customer_cdc_feed (customer_id, op, email, tier, source_ts, ingest_seq) is a change feed from the customer database. Each row is a full version of a customer as of source_ts (when the change happened in the source); ingest_seq is the order rows reached the warehouse. op is 'U' for an upsert or 'D' for a delete (a *tombstone*, with no email or tier).
The feed is at-least-once and unordered: old versions can be re-delivered later (with a higher ingest_seq but their original source_ts).
A customer's current version is the row with the latest source_ts; when two rows share it, the one with the higher ingest_seq was applied last. If the current version is a delete, the customer no longer exists — even if older versions say otherwise. A customer deleted and then upserted again exists.
Return every customer that currently exists, with its current version.
Columns: customer_id, email, tier, source_ts. Sort by customer_id.