Say we have a table with average item size of 1 KB. We perform a query which reads 3 such items. Now according to what I have read, the number of RCUs should be (strongly consistent reads) : (Number of items read) * ceil(item_size/4) = 3 * ceil(1/4) = 3*1 = 3.
So wanted to confirm : is this correct? Or do we use a single RCU as total size of messages read is 3, which is less than 4.
An RCU is good for 1 strongly consistent read of up to 4KB.
Thus you can query() four 1KB items for 1 RCU.
Since you have only 3 to read, 1 RCU will be consumed.
Using GetItem() to get those same 3 records would cost 3 RCU.
Let say you had 100 items that matched (HK+SK) the query, but you're also using filter to further select records to be returned; so you're only getting 4 records back. That query would take 25 RCU, as the records still have to be read even if not returned.
Reference can be found here :
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.
