**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Daily upsert with Spark: Use Delta Lake `MERGE` or `foreachBatch` for streaming. Batch example:
```python
deltaTable.alias('t').merge(df.alias('s'), 't.id = s.id')
.whenMatchedUpdateAll().whenNotMatchedInsertAll().execute()
```
For partitioned tables, include partition columns in merge condition....
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