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A query operation as specified in DynamoDb documentation:
A query operation searches only primary key attribute values and supports a subset of comparison operators on key attribute values to refine the search process.
and the scan operation:
A scan operation scans the entire table. You can specify filters to apply to the results to refine the values returned to you, after the complete scan.
Which is best based on Performance and Cost Considerations.
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A scan query is a simple search query used to retrieve data from a cache in a distributed manner. When executed without parameters, a scan query returns all entries from the cache.
getItem retrieve via hash and range key is a 1:1 fit, the time it takes (hence performance) to retrieve it is limited by the hash and sharding internally. Query results in a search on "all" range keys. It adds computational work, thus considered slower.
A Scan operation in Amazon DynamoDB reads every item in a table or a secondary index. By default, a Scan operation returns all of the data attributes for every item in the table or index. You can use the ProjectionExpression parameter so that Scan only returns some of the attributes, rather than all of them.
When creating a Dynamodb table select Primary Keys and Local Secondary Indexes (LSIs) so that a Query operation returns the items you want.
Query operations only support an equal operator evaluation of the Primary Key, but conditional (=, <, <=, >, >=, Between, Begin) on the Sort Key.
Scan operations are generally slower and more expensive as the operation has to iterate through each item in your table to get the items you are requesting.
Example:
Table: CustomerId, AccountType, Country, LastPurchase Primary Key: CustomerId + AccountType In this example, you can use a Query operation to get:
A Scan operation would need to be used to return:
To avoid scan operations on frequently used operations create a Local Secondary Index (LSI) or Global Secondary Index (GSI).
Example:
Table: CustomerId, AccountType, Country, LastPurchase Primary Key: CustomerId + AccountType GSI: AccountType + CustomerId LSI: CustomerId + LastPurchase In this example a Query operation can allow you to get:
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