SimpleDB: Finally Supports Sorting


The latest version of Amazon's SimpleDB service is available, and happily it now allows for sorting of query results. This highly-anticipated feature will make the service much more useful.

Sorting still suffers from some limitations: you can only sort by a single attribute, you cannot perform sorting in queries that contain union or not predicates, and the attribute against which you are sorting must be present in a query predicate. However, it is still a massive improvement on sorting query results in your own client code.

For example, given the following SimpleDB query that finds historic stock records for a given time period:

['Code' = 'AAPL'] intersection ['Date' > '2007-12-01']

With the new sort operator, you can now instruct the service to return the results in reverse (descending) date order like so:

['Code' = 'AAPL'] intersection ['Date' > '2007-12-01'] sort 'Date' desc

Although you can only sort by an attribute that is also mentioned in a predicate, you can easily work around this restriction by including a starts-with '' predicate for that criteria, which will always evaluate to true. Here is a query to return stock records sorted according to the closing price, from lowest to highest:

['Code' = 'AAPL'] intersection ['Close' starts-with ''] sort 'Close' asc

The SimpleDB update also includes some other new features, such as a new does-not-start-with operator and support for 10 predicates per query instead of 5, but I think it is the sorting that will have the most impact.

Tags: AWS Coding