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Some applications only need to query data using the base table's primary key. However, there might be situations where an alternative sort key would be helpful. To give your application a choice of sort keys, you can create one or more local secondary indexes on an Amazon DynamoDB table and issue Show
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Scenario: Using a Local Secondary IndexAs an example, consider the DynamoDB stores all of the items with the same partition key value continuously. In this example, given a particular Some requests might require more complex data access patterns. For example:
To answer these questions, the However, you can specify one or more local secondary indexes on non-key attributes, such as A local secondary index maintains an alternate sort key for a given partition key value. A local secondary index also contains a copy of some or all of the attributes from its base table. You specify which attributes are projected into the local secondary index when you create the table. The data in a local secondary index is organized by the same partition key as the base table, but with a different sort key. This lets you access data items efficiently across this different dimension. For greater query or scan flexibility, you can create up to five local secondary indexes per table. Suppose that an application needs to find all of the threads that have been posted within the last three months in a particular forum. Without a local secondary index, the application would have to The following diagram shows a local secondary index named Every local secondary index must meet the following conditions:
In this example, the partition key is Every local secondary index automatically contains the partition and sort keys from its base table; you can optionally project non-key attributes into the index. When you query the index, DynamoDB can retrieve these projected attributes efficiently. When you query a local secondary index, the query can also retrieve attributes that are not projected into the index. DynamoDB automatically fetches these attributes from the base table, but at a greater latency and with higher provisioned throughput costs. For any local secondary index, you can store up to 10 GB of data per distinct partition key value. This figure includes all of the items in the base table, plus all of the items in the indexes, that have the same partition key value. For more information, see Item collections in Local Secondary Indexes. Attribute projectionsWith Suppose that you wanted to populate a webpage with a list of all the threads in "S3" and the number of replies for each thread, sorted by the last reply date/time beginning with the most recent reply. To populate this list, you would need the following attributes:
The most efficient way to query this data and
to avoid fetch operations would be to project the A projection is the set of attributes that is copied from a table into a secondary index. The partition key and sort key of the table are always projected into the index; you can project other attributes to support your application's query requirements. When you query an index, Amazon DynamoDB can access any attribute in the projection as if those attributes were in a table of their own. When you create a secondary index, you need to specify the attributes that will be projected into the index. DynamoDB provides three different options for this:
In the previous diagram, the non-key attribute From an application's point of view, fetching additional attributes from the base table is automatic and transparent, so there is no need to rewrite any application logic. However, such fetching can greatly reduce the performance advantage of using a local secondary index. When you choose the attributes to project into a local secondary index, you must consider the tradeoff between provisioned throughput costs and storage costs:
Creating a Local Secondary IndexTo create one or more local secondary indexes on a table, use the You must specify one non-key attribute to act as the sort key of the local secondary index. The attribute that you choose must be a scalar For tables with local secondary indexes, there is a 10 GB size limit per partition key value. A table with local secondary indexes can store any number of items, as long as the total size for any one partition key value does not exceed 10 GB. For more information, see Item collection size limit. You can project attributes of any data type into a local secondary index. This includes scalars, documents, and sets. For a complete list of data types, see Data types. Reading data from a Local Secondary IndexYou can retrieve items from a local secondary index using the Querying a Local Secondary IndexIn a DynamoDB table, the combined partition key value and sort key value for each item must be unique. However, in a local secondary index, the sort key value does not need to be unique for
a given partition key value. If there are multiple items in the local secondary index that have the same sort key value, a You can query a local secondary index using either eventually consistent or strongly consistent reads. To specify which type of consistency you want, use the Example Consider the following data returned from a
In this query:
Because the Scanning a Local Secondary IndexYou can use Item writes and Local Secondary IndexesDynamoDB automatically keeps all local secondary indexes synchronized with their respective base tables. Applications never write directly to an index. However, it is important that you understand the implications of how DynamoDB maintains these indexes. When you create a local secondary index, you specify an attribute to serve as the sort key for the index. You also specify a data type for that attribute. This
means that whenever you write an item to the base table, if the item defines an index key attribute, its type must match the index key schema's data type. In the case of There is no requirement for a one-to-one relationship between the items in a base table and the items in a local secondary index. In fact, this behavior can be advantageous for many applications. A table with many local secondary indexes incurs higher costs for write activity than tables with fewer indexes. For more information, see Provisioned throughput considerations for Local Secondary Indexes. For tables with local secondary indexes, there is a 10 GB size limit per partition key value. A table with local secondary indexes can store any number of items, as long as the total size for any one partition key value does not exceed 10 GB. For more information, see Item collection size limit. Provisioned throughput considerations for Local Secondary IndexesWhen you create a table in DynamoDB, you provision read and write capacity units for the table's expected workload. That workload includes read and write activity on the table's local secondary indexes. To view the current rates for provisioned throughput capacity, see Amazon DynamoDB pricing. Read capacity unitsWhen you query a local secondary index, the number of read capacity units consumed depends on how the data is accessed. As with table queries, an index query can use either eventually consistent or strongly consistent reads depending on the value of For index queries that request only index keys and projected attributes, DynamoDB calculates the provisioned read activity in the same way as it does for queries against tables. The only difference is that the calculation is based on the sizes of the index entries, rather than the size of the item in the base table. The number of read capacity units is the sum of all projected attribute sizes across all of the items returned; the result is then rounded up to the next 4 KB boundary. For more information about how DynamoDB calculates provisioned throughput usage, see Managing settings on DynamoDB provisioned capacity tables. For index queries that read attributes that are not projected into the local secondary index, DynamoDB needs to fetch those attributes from the base table, in addition to
reading the projected attributes from the index. These fetches occur when you include any non-projected attributes in the The maximum size of the results returned by a
Using this formula, the maximum size of the results returned by a Query operation is still 1 MB. For example, consider a table where the size of each item is 300 bytes. There is a local secondary index on that table, but only 200 bytes of each item is projected into the index. Now suppose that you
The total size of the data in the result is therefore 20 KB. Write capacity unitsWhen an item in a table is added, updated, or deleted, updating the local secondary indexes consumes provisioned write capacity units for the table. The total provisioned throughput cost for a write is the sum of write capacity units consumed by writing to the table and those consumed by updating the local secondary indexes. The cost of writing an item to a local secondary index depends on several factors:
All of these factors assume that the size of each item in the index is less than or equal to the 1 KB item size for calculating write capacity units. Larger index entries require additional write capacity units. You can minimize your write costs by considering which attributes your queries need to return and projecting only those attributes into the index. Storage considerations for Local Secondary IndexesWhen an application writes an item to a table, DynamoDB automatically copies the correct subset of attributes to any local secondary indexes in which those attributes should appear. Your AWS account is charged for storage of the item in the base table and also for storage of attributes in any local secondary indexes on that table. The amount of space used by an index item is the sum of the following:
To estimate the storage requirements for a local secondary index, you can estimate the average size of an item in the index and then multiply by the number of items in the base table. If a table contains an item where a particular attribute is not defined, but that attribute is defined as an index sort key, then DynamoDB does not write any data for that item to the index. Item collections in Local Secondary IndexesThis section pertains only to tables that have local secondary indexes. In DynamoDB, an item collection is any group of items that have the same partition key value in a table and all of its local secondary indexes. In the examples used throughout this section, the partition key for the The following diagram shows the item collection for forum In this diagram, the item collection consists of all the items in You can use any of the following operations in DynamoDB to return information about item collections:
Each of these operations supports the Example The following is an example from the output of an
The Item collection size limitThe maximum size of any item collection for a table which has one or more local secondary indexes is 10 GB. This does not apply to item collections in tables without local secondary indexes, and also does not apply to item collections in global secondary indexes. Only tables that have one or more local secondary indexes are affected. If an
item collection exceeds the 10 GB limit, DynamoDB returns an To reduce the size of an item collection, you can do one of the following:
When the total size of the item collection drops below 10 GB, you can once again add items with the same partition key value. We recommend as a best practice that you instrument your application to monitor the sizes of your item collections. One way to do so is
to set the Item collections and partitionsIn a table with one or more local secondary indexes, each item collection is stored in one partition. The total size of such an item collection is limited to the capability of that partition: 10 GB. For an application where the data model includes item collections which are unbounded in size, or where you might reasonably expect some item collections to grow beyond 10 GB in the future, you should consider using a global secondary index instead. You should design your applications so that table data is evenly distributed across distinct partition key values. For tables with local secondary indexes, your applications should not create "hot spots" of read and write activity within a single item collection on a single partition. What is minimum attribute?The min attribute defines the minimum value that is acceptable and valid for the input containing the attribute. If the value of the element is less than this, the element fails constraint validation. This value must be less than or equal to the value of the max attribute. Some input types have a default minimum.
What is the attributes for tag?Attributes define additional characteristics or properties of the element such as width and height of an image. Attributes are always specified in the start tag (or opening tag) and usually consists of name/value pairs like name="value" .
What are the three attributes of a product?What are product attributes? Product attributes are the properties that describe a product. They include details that are tangible and intangible, subjective and objective.
What are the boolean attributes?A Boolean attribute is an attribute that can only be true or false. How does a Boolean attribute work? According to the HTML specification: The presence of a boolean attribute on an element represents the “true” value, and the absence of the attribute represents the “false” value.
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