DynamoDB query row number

Question

Can i smhow get index of a query response in DyanmoDB?

[hashKey exists, sortKey exists]

query { KeyCondExp = "hashKey = smthin1", FilterExp = "nonPrimeKey = smthin2" }

I need index of row according to sortKey for that selected document

Answer 1

When a DynamoDB Query request returns an item - in your example chosen by a specific filter - it will return the full item, including the sort key. If that is what you call "the index of row according to sortKey", then you are done.

If, however, by "index" you mean the numeric index - ie, if the item is the 100th sort key in this partition (hash key), you want to return the number 100 - well, that you can't do. DynamoDB keeps rows inside a partition sorted by the sort key, but not numbered. You can insert an item in the middle of a million-row partition, and it will be inserted in the right place but DynamoDB won't bother to renumber the million-row list just to maintain numeric indexes.

But there is something else you should know. In the query you described, you are using a FilterExpression to return only specific rows out of the entire partition. With such a request, Amazon will charge you for reading the entire partition , not just the specific rows returned after the filter. If you're charged for reading the entire partition, you might as well just read it all, without a filter, and then you can actually count the rows and get the numeric index of the match if that's what you want. Reading the entire partition will cause you more work at the client (and more.network traffic), but will not increase your DynamoDB RCU bill.