DynamoDB secondary sort

Question

I'm assessing whether if I can use DynamoDB for our next project, what we are building is quite similar to a blogging platform, here is a simple table

Blog Post
  ID - primary hash key
  Title
  DateCreated - primary range key
  Votes

I've read enough to know how to List - list of blog posts, Paging - using last fetched index, Get post details - get a row, I will be sorting using DateCreate, which is my range key.

I'm struggling on how do do sort on a secondary index. For example, if we have a column called Votes, how do you do Most Votes? My interpretation is that you can only sort using the range index which I'm already using.

Answer 1

Update

AWS has just announced general availability of the much anticipated Global Secondary Indexes for Amazon DynamoDB, which are addressing the limitations of Local Secondary Indexes discussed further below:

You can now create indexes and perform lookups using attributes other than the item's primary key. [...]

You can now create up to five Global Secondary Indexes when you create a table, each referencing either a hash key or a hash key and a range key. You can also create up to five Local Secondary Indexes, and you can choose to project some or all of the table's attributes into each of the table’s indexes.

Please refer to the blog post for more details on the choice between these two models.

---

Correction

As rightly pointed out by vartec, I've been getting ahead of myself adding this information at the day Local Secondary Indexes had been announced without properly analyzing the problem at hand, where those are in fact not applicable - ironically I've stressed just that myself in a later comment on another question:

[...] however, please note that local is a crucial limitation: A local secondary index is a data structure that maintains an alternate range key for a given hash key - while this covers many real world scenarios, it doesn't apply to arbitrary non primary key field queries like those of the question at hand.

Thanks vartec for spotting this error and apologies for being misleading here.

---

Initial (erroneous) answer

Amazon DynamoDB has just announced Support for Local Secondary Indexes to address your use case:

[...] We call the newest capability Local Secondary Indexes (LSI). While DynamoDB already allows you to perform low-latency queries based on your table’s primary key, even at tremendous scale, LSI will now give you the ability to perform fast queries against other attributes (or columns) in your table. This gives you the ability to perform richer queries while still meeting the low-latency demands of responsive, scalable applications.

See also the introductory blog post Local Secondary Indexes for Amazon DynamoDB for a more detailed explanation.

As usual for AWS, the new functionality is released with a constrained feature set at first, which is going to be expanded over time:

Today, local secondary indexes must be defined at the time you create your DynamoDB tables. In the future, we plan to provide you with an ability to add or drop LSI for existing tables. If you want to equip an existing DynamoDB table to local secondary indexes immediately, you can export the data from your existing table using Elastic Map Reduce, and import it to a new table with LSI. [emphasis mine]