在DynamoDB表中使用GSI时估算额外的维护成本

Question

I have a Users table in DynamoDB that has a unique hash key username . I want, however, to be able to find a specific user in the most efficient way possible by providing either just the username, or just the email (the email is also unique). I can make the email a global secondary index, but I have a trouble estimating the additional cost of this approach. Will using the index to retrieve a user result in two read operations? Or how many operations exactly?

Also, I want read and write throughput of the index to equal those of the table (and ideally, scale automatically), can I do that by not providing specific throughput values when I create the index with API, or do I have to provide them?

Answer 1

The number of read operations you will need to retrieve values from the index will depend on what values you want to read (all of them vs just a subset) and what the projection type for the index is. If the projection is ALL then it only takes 1 read, but it may cost more. If the projection is KEYS_ONLY you will only get back the table's primary key, then you would have to query the table again by that. That takes more than 1 read, but may be cheaper. It will all depend on your use cases and usage patterns.

See "Attribute Projections" at https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/GSI.html

I think you need to provide the read capacity and write capacity for the index when it is created - it will not inherit any values from the parent table. Although if the table is using autoscaling, the autoscaling configuration can be automatically applied to the GSI. See https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/AutoScaling.Console.html#AutoScaling.Console.ExistingTable