From community experience, vector index sync behavior depends heavily on how the Delta table is updated. With OVERWRITE, the table is effectively replaced, so the vector index typically treats this as a full refresh. Existing embeddings are dropped and rebuilt, which can be expensive and cause temporary unavailability. In contrast, MERGE is incremental: inserts, updates, and deletes are tracked at the row level, allowing the vector index to sync only changed records. This makes MERGE far more efficient and reliable for production pipelines. Best practice is to use MERGE for ongoing updates and reserve OVERWRITE for rare, full reprocessing scenarios.