How are consumed read capacity units calculated in DynamoDB query

Query—Reads multiple items that have the same partition key value. All items returned are treated as a single read operation, where DynamoDB computes the total size of all items and then rounds up to the next 4 KB boundary. For example, suppose your query returns 10 items whose combined size is 40.8 KB. DynamoDB rounds the item size for the operation to 44 KB. If a query returns 1500 items of 64 bytes each, the cumulative size is 96 KB.

Ref: https://docs.amazonaws.cn/en_us/amazondynamodb/latest/developerguide/ProvisionedThroughput.html

Scanning a table that contains 50 items will consume 50 RCU only if the total size of the 50 items combined equal 200KB (for a strongly consistent read, or 400KB for an eventual consistent read). Most items are not that big, so a 50 items typically only require about 10KB to store meaning a full scan for a table of 50 items, with eventual consistency, would only cost about 3 RCU.

The consumed Read Capacity Units (RCU) depends on multiple factors:

• the operation (ie. Get vs. Query/Scan)
• the size of the items
• whether the read is strongly consistent or eventually consistent

If an item is read using a GetItem operation than the consumed capacity is billed in increments of 4KB, based on the size of the item (ie. a 200B item and a 3KB item would each consume 1RCU, while a 5KB item would consume 2 RCU)

If you read multiple items using a Query or Scan operation, then the capacity consumed depends on the cumulative size of items being accessed (you get billed even for items filtered out of a query or scan when using filters). So, if your query or scan accesses 10 items, that are approximately 200 bytes each in size, then it will consume only 1 RCU. If you read 10 items but each item is about 5KB in size, then the total consumed capacity will be 13 RCU (50KB / 4KB = 12.5, rounded up, is 13)

What's more, if you perform an eventual consistent read, then you can double the size per capacity unit. So it would only cost 7 RCU to read the 10 5KB items.

You can read more about throughput capacity here.

A couple of things to note:

• a single item may be as large as 400KB, so reading an item could consume as much as 100 RCU.
• when calculating item size, attribute names count towards the item size as well, not just their values!