What a write to an indexed DynamoDB table costs

My conformance suite has had a per-index capacity assertion since fairly early on. It was on a Query . Which is the wrong half. Reading through an index is the cheap, obvious, well-understood part. The write side is where a secondary index actually costs you money, because every write to the base table fans out to every index the item lands in, and you pay for each one. That went unmeasured for…