Scan
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
.
Note
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.
Note
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.
Request Syntax
{
"AttributesToGet": [ "string
" ],
"ConditionalOperator": "string
",
"ConsistentRead": boolean
,
"ExclusiveStartKey": {
"string
" : {
"B": blob
,
"BOOL": boolean
,
"BS": [ blob
],
"L": [
"AttributeValue"
],
"M": {
"string
" : "AttributeValue"
},
"N": "string
",
"NS": [ "string
" ],
"NULL": boolean
,
"S": "string
",
"SS": [ "string
" ]
}
},
"ExpressionAttributeNames": {
"string
" : "string
"
},
"ExpressionAttributeValues": {
"string
" : {
"B": blob
,
"BOOL": boolean
,
"BS": [ blob
],
"L": [
"AttributeValue"
],
"M": {
"string
" : "AttributeValue"
},
"N": "string
",
"NS": [ "string
" ],
"NULL": boolean
,
"S": "string
",
"SS": [ "string
" ]
}
},
"FilterExpression": "string
",
"IndexName": "string
",
"Limit": number
,
"ProjectionExpression": "string
",
"ReturnConsumedCapacity": "string
",
"ScanFilter": {
"string
" : {
"AttributeValueList": [
{
"B": blob
,
"BOOL": boolean
,
"BS": [ blob
],
"L": [
"AttributeValue"
],
"M": {
"string
" : "AttributeValue"
},
"N": "string
",
"NS": [ "string
" ],
"NULL": boolean
,
"S": "string
",
"SS": [ "string
" ]
}
],
"ComparisonOperator": "string
"
}
},
"Segment": number
,
"Select": "string
",
"TableName": "string
",
"TotalSegments": number
}
Request Parameters
The request accepts the following data in JSON format.
Note
In the following list, the required parameters are described first.
- TableName
-
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.
Type: String
Length Constraints: Minimum length of 1. Maximum length of 1024.
Required: Yes
- AttributesToGet
-
This is a legacy parameter. Use
ProjectionExpression
instead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.Type: Array of strings
Array Members: Minimum number of 1 item.
Length Constraints: Maximum length of 65535.
Required: No
- ConditionalOperator
-
This is a legacy parameter. Use
FilterExpression
instead. For more information, see ConditionalOperator in the Amazon DynamoDB Developer Guide.Type: String
Valid Values:
AND | OR
Required: No
- 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 (PutItem
,UpdateItem
, orDeleteItem
). -
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
.Type: Boolean
Required: No
-
- 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
.Type: String to AttributeValue object map
Key Length Constraints: Maximum length of 65535.
Required: No
- 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
Note
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.
Type: String to string map
Value Length Constraints: Maximum length of 65535.
Required: No
-
- 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.
Type: String to AttributeValue object map
Required: No
- 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.Note
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.
Type: String
Required: No
- 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
.Type: String
Length Constraints: Minimum length of 3. Maximum length of 255.
Pattern:
[a-zA-Z0-9_.-]+
Required: No
- 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.Type: Integer
Valid Range: Minimum value of 1.
Required: No
- 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.
Type: String
Required: No
- ReturnConsumedCapacity
-
Determines the level of detail about either provisioned or on-demand throughput consumption that is returned in the response:
-
INDEXES
- The response includes the aggregateConsumedCapacity
for the operation, together withConsumedCapacity
for each table and secondary index that was accessed.Note that some operations, such as
GetItem
andBatchGetItem
, do not access any indexes at all. In these cases, specifyingINDEXES
will only returnConsumedCapacity
information for table(s). -
TOTAL
- The response includes only the aggregateConsumedCapacity
for the operation. -
NONE
- NoConsumedCapacity
details are included in the response.
Type: String
Valid Values:
INDEXES | TOTAL | NONE
Required: No
-
- ScanFilter
-
This is a legacy parameter. Use
FilterExpression
instead. For more information, see ScanFilter in the Amazon DynamoDB Developer Guide.Type: String to Condition object map
Key Length Constraints: Maximum length of 65535.
Required: No
- 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
.Type: Integer
Valid Range: Minimum value of 0. Maximum value of 999999.
Required: No
- 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
.)Note
If you use the
ProjectionExpression
parameter, then the value forSelect
can only beSPECIFIC_ATTRIBUTES
. Any other value forSelect
will return an error.Type: String
Valid Values:
ALL_ATTRIBUTES | ALL_PROJECTED_ATTRIBUTES | SPECIFIC_ATTRIBUTES | COUNT
Required: No
-
- 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
.Type: Integer
Valid Range: Minimum value of 1. Maximum value of 1000000.
Required: No
Response Syntax
{
"ConsumedCapacity": {
"CapacityUnits": number,
"GlobalSecondaryIndexes": {
"string" : {
"CapacityUnits": number,
"ReadCapacityUnits": number,
"WriteCapacityUnits": number
}
},
"LocalSecondaryIndexes": {
"string" : {
"CapacityUnits": number,
"ReadCapacityUnits": number,
"WriteCapacityUnits": number
}
},
"ReadCapacityUnits": number,
"Table": {
"CapacityUnits": number,
"ReadCapacityUnits": number,
"WriteCapacityUnits": number
},
"TableName": "string",
"WriteCapacityUnits": number
},
"Count": number,
"Items": [
{
"string" : {
"B": blob,
"BOOL": boolean,
"BS": [ blob ],
"L": [
"AttributeValue"
],
"M": {
"string" : "AttributeValue"
},
"N": "string",
"NS": [ "string" ],
"NULL": boolean,
"S": "string",
"SS": [ "string" ]
}
}
],
"LastEvaluatedKey": {
"string" : {
"B": blob,
"BOOL": boolean,
"BS": [ blob ],
"L": [
"AttributeValue"
],
"M": {
"string" : "AttributeValue"
},
"N": "string",
"NS": [ "string" ],
"NULL": boolean,
"S": "string",
"SS": [ "string" ]
}
},
"ScannedCount": number
}
Response Elements
If the action is successful, the service sends back an HTTP 200 response.
The following data is returned in JSON format by the service.
- ConsumedCapacity
-
The capacity units consumed by the
Scan
operation. The data returned includes the total provisioned throughput consumed, along with statistics for the table and any indexes involved in the operation.ConsumedCapacity
is only returned if theReturnConsumedCapacity
parameter was specified. For more information, see Capacity unit consumption for read operations in the Amazon DynamoDB Developer Guide.Type: ConsumedCapacity object
- Count
-
The number of items in the response.
If you set
ScanFilter
in the request, thenCount
is the number of items returned after the filter was applied, andScannedCount
is the number of matching items before the filter was applied.If you did not use a filter in the request, then
Count
is the same asScannedCount
.Type: Integer
- Items
-
An array of item attributes that match the scan criteria. Each element in this array consists of an attribute name and the value for that attribute.
Type: Array of string to AttributeValue object maps
Key Length Constraints: Maximum length of 65535.
- LastEvaluatedKey
-
The primary key of the item where the operation stopped, inclusive of the previous result set. Use this value to start a new operation, excluding this value in the new request.
If
LastEvaluatedKey
is empty, then the "last page" of results has been processed and there is no more data to be retrieved.If
LastEvaluatedKey
is not empty, it does not necessarily mean that there is more data in the result set. The only way to know when you have reached the end of the result set is whenLastEvaluatedKey
is empty.Type: String to AttributeValue object map
Key Length Constraints: Maximum length of 65535.
- ScannedCount
-
The number of items evaluated, before any
ScanFilter
is applied. A highScannedCount
value with few, or no,Count
results indicates an inefficientScan
operation. For more information, see Count and ScannedCount in the Amazon DynamoDB Developer Guide.If you did not use a filter in the request, then
ScannedCount
is the same asCount
.Type: Integer
Errors
For information about the errors that are common to all actions, see Common Errors.
- InternalServerError
-
An error occurred on the server side.
HTTP Status Code: 500
- ProvisionedThroughputExceededException
-
Your request rate is too high. The Amazon SDKs for DynamoDB automatically retry requests that receive this exception. Your request is eventually successful, unless your retry queue is too large to finish. Reduce the frequency of requests and use exponential backoff. For more information, go to Error Retries and Exponential Backoff in the Amazon DynamoDB Developer Guide.
HTTP Status Code: 400
- RequestLimitExceeded
-
Throughput exceeds the current throughput quota for your account. Please contact Amazon Web Services Support
to request a quota increase. HTTP Status Code: 400
- ResourceNotFoundException
-
The operation tried to access a nonexistent table or index. The resource might not be specified correctly, or its status might not be
ACTIVE
.HTTP Status Code: 400
Examples
Return All Items
The following example returns all of the items in a table. No scan filter is applied.
Sample Request
POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date>
X-Amz-Target: DynamoDB_20120810.Scan
{
"TableName": "Reply",
"ReturnConsumedCapacity": "TOTAL"
}
Sample Response
HTTP/1.1 200 OK
x-amzn-RequestId: <RequestId>
x-amz-crc32: <Checksum>
Content-Type: application/x-amz-json-1.0
Content-Length: <PayloadSizeBytes>
Date: <Date>
{
"ConsumedCapacity": {
"CapacityUnits": 0.5,
"TableName": "Reply"
},
"Count": 4,
"Items": [
{
"PostedBy": {
"S": "joe@example.com"
},
"ReplyDateTime": {
"S": "20130320115336"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "Have you looked at BatchWriteItem?"
}
},
{
"PostedBy": {
"S": "fred@example.com"
},
"ReplyDateTime": {
"S": "20130320115342"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "No, I didn't know about that. Where can I find more information?"
}
},
{
"PostedBy": {
"S": "joe@example.com"
},
"ReplyDateTime": {
"S": "20130320115347"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "BatchWriteItem is documented in the Amazon DynamoDB API Reference."
}
},
{
"PostedBy": {
"S": "fred@example.com"
},
"ReplyDateTime": {
"S": "20130320115352"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "OK, I'll take a look at that. Thanks!"
}
}
],
"ScannedCount": 4
}
Use a Filter Expression
The following example returns only those items matching specific criteria.
Sample Request
POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date>
X-Amz-Target: DynamoDB_20120810.Scan
{
"TableName": "Reply",
"FilterExpression": "PostedBy = :val",
"ExpressionAttributeValues": {":val": {"S": "joe@example.com"}},
"ReturnConsumedCapacity": "TOTAL"
}
Sample Response
HTTP/1.1 200 OK
x-amzn-RequestId: <RequestId>
x-amz-crc32: <Checksum>
Content-Type: application/x-amz-json-1.0
Content-Length: <PayloadSizeBytes>
Date: <Date>
{
"ConsumedCapacity": {
"CapacityUnits": 0.5,
"TableName": "Reply"
},
"Count": 2,
"Items": [
{
"PostedBy": {
"S": "joe@example.com"
},
"ReplyDateTime": {
"S": "20130320115336"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "Have you looked at BatchWriteItem?"
}
},
{
"PostedBy": {
"S": "joe@example.com"
},
"ReplyDateTime": {
"S": "20130320115347"
},
"Id": {
"S": "Amazon DynamoDB#How do I update multiple items?"
},
"Message": {
"S": "BatchWriteItem is documented in the Amazon DynamoDB API Reference."
}
}
],
"ScannedCount": 4
}
See Also
For more information about using this API in one of the language-specific Amazon SDKs, see the following: