I know BatchGetItems allows for retrieval of multiple hash keys. To save on the read capacity, I like to know if Query provide same functionality via some "IN" keyword I can use? ie, all primary keys will be inserted into an array for Query to search "IN" in the array.
Query doesn't provide what you want. As per the documentation here:
KeyConditionExpression: The condition must perform an equality test on a single partition key value. The condition can also perform one of several comparison tests on a single sort key value. Query can use KeyConditionExpression to retrieve one item with a given partition key value and sort key value, or several items that have the same partition key value but different sort key values.
BatchGetItem is the only option that you have.
Related
I'd like to list records from my DDB table ordered by creation date.
My table has an attribute DateCreated.
All examples I can find describe ordering within some partition.
But I want global ordering.
Am I supposed to create an artificial attribute which will have the same value across all records, just to use it as a partition key? E.g. add new attribute GlobalPartition with value 1 to every record in the table, and create a GSI with partition key GlobalPartition and sort key DateCreated. Isn't there a better way?
Thx!
As you noticed, DynamoDB indeed does not have an option to sort items "globally". In other words, there is no way to Scan the database in sorted partition-key order. You can only sort items inside one partition, sorted by the "sort key".
When you have a small amount of data, you can indeed do what you said: Have a single partition with everything in this partition. However it's not clear how practical this approach becomes as your single partition grows - to gigabytes or terabytes, and how well DynamoDB can load-balance when you have just a single partition (I never saw any DynamoDB documentation which answer this question).
So another option is not to have a single partition but rather have a number of them. For example, consider that you want to sort items by date. Now insead of having a single partition, have a partition per month, i.e., the partition key is the month number. Now, if you want to sort everything within a month, you can do it directly, but if you want to get a sorted list of a full year, you need to Query twelve partitions, in order, getting a sorted list in each one and combining it to a sorted list for the full year. So-called time-series databases are often modeled this way.
If you want to sort any data in DynamoDB you need to add Sort Key index on that attribute. If value is not in attribute which maps to tables' sort key, or table does not have sort key, then you need to create GSI and put GSI's sort key on that attribute. You can use LSI too. Any attribute, which maps to "Sort Key" of any index. Table, LSI, GSI.
Check for more details "ScanIndexForward" param of the query request.
If ScanIndexForward is true, DynamoDB returns the results in the order in which they are stored (by sort key value). This is the default behavior. If ScanIndexForward is false, DynamoDB reads the results in reverse order by sort key value, and then returns the results to the client.
https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Query.html#API_Query_RequestSyntax
UI has checkbox too for this:
"Global sort" is not possible, while "global" would mean scan operation and it just runs through all rows in database and filters by filters, yet it does not have sorting option. On query on attribute mapped to sort key has ScanIndexForward option to change sort direction.
Is it possible to Query a DynamoDB table using both the hash & range key AND a local secondary index?
I have three attributes I want to compare against in my query. Two are the main hash and range keys and the third is the range key of the local secondary index.
No, but that shouldn't be necessary based on your description of what you are trying to accomplish.
If you are trying to access an object based on the hash and range key (of the main table) as well as an additional attribute, selecting on only the hash and range of the main table (which is required to return a single record by definition) will return that record.
If your concern is that the third attribute may be a value that you want to ignore the entire record you can use a query filter to have that item filtered out by DynamoDB or you can use logic in your application to ignore that object.
I have a AWS DynamoDB table storing books information, the hash key is book id. There is an attribute for book price.
Now I want to perform a query to return all the books whose price is lower than a certain value. How to do this efficiently, without scanning the whole table?
The query on secondary-index seems only could return a set of entries with the index being a certain value, so I am confused about how to perform a range query efficiently. Thank you very much!
There are two things that maybe you are confusing. The range key with a range on an attribute.
To clarify, in this case you would need a secondary index and when querying the index you would specify a key condition (assuming java and assuming secondary index on value - this in pretty much any sdk supported language)
see http://docs.amazonaws.cn/en_us/AWSJavaSDK/latest/javadoc/index.html?com/amazonaws/services/dynamodbv2/model/QueryRequest.html w/ a BETWEEN condition.
You can't do query of that kind. DynamoDB is sharded across many nodes by hash key, so doing a query without hash key (on all hash keys) is essentially a full scan.
A hack for your case would be to have a hash key with only one value for the whole table, but this is fundamentally wrong because you loose all the pros of using DynamoDB. See hot hash key issue for more info: http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GuidelinesForTables.html
I've been going through AWS DynamoDB docs and cannot figure out what's the difference between batchGetItem() and Query().
My use case: I have a table which has Id as primary hash key, and attribute values are Name and Marks.
I would like to perform batch query which returns list of names and marks by providing list of Id's which are primary keys.
Should I use batchGetItem() or Query()?
BatchGetItem: Allows to you parallelize "GetItem" requests for languages that don't support parallelism (i.e. javascript). This includes retrieving items from different tables (doesn't support indexes though).
Query: Allows you to page through tables with a Hash-Range schema (where you'll have multiple results associated with a Hash key) and allows you to retrieve items from the indexes on your table. Note you can also add an additional condition on range key in your KeyConditions and add conditions on any non primary key attribute in your QueryFilter.
It seems like that your use case calls for a BatchGetItem request, as you are trying to retrieve items from your base table by way of a Hash key.
Hope that helps!
Is there any way to get sorted result out of Dynamodb when using Scan/Query APIs? I know in Query API you can sort by Rangekey and ScanIndexForward which sorts the result ascending if the value is true and descending if false;
+But as far as I understood you can have one range key, so how if I want to sort based on different fields?
+Also if I'm using scan, it seems there is no option to sort the result either!
Any help is appreciated!
For the first question about having only one range key, you can use Local secondary Index. You assign a normal attribute as the range key of the LSI and DynamoDB will sort your rows (with the same hashkey) by comparing that attribute.
So essentially LSI gives you "additional rangeKey". You can create up to 5 LSIs.
See here and here for example of querying LSI. You can treat an Index just like a regular table. You can do query & scan on index (but not put).
For your second question about sorting the rows globally instead of sorting items with the same hashkey, I don't think DynamoDB supports this feature out-of-the-box. You will have to
a) scan and sort the items on your own
b) or create a global secondary index with just one hash key and dump all your items into that hashkey. It is not recommended because this creates a hot partition in GSI.
c) or design your schema to avoid having to sort items globally.