DISTINCT vs GROUP BY vs ROW_NUMBER

Concepts covered: sqlWindowDedup

CDC streams are the canonical case for production dedup. Debezium, AWS DMS, Fivetran, Stitch, and Snowflake Streams all emit row-level changes from a source database to a downstream system. During failovers, retries, and reconnections, the same logical change can be emitted multiple times with different ingestion timestamps. The dedup pattern has to handle this without dropping legitimate updates and without keeping spurious duplicates. The CDC duplicate scenario Imagine the upstream MySQL primary fails over to a replica. The CDC connector reconnects and replays the last batch of changes it had not confirmed. The same order_update event for order_id=5000 arrives twice: once before the failover (created_at = 10:00:00) and once after (created_at = 10:00:00, but with a different ingestion_tim

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