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 provideTableName
.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 inLastEvaluatedKey
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 specifyingALL_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 inProjectionExpression
. This return value is equivalent to specifyingProjectionExpression
without specifying any value forSelect
.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
norProjectionExpression
are specified, DynamoDB defaults toALL_ATTRIBUTES
when accessing a table, andALL_PROJECTED_ATTRIBUTES
when accessing an index. You cannot use bothSelect
andProjectionExpression
together in a single request, unless the value forSelect
isSPECIFIC_ATTRIBUTES
. (This usage is equivalent to specifyingProjectionExpression
without any value forSelect
.)If you use the
ProjectionExpression
parameter, then the value forSelect
can only beSPECIFIC_ATTRIBUTES
. Any other value forSelect
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 includesExclusiveStartKey
must specify the same segment whose previousscan
returned the corresponding value ofLastEvaluatedKey
.ReturnConsumedCapacity
TotalSegments
For a parallel
scan
request,TotalSegments
represents the total number of segments into which thescan
operation will be divided. The value ofTotalSegments
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 aTotalSegments
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 aTotalSegments
value of 1, thescan
operation will be sequential rather than parallel.If you specify
TotalSegments
, you must also specifySegment
.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 parallelscan
request must be used asExclusiveStartKey
with the same segment ID in a subsequentscan
operation.The value for
Segment
must be greater than or equal to 0, and less than the value provided forTotalSegments
.If you provide
Segment
, you must also provideTotalSegments
.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 theFilterExpression
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
isfalse
, then the data returned fromscan
might not contain the results from other recently completed write operations (put_item
,update_item
, ordelete_item
).If
ConsistentRead
istrue
, then all of the write operations that completed before thescan
began are guaranteed to be contained in thescan
response.
The default setting for
ConsistentRead
isfalse
.The
ConsistentRead
parameter is not supported on global secondary indexes. If you scan a global secondary index withConsistentRead
set to true, you will receive aValidationException
.
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)