Get Max Partition Key Value from DynamoDb Table

Question

I have MenuItem table in DynamoDb which has a partition key "Id" and I want the maximum value of Id column before inserting new record. Please share the code for the same.

Accepted Answer

You'd first need a Global Secondary Index with Id as the sort key value, and a static value as the partition key:

| GSI_PK | GSI_SK | Data |
| --- | --- | --- |
| 1 | ID_01 | Data |
| 1 | ID_02 | Data |
| 1 | ID_03 | Data |
| 1 | ID_14 | Data |

Then you would `Query` your index with `ScanIndexForward=False` (DESC order) with a `Limit=1` (return biggest ID).

```
        var client = new AmazonDynamoDBClient();

var request = new QueryRequest
        {
            TableName = "Table_Name",
            IndexName = "Index_Name",
            KeyConditionExpression = "#pk = :pk",
            ExpressionAttributeValues = new Dictionary
            {
                { ":pk", new AttributeValue { N = "1" } }
            },
            ExpressionAttributeNames = new Dictionary
            {
                { "#pk", "GSI_PK" }
            },
            ProjectionExpression = "GSI_SK",
            Limit = 1
        };

var response = await client.QueryAsync(request);

Console.WriteLine(response);
```

Answer

You could scan the whole table, looking for the max value, but I'm assuming you want to avoid that.  The best option I can think of is to add a Global Secondary Index (GSI) to your table that has "Id" as a Sort (Range) key, and query that index, e.g.:

```
# Use the Query operation to retrieve the first item in the index in descending order
response = dynamodb.query(
    TableName=table_name,
    IndexName=index_name,
    Select='SPECIFIC_ATTRIBUTES',
    AttributesToGet=[partition_key_name],
    Limit=1,
    ScanIndexForward=False
)

# Extract the maximum partition key value from the response
max_partition_key_value = response['Items'][0][partition_key_name]
```
You may need a dummy Hash (Partition) key for the GSI, always set to the same value, if nothing else suits your use case.

Get Max Partition Key Value from DynamoDb Table

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