DynamoDB 1MB limit on full item or only requested attributes?

Question

Suppose I make a Query in which I use KeyConditionExpression and I only want to return a subset of all attributes with ProjectionExpression. Does the full item size count toward the 1MB limit, or only what I'm fetching?

There is a slight ambiguity which I would like to clarify with respect to this question, because there it mentions read capacity, but not size limit.

Answer 1

That is a good question. I've decided to test it.

It seems it doesn't change the page size, but it does help with performance. My objects have a dummy data field that is quite large, so it's normal to get much better response times, given the decrease in bytes that need to be transferred.

Here is my code

// Without Projection
long start = System.currentTimeMillis();
double avgPageSize = 0;

for (var page : table.scan()) {
  avgPageSize = (avgPageSize + page.items().size()) / 2;
}

System.out.println("Without Projection ms " + (System.currentTimeMillis() - start));
System.out.println("Average Page size " + avgPageSize);


// With projection
start = System.currentTimeMillis();
avgPageSize = 0;
scanEnhancedRequest = ScanEnhancedRequest.builder()
  .addAttributeToProject("PK")
  .build();

for (var page : table.scan(scanEnhancedRequest)) {
  avgPageSize = (avgPageSize + page.items().size()) / 2;
}

System.out.println("With Projection ms " + (System.currentTimeMillis() - start));
System.out.println("Average Page size " + avgPageSize);

And the results

Without Projection ms 13862
Average Page size 3062.7655089609325

With Projection ms 2241
Average Page size 3062.7655089609325

So it seems the sequence is query -> calculate capacity units -> filters -> pagination + projection

The last one is just an assumption