Can we avoid scan in dynamodb

Question

I am new the noSQL data modelling so please excuse me if my question is trivial. One advise I found in dynamodb is always supply 'PartitionId' while querying otherwise, it will scan the whole table. But there could be cases where we need listing our items, for instance in case of ecom website, where we need to list our products on list page (with pagination).

How should we perform this listing by avoiding scan or using is efficiently?

Answer 1

Basically, there are three ways of reading data from DynamoDB:

• GetItem – Retrieves a single item from a table. This is the most efficient way to read a single item, because it provides direct access to the physical location of the item.
• Query – Retrieves all of the items that have a specific partition key. Within those items, you can apply a condition to the sort key and retrieve only a subset of the data. Query provides quick, efficient access to the partitions where the data is stored.
• Scan – Retrieves all of the items in the specified table. (This operation should not be used with large tables, because it can consume large amounts of system resources.

And that's it. As you see, you should always prefer GetItem (BatchGetItem) to Query, and Query — to Scan.

You could use queries if you add a sort key to your data. I.e. you can use category as a hash key and product name as a sort key, so that the page showing items for a particular category could use querying by that category and product name. But that design is fragile, as you may need other keys for other pages, for example, you may need a vendor + price query if the user looks for a particular mobile phones. Indexes can help here, but they come with their own tradeofs and limitations.

Moreover, filtering by arbitrary expressions is applied after the query / scan operation completes but before you get the results, so you're charged for the whole query / scan. It's literally like filtering the data yourself in the application and not on the database side.

I would say that DynamoDB just is not intended for many kinds of workloads. Probably, it's not suited for your case too. Think of it as of a rich key-value (key to object) store, and not a "classic" RDBMS where indexes come at a lower cost and with less limitations and who provide developers rich querying capabilities.

There is a good article describing potential issues with DynamoDB, take a look. It contains an awesome decision tree that guides you through the DynamoDB argumentation. I'm pasting it here, but please note, that the original author is Forrest Brazeal.

Another article worth reading.

Finally, check out this short answer on SO about DynamoDB usecases and issues.

P.S. There is nothing criminal in doing scans (and I actually do them by schedule once per day in one of my projects), but that's an exceptional case and I regret about the decision to use DynamoDB in that case. It's not efficient in terms of speed, money, support and "dirtiness". I had to increase the capacity before the job and reduce it after, but that's another story…