the result set
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
I have taken over some code and I see that database updates are performed like this:
dbcon = DependencyService.Get<ISQLite>().GetConnection();
public void UpdateAnswered(string id)
{
lock(locker)
{
dbcon.Query<Phrase>("UPDATE Phrase SET Answered = Answered + 1 " +
"WHERE Id = ?", id);
}
}
I am new to using SQLite with Xamarin but it looks strange to me that this update is handled with a dbcon.Query and that the table name is passed as . Can someone confirm is this the optimal way to handle a table update? Also why is it coded as a query with the table name being passed?
Update<T>
This method allows you to pass in an instance of an object that this stored in the database which has a primary key. SQLite then recognizes the primary key and updates the rest of the object's values.
You would just call connection.Update( phrase ); where the phrase is an instance of the Phrase class with properties you want to set. Be aware that all columns except ID will be updated.
Query<T>
Performs a query and returns the results. The type parameter specifies the type of the items returned. This is most appropriate for SELECT queries.
Execute
This returns the number of affected rows by the query as an int. This is probably the best choice for your UPDATE query after the Update<T> method.
ExecuteScalar<T>
Use for queries that return scalar types - like COUNT, etc., where T is the type of the value.
In summary, Update is the most natural way to update a row in the database (with an instance you have), but Query<T> and Execute<T> are very useful if you just want to UPDATE one column like in your example.
Simple thing. While inserting data to table A I have a HashKey id and additional hash index for column ex_id, which is kind of a foreign key in table B.
When inserting a new data into table A I would like to create an exception whenever data is inserted with value in column ex_id that doesn't have a correspondent entry in table B.
I thought that ConditionExpression is the way to go, but can't make it work - probably missing something obvious. Tried to use contains()...
Any ideas?
As per my knowledge this would not be possible at DynamoDB end because there are no relationship between the tables.
What you can do is that you can have a condition at the application level, which checks on its own and throw an exception before inserting the value in table A. (You can query table B for that "Id" if found then insert else throw exception)
DynamoDB does not natively support any kind of foreign key support, everything works on a per table basis, per key basis. DynamoDB's approach is to handle such logic at the client level. For example see the dynamodb transactions client. This library allows you to perform transactions across tables which either all succeed or all rollback.
For your case, I would first make a getItem request to table B (use consistent read) if it exists then write to table A.
Then I would enable streams on table A and write a lambda function to check if any data violations get written to the table.
Is it possible to modify the Rangekey column after table creation. Such as adding new column/attribute and assigning as RangeKey for the table. Tried searching but cant ble to find any articles about changing the Range or Hash key
No, unfortunately it's not possible to change the hash key, range key, or indexes after a table is created in DynamoDB. The DynamoDB UpdateItem API Documentation is clear about the fact that indexes cannot be modified. I can't find a reference to anywhere in the docs that explicitly states that the table keys cannot be modified, but at present they cannot be changed.
Note that DynamoDB is schema-less other than the hash and range key, and you can add other attributes to new items with no problems. Unfortunately, if you need to modify either your hash key or range key, you'll have to make a new table and migrate the data.
Edit (January 2014): DynamoDB now has support for on the fly global secondary indexes
To change or create an additional sort key, you will need to create a new table and migrate over to it, as both actions cannot be done on existing tables.
DynamoDB streams enable us to migrate tables without any downtime. I've done this to great effective, and the steps I've followed are:
Create a new table (let us call this NewTable), with the desired key structure, LSIs, GSIs.
Enable DynamoDB Streams on the original table
Associate a Lambda to the Stream, which pushes the record into NewTable. (This Lambda should trim off the migration flag in Step 5)
[Optional] Create a GSI on the original table to speed up scanning items. Ensure this GSI only has attributes: Primary Key, and Migrated (See Step 5).
Scan the GSI created in the previous step (or entire table) and use the following Filter:
FilterExpression = "attribute_not_exists(Migrated)"
Update each item in the table with a migrate flag (ie: “Migrated”: { “S”: “0” }, which sends it to the DynamoDB Streams (using UpdateItem API, to ensure no data loss occurs).
NOTE: You may want to increase write capacity units on the table during the updates.
The Lambda will pick up all items, trim off the Migrated flag and push it into NewTable.
Once all items have been migrated, repoint the code to the new table
Remove original table, and Lambda function once happy all is good.
Following these steps should ensure you have no data loss and no downtime.
I've documented this on my blog, with code to assist:
https://www.abhayachauhan.com/2018/01/dynamodb-changing-table-schema/