A strongly consistent read returns the most recent successfully completed write for the requested data. DynamoDB reads are eventually consistent by default, so a read immediately after an update can return an older version. section 12.4.6
Set ConsistentRead to true to request strongly consistent results from getItem, query, or scan. This option improves freshness but takes longer and consumes more read capacity than an eventually consistent read. section 12.4.6
Comparing DynamoDB read requests
An AWS CLI get-item request without --consistent-read consumes 0.5 capacity units in the example; adding --consistent-read consumes 1.0 unit. A strongly consistent getItem for an item up to 4 KB requires one read capacity unit. section 12.11.1
Global secondary indexes are always eventually consistent because table-to-index synchronization is asynchronous. Strong consistency therefore cannot be used to obtain fresh reads from such an index. section 12.4.6 section 12.15
