How to filter a DynamoDB Scan in a StepFunction?

Question

Trying to perform a DynamoDB scan and Filter in a Step Function, but the filter matches no items.

My Task is;
```
"Scan (2)": {
      "Type": "Task",
      "Parameters": {
        "TableName": "Users",
        "FilterExpression": "#Scope = :Global",
        "ExpressionAttributeValues": {
          ":Global": {
            "S": "Global"
          }
        },
        "ExpressionAttributeNames": {
          "#Scope": {
            "S": "Scope"
          }
        }
      },
      "Resource": "arn:aws:states:::aws-sdk:dynamodb:scan",
      "Next": "Pass"
    },
```
the task runs successfully and produces the following output;
```
{
  "Count": 0,
  "Items": [],
  "ScannedCount": 4
}
```

Whereas this task, running a Scan (without a Filter) against the same table;
```
{
  "Type": "Task",
  "Parameters": {
    "TableName": "Users"
  },
  "Resource": "arn:aws:states:::aws-sdk:dynamodb:scan",,
  "Next": "Scan (2)"
}
```
Produces this output;
```
{
  "Count": 4,
  "Items": [ 
      {
      "Scope": {
        "S": "Global"
      },
      "ServiceName": {
        "S": "Driver"
      },
      "lastVisitDateTime": {
        "N": "1680022000"
      }
    },
      {
      "Scope": {
        "S": "us-east-1"
      },
      "ServiceName": {
        "S": "Driver"
      },
      "lastVisitDateTime": {
        "N": "1680022106"
      }
    },
      {
      "Scope": {
        "S": "Global"
      },
      "ServiceName": {
        "S": "Rider"
      },
      "lastVisitDateTime": {
        "N": "1680022106"
      }
    },
      {
      "Scope": {
        "S": "Global"
      },
      "ServiceName": {
        "S": "Driver"
      },
      "lastVisitDateTime": {
        "N": "1680022200"
      }
    }
  ],
  "ScannedCount": 4
}
```

Leeroy Hannigan · Accepted Answer

Your expression should look like the following:

```
        "FilterExpression": "#Scope = :Global",
        "ExpressionAttributeValues": {
          ":Global": {
            "S": "Global"
          }
        },
        "ExpressionAttributeNames": {
          "#Scope": "Scope"
        }
```

However, its important to not how in-efficient your `Scan` is. Its not scalable, and as a result will become expensive and slow over time. It would be much more efficient to create a Global Secondary Index for `Scope` which will allow you to use a `Query` which is more efficient in both performance and cost over `Scan`.

Steve Kinsman · Answer

I'm way out of practice with filter expressions but I think it should be something like:

```
        "FilterExpression": "#Scope = :Global",
        "ExpressionAttributeValues": {
          ":Global": {
            "S": "Global"
          }
        },
        "ExpressionAttributeNames": {
          "#Scope": "Scope"
        }
```

How to filter a DynamoDB Scan in a StepFunction?

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