In SQL, a SELECT statement without a WHERE clause will
return every row in a table. In Amazon DynamoDB, the Scan operation does the
same thing. In both cases, you can retrieve all of the items or just some of
them.
Whether you are using a SQL or a NoSQL database, scans should be used sparingly because they can consume large amounts of system resources. Sometimes a scan is appropriate (such as scanning a small table) or unavoidable (such as performing a bulk export of data). However, as a general rule, you should design your applications to avoid performing scans. For more information, see Querying tables in DynamoDB.
Note
Doing a bulk export also creates at least 1 file per partition. All of the items in each file are from that particular partition's hashed keyspace.
Scanning a table with SQL
When using SQL you can scan a table and retrieve all of its data by using a
SELECT statement without specifying a WHERE
clause. You can request one or more columns in the result. Or you can request
all of them if you use the wildcard character (*).
The following are examples of using a SELECT statement.
/* Return all of the data in the table */ SELECT * FROM Music;
/* Return all of the values for Artist and Title */ SELECT Artist, Title FROM Music;
Scanning a table in
DynamoDB
In Amazon DynamoDB, you can use either the DynamoDB API or PartiQL (a SQL-compatible query language) to perform a scan on a table.
With the DynamoDB API, you use the Scan operation to
return one or more items and item attributes by accessing every item
in a table or a secondary index.
// Return all of the data in the table { TableName: "Music" }
// Return all of the values for Artist and Title { TableName: "Music", ProjectionExpression: "Artist, Title" }
The Scan operation also provides a
FilterExpression parameter, which you can use to
discard items that you do not want to appear in the results. A
FilterExpression is applied after the scan is
performed, but before the results are returned to you. (This is not
recommended with large tables. You are still charged for the entire
Scan, even if only a few matching items are
returned.)
