Usage contains filter in DynamoDB scan using Java

Question

Suppose I have a working request, for example:

ScanRequest scanRequest = new ScanRequest() .withTableName("myTable") .withFilterExpression("attr1 = :val1 and attr2 = :val2") .withExpressionAttributeValues(vm) //contains values for :val1 and :val2 .withLimit(PAGE_SIZE) .withConsistentRead(READ_TYPE); 

Now I would like to expand this scan. Suppose my table also has an attr3 attribute, which takes the form:

 "attr3": { "S": "AAA BBB CCC DDD" } 

How to filter elements that attr3 contain AAA? Or AAA and BBB?

Answer 1

DynamoDB Expression Terms Reference documentation is your friend!

In your specific case, you can use the contains function to search for substrings. Your filter expression might look something like this:

 "attr1 = :val1 and attr2 = :val2 and (contains(attr3, :val3a) or contains(attr3, :val3b))" // where :val3a and :val3b are value placeholders for say AAA and BBB 

But I suspect that what you want to achieve is a bit more complicated than the DynamoDB filters can handle the server, so you have two options:

• perform filtering in the application logic (i.e., remove the results to the client and the filter there), or;
• change your attribute from row to list or rowset (if duplicates are not allowed)

In any case, you should know that scanning with filters is more effective than conventional scanning in terms of network bandwidth (in the case when most of the results are excluded by the filter). But in terms of power consumption, scanning is equally expensive with or without filters. Therefore, if you can avoid this, do not rely too much on scanning the table!