Working with Local Secondary Indexes: .NET

You can use the Amazon SDK for .NET low-level API to create an Amazon DynamoDB table with one or more local secondary indexes, describe the indexes on the table, and perform queries using the indexes. These operations map to the corresponding low-level DynamoDB API actions. For more information, see .NET code examples.

The following are the common steps for table operations using the .NET low-level API.

Create a table with a Local Secondary Index

Local secondary indexes must be created at the same time that you create a table. To do this, use CreateTable and provide your specifications for one or more local secondary indexes. The following C# code example creates a table to hold information about songs in a music collection. The partition key is Artist and the sort key is SongTitle. A secondary index, AlbumTitleIndex, facilitates queries by album title.

The following are the steps to create a table with a local secondary index, using the .NET low-level API.

The following C# code example demonstrates the preceding steps. The code creates a table (Music) with a secondary index on the AlbumTitle attribute. The table partition key and sort key, plus the index sort key, are the only attributes projected into the index.

AmazonDynamoDBClient client = new AmazonDynamoDBClient();
string tableName = "Music";

CreateTableRequest createTableRequest = new CreateTableRequest()
{
    TableName = tableName
};

//ProvisionedThroughput
createTableRequest.ProvisionedThroughput = new ProvisionedThroughput()
{
    ReadCapacityUnits = (long)5,
    WriteCapacityUnits = (long)5
};

//AttributeDefinitions
List<AttributeDefinition> attributeDefinitions = new List<AttributeDefinition>();

attributeDefinitions.Add(new AttributeDefinition()
{
    AttributeName = "Artist",
    AttributeType = "S"
});

attributeDefinitions.Add(new AttributeDefinition()
 {
     AttributeName = "SongTitle",
     AttributeType = "S"
 });

attributeDefinitions.Add(new AttributeDefinition()
 {
     AttributeName = "AlbumTitle",
     AttributeType = "S"
 });

createTableRequest.AttributeDefinitions = attributeDefinitions;

//KeySchema
List<KeySchemaElement> tableKeySchema = new List<KeySchemaElement>();

tableKeySchema.Add(new KeySchemaElement() { AttributeName = "Artist", KeyType = "HASH" });  //Partition key
tableKeySchema.Add(new KeySchemaElement() { AttributeName = "SongTitle", KeyType = "RANGE" });  //Sort key

createTableRequest.KeySchema = tableKeySchema;

List<KeySchemaElement> indexKeySchema = new List<KeySchemaElement>();
indexKeySchema.Add(new KeySchemaElement() { AttributeName = "Artist", KeyType = "HASH" });  //Partition key
indexKeySchema.Add(new KeySchemaElement() { AttributeName = "AlbumTitle", KeyType = "RANGE" });  //Sort key

Projection projection = new Projection() { ProjectionType = "INCLUDE" };

List<string> nonKeyAttributes = new List<string>();
nonKeyAttributes.Add("Genre");
nonKeyAttributes.Add("Year");
projection.NonKeyAttributes = nonKeyAttributes;

LocalSecondaryIndex localSecondaryIndex = new LocalSecondaryIndex()
{
    IndexName = "AlbumTitleIndex",
    KeySchema = indexKeySchema,
    Projection = projection
};

List<LocalSecondaryIndex> localSecondaryIndexes = new List<LocalSecondaryIndex>();
localSecondaryIndexes.Add(localSecondaryIndex);
createTableRequest.LocalSecondaryIndexes = localSecondaryIndexes;

CreateTableResponse result = client.CreateTable(createTableRequest);
Console.WriteLine(result.CreateTableResult.TableDescription.TableName);
Console.WriteLine(result.CreateTableResult.TableDescription.TableStatus);

You must wait until DynamoDB creates the table and sets the table status to ACTIVE. After that, you can begin putting data items into the table.

Describe a table with a Local Secondary Index

To get information about local secondary indexes on a table, use the DescribeTable API. For each index, you can access its name, key schema, and projected attributes.

The following are the steps to access local secondary index information a table using the .NET low-level API.

The following C# code example demonstrates the preceding steps.

AmazonDynamoDBClient client = new AmazonDynamoDBClient();
string tableName = "Music";

DescribeTableResponse response = client.DescribeTable(new DescribeTableRequest() { TableName = tableName });
List<LocalSecondaryIndexDescription> localSecondaryIndexes =
    response.DescribeTableResult.Table.LocalSecondaryIndexes;

// This code snippet will work for multiple indexes, even though
// there is only one index in this example.
foreach (LocalSecondaryIndexDescription lsiDescription in localSecondaryIndexes)
{
    Console.WriteLine("Info for index " + lsiDescription.IndexName + ":");

    foreach (KeySchemaElement kse in lsiDescription.KeySchema)
    {
        Console.WriteLine("\t" + kse.AttributeName + ": key type is " + kse.KeyType);
    }

    Projection projection = lsiDescription.Projection;

    Console.WriteLine("\tThe projection type is: " + projection.ProjectionType);

    if (projection.ProjectionType.ToString().Equals("INCLUDE"))
    {
        Console.WriteLine("\t\tThe non-key projected attributes are:");

        foreach (String s in projection.NonKeyAttributes)
        {
            Console.WriteLine("\t\t" + s);
        }

    }
}

Query a Local Secondary Index

You can use Query on a local secondary index in much the same way you Query a table. You must specify the index name, the query criteria for the index sort key, and the attributes that you want to return. In this example, the index is AlbumTitleIndex, and the index sort key is AlbumTitle.

The only attributes returned are those that have been projected into the index. You could modify this query to select non-key attributes too, but this would require table fetch activity that is relatively expensive. For more information about table fetches, see Attribute projections

The following are the steps to query a local secondary index using the .NET low-level API.

The following C# code example demonstrates the preceding steps.

QueryRequest queryRequest = new QueryRequest
{
    TableName = "Music",
    IndexName = "AlbumTitleIndex",
    Select = "ALL_ATTRIBUTES",
    ScanIndexForward = true,
    KeyConditionExpression = "Artist = :v_artist and AlbumTitle = :v_title",
    ExpressionAttributeValues = new Dictionary<string, AttributeValue>()
    {
        {":v_artist",new AttributeValue {S = "Acme Band"}},
        {":v_title",new AttributeValue {S = "Songs About Life"}}
    },
};

QueryResponse response = client.Query(queryRequest);

foreach (var attribs in response.Items)
{
    foreach (var attrib in attribs)
    {
        Console.WriteLine(attrib.Key + " ---> " + attrib.Value.S);
    }
    Console.WriteLine();
}