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Scan

dynamodb_scan R Documentation

The Scan operation returns one or more items and item attributes by accessing every item in a table or a secondary index

Description

The scan operation returns one or more items and item attributes by accessing every item in a table or a secondary index. To have DynamoDB return fewer items, you can provide a FilterExpression operation.

If the total size of scanned items exceeds the maximum dataset size limit of 1 MB, the scan completes and results are returned to the user. The LastEvaluatedKey value is also returned and the requestor can use the LastEvaluatedKey to continue the scan in a subsequent operation. Each scan response also includes number of items that were scanned (ScannedCount) as part of the request. If using a FilterExpression, a scan result can result in no items meeting the criteria and the Count will result in zero. If you did not use a FilterExpression in the scan request, then Count is the same as ScannedCount.

Count and ScannedCount only return the count of items specific to a single scan request and, unless the table is less than 1MB, do not represent the total number of items in the table.

A single scan operation first reads up to the maximum number of items set (if using the Limit parameter) or a maximum of 1 MB of data and then applies any filtering to the results if a FilterExpression is provided. If LastEvaluatedKey is present in the response, pagination is required to complete the full table scan. For more information, see Paginating the Results in the Amazon DynamoDB Developer Guide.

scan operations proceed sequentially; however, for faster performance on a large table or secondary index, applications can request a parallel scan operation by providing the Segment and TotalSegments parameters. For more information, see Parallel Scan in the Amazon DynamoDB Developer Guide.

By default, a scan uses eventually consistent reads when accessing the items in a table. Therefore, the results from an eventually consistent scan may not include the latest item changes at the time the scan iterates through each item in the table. If you require a strongly consistent read of each item as the scan iterates through the items in the table, you can set the ConsistentRead parameter to true. Strong consistency only relates to the consistency of the read at the item level.

DynamoDB does not provide snapshot isolation for a scan operation when the ConsistentRead parameter is set to true. Thus, a DynamoDB scan operation does not guarantee that all reads in a scan see a consistent snapshot of the table when the scan operation was requested.

Usage

dynamodb_scan(TableName, IndexName, AttributesToGet, Limit, Select,
  ScanFilter, ConditionalOperator, ExclusiveStartKey,
  ReturnConsumedCapacity, TotalSegments, Segment, ProjectionExpression,
  FilterExpression, ExpressionAttributeNames, ExpressionAttributeValues,
  ConsistentRead)

Arguments

TableName

[required] The name of the table containing the requested items or if you provide IndexName, the name of the table to which that index belongs.

You can also provide the Amazon Resource Name (ARN) of the table in this parameter.

IndexName

The name of a secondary index to scan. This index can be any local secondary index or global secondary index. Note that if you use the IndexName parameter, you must also provide TableName.

AttributesToGet

This is a legacy parameter. Use ProjectionExpression instead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.

Limit

The maximum number of items to evaluate (not necessarily the number of matching items). If DynamoDB processes the number of items up to the limit while processing the results, it stops the operation and returns the matching values up to that point, and a key in LastEvaluatedKey to apply in a subsequent operation, so that you can pick up where you left off. Also, if the processed dataset size exceeds 1 MB before DynamoDB reaches this limit, it stops the operation and returns the matching values up to the limit, and a key in LastEvaluatedKey to apply in a subsequent operation to continue the operation. For more information, see Working with Queries in the Amazon DynamoDB Developer Guide.

Select

The attributes to be returned in the result. You can retrieve all item attributes, specific item attributes, the count of matching items, or in the case of an index, some or all of the attributes projected into the index.

  • ALL_ATTRIBUTES - Returns all of the item attributes from the specified table or index. If you query a local secondary index, then for each matching item in the index, DynamoDB fetches the entire item from the parent table. If the index is configured to project all item attributes, then all of the data can be obtained from the local secondary index, and no fetching is required.

  • ALL_PROJECTED_ATTRIBUTES - Allowed only when querying an index. Retrieves all attributes that have been projected into the index. If the index is configured to project all attributes, this return value is equivalent to specifying ALL_ATTRIBUTES.

  • COUNT - Returns the number of matching items, rather than the matching items themselves. Note that this uses the same quantity of read capacity units as getting the items, and is subject to the same item size calculations.

  • SPECIFIC_ATTRIBUTES - Returns only the attributes listed in ProjectionExpression. This return value is equivalent to specifying ProjectionExpression without specifying any value for Select.

    If you query or scan a local secondary index and request only attributes that are projected into that index, the operation reads only the index and not the table. If any of the requested attributes are not projected into the local secondary index, DynamoDB fetches each of these attributes from the parent table. This extra fetching incurs additional throughput cost and latency.

    If you query or scan a global secondary index, you can only request attributes that are projected into the index. Global secondary index queries cannot fetch attributes from the parent table.

If neither Select nor ProjectionExpression are specified, DynamoDB defaults to ALL_ATTRIBUTES when accessing a table, and ALL_PROJECTED_ATTRIBUTES when accessing an index. You cannot use both Select and ProjectionExpression together in a single request, unless the value for Select is SPECIFIC_ATTRIBUTES. (This usage is equivalent to specifying ProjectionExpression without any value for Select.)

If you use the ProjectionExpression parameter, then the value for Select can only be SPECIFIC_ATTRIBUTES. Any other value for Select will return an error.

ScanFilter

This is a legacy parameter. Use FilterExpression instead. For more information, see ScanFilter in the Amazon DynamoDB Developer Guide.

ConditionalOperator

This is a legacy parameter. Use FilterExpression instead. For more information, see ConditionalOperator in the Amazon DynamoDB Developer Guide.

ExclusiveStartKey

The primary key of the first item that this operation will evaluate. Use the value that was returned for LastEvaluatedKey in the previous operation.

The data type for ExclusiveStartKey must be String, Number or Binary. No set data types are allowed.

In a parallel scan, a scan request that includes ExclusiveStartKey must specify the same segment whose previous scan returned the corresponding value of LastEvaluatedKey.

ReturnConsumedCapacity
TotalSegments

For a parallel scan request, TotalSegments represents the total number of segments into which the scan operation will be divided. The value of TotalSegments corresponds to the number of application workers that will perform the parallel scan. For example, if you want to use four application threads to scan a table or an index, specify a TotalSegments value of 4.

The value for TotalSegments must be greater than or equal to 1, and less than or equal to 1000000. If you specify a TotalSegments value of 1, the scan operation will be sequential rather than parallel.

If you specify TotalSegments, you must also specify Segment.

Segment

For a parallel scan request, Segment identifies an individual segment to be scanned by an application worker.

Segment IDs are zero-based, so the first segment is always 0. For example, if you want to use four application threads to scan a table or an index, then the first thread specifies a Segment value of 0, the second thread specifies 1, and so on.

The value of LastEvaluatedKey returned from a parallel scan request must be used as ExclusiveStartKey with the same segment ID in a subsequent scan operation.

The value for Segment must be greater than or equal to 0, and less than the value provided for TotalSegments.

If you provide Segment, you must also provide TotalSegments.

ProjectionExpression

A string that identifies one or more attributes to retrieve from the specified table or index. These attributes can include scalars, sets, or elements of a JSON document. The attributes in the expression must be separated by commas.

If no attribute names are specified, then all attributes will be returned. If any of the requested attributes are not found, they will not appear in the result.

For more information, see Specifying Item Attributes in the Amazon DynamoDB Developer Guide.

FilterExpression

A string that contains conditions that DynamoDB applies after the scan operation, but before the data is returned to you. Items that do not satisfy the FilterExpression criteria are not returned.

A FilterExpression is applied after the items have already been read; the process of filtering does not consume any additional read capacity units.

For more information, see Filter Expressions in the Amazon DynamoDB Developer Guide.

ExpressionAttributeNames

One or more substitution tokens for attribute names in an expression. The following are some use cases for using ExpressionAttributeNames:

  • To access an attribute whose name conflicts with a DynamoDB reserved word.

  • To create a placeholder for repeating occurrences of an attribute name in an expression.

  • To prevent special characters in an attribute name from being misinterpreted in an expression.

Use the # character in an expression to dereference an attribute name. For example, consider the following attribute name:

  • Percentile

The name of this attribute conflicts with a reserved word, so it cannot be used directly in an expression. (For the complete list of reserved words, see Reserved Words in the Amazon DynamoDB Developer Guide). To work around this, you could specify the following for ExpressionAttributeNames:

  • {"#P":"Percentile"}

You could then use this substitution in an expression, as in this example:

  • ⁠#P = :val⁠

Tokens that begin with the : character are expression attribute values, which are placeholders for the actual value at runtime.

For more information on expression attribute names, see Specifying Item Attributes in the Amazon DynamoDB Developer Guide.

ExpressionAttributeValues

One or more values that can be substituted in an expression.

Use the : (colon) character in an expression to dereference an attribute value. For example, suppose that you wanted to check whether the value of the ProductStatus attribute was one of the following:

Available | Backordered | Discontinued

You would first need to specify ExpressionAttributeValues as follows:

⁠{ ":avail":{"S":"Available"}, ":back":{"S":"Backordered"}, ":disc":{"S":"Discontinued"} }⁠

You could then use these values in an expression, such as this:

⁠ProductStatus IN (:avail, :back, :disc)⁠

For more information on expression attribute values, see Condition Expressions in the Amazon DynamoDB Developer Guide.

ConsistentRead

A Boolean value that determines the read consistency model during the scan:

  • If ConsistentRead is false, then the data returned from scan might not contain the results from other recently completed write operations (put_item, update_item, or delete_item).

  • If ConsistentRead is true, then all of the write operations that completed before the scan began are guaranteed to be contained in the scan response.

The default setting for ConsistentRead is false.

The ConsistentRead parameter is not supported on global secondary indexes. If you scan a global secondary index with ConsistentRead set to true, you will receive a ValidationException.

Value

A list with the following syntax: 

list(
  Items = list(
    list(
      list(
        S = "string",
        N = "string",
        B = raw,
        SS = list(
          "string"
        ),
        NS = list(
          "string"
        ),
        BS = list(
          raw
        ),
        M = list(
          list()
        ),
        L = list(
          list()
        ),
        NULL = TRUE|FALSE,
        BOOL = TRUE|FALSE
      )
    )
  ),
  Count = 123,
  ScannedCount = 123,
  LastEvaluatedKey = list(
    list(
      S = "string",
      N = "string",
      B = raw,
      SS = list(
        "string"
      ),
      NS = list(
        "string"
      ),
      BS = list(
        raw
      ),
      M = list(
        list()
      ),
      L = list(
        list()
      ),
      NULL = TRUE|FALSE,
      BOOL = TRUE|FALSE
    )
  ),
  ConsumedCapacity = list(
    TableName = "string",
    CapacityUnits = 123.0,
    ReadCapacityUnits = 123.0,
    WriteCapacityUnits = 123.0,
    Table = list(
      ReadCapacityUnits = 123.0,
      WriteCapacityUnits = 123.0,
      CapacityUnits = 123.0
    ),
    LocalSecondaryIndexes = list(
      list(
        ReadCapacityUnits = 123.0,
        WriteCapacityUnits = 123.0,
        CapacityUnits = 123.0
      )
    ),
    GlobalSecondaryIndexes = list(
      list(
        ReadCapacityUnits = 123.0,
        WriteCapacityUnits = 123.0,
        CapacityUnits = 123.0
      )
    )
  )
)

Request syntax

svc$scan(
  TableName = "string",
  IndexName = "string",
  AttributesToGet = list(
    "string"
  ),
  Limit = 123,
  Select = "ALL_ATTRIBUTES"|"ALL_PROJECTED_ATTRIBUTES"|"SPECIFIC_ATTRIBUTES"|"COUNT",
  ScanFilter = list(
    list(
      AttributeValueList = list(
        list(
          S = "string",
          N = "string",
          B = raw,
          SS = list(
            "string"
          ),
          NS = list(
            "string"
          ),
          BS = list(
            raw
          ),
          M = list(
            list()
          ),
          L = list(
            list()
          ),
          NULL = TRUE|FALSE,
          BOOL = TRUE|FALSE
        )
      ),
      ComparisonOperator = "EQ"|"NE"|"IN"|"LE"|"LT"|"GE"|"GT"|"BETWEEN"|"NOT_NULL"|"NULL"|"CONTAINS"|"NOT_CONTAINS"|"BEGINS_WITH"
    )
  ),
  ConditionalOperator = "AND"|"OR",
  ExclusiveStartKey = list(
    list(
      S = "string",
      N = "string",
      B = raw,
      SS = list(
        "string"
      ),
      NS = list(
        "string"
      ),
      BS = list(
        raw
      ),
      M = list(
        list()
      ),
      L = list(
        list()
      ),
      NULL = TRUE|FALSE,
      BOOL = TRUE|FALSE
    )
  ),
  ReturnConsumedCapacity = "INDEXES"|"TOTAL"|"NONE",
  TotalSegments = 123,
  Segment = 123,
  ProjectionExpression = "string",
  FilterExpression = "string",
  ExpressionAttributeNames = list(
    "string"
  ),
  ExpressionAttributeValues = list(
    list(
      S = "string",
      N = "string",
      B = raw,
      SS = list(
        "string"
      ),
      NS = list(
        "string"
      ),
      BS = list(
        raw
      ),
      M = list(
        list()
      ),
      L = list(
        list()
      ),
      NULL = TRUE|FALSE,
      BOOL = TRUE|FALSE
    )
  ),
  ConsistentRead = TRUE|FALSE
)

Examples

## Not run: 
# This example scans the entire Music table, and then narrows the results
# to songs by the artist "No One You Know". For each item, only the album
# title and song title are returned.
svc$scan(
  ExpressionAttributeNames = list(
    `#AT` = "AlbumTitle",
    `#ST` = "SongTitle"
  ),
  ExpressionAttributeValues = list(
    `:a` = list(
      S = "No One You Know"
    )
  ),
  FilterExpression = "Artist = :a",
  ProjectionExpression = "#ST, #AT",
  TableName = "Music"
)

## End(Not run)