Explanation

Correct Choice: Use a MERGE INTO operation with a WHEN NOT MATCHED clause based on a unique key.

• Atomic Idempotency: Delta Lake's MERGE INTO statement allows for complex conditional logic within a single transaction. By specifying a WHEN NOT MATCHED THEN INSERT clause based on a primary or unique key, the system checks incoming records against the target table. If the key already exists, the record is ignored; if not, it is inserted. This ensures that late-arriving duplicates or re-run batches do not result in duplicate entries in the target table.
• Efficiency: This method is highly efficient for incremental loads because it only targets new records rather than rewriting the entire table.

Why other options are incorrect:

• Schema Enforcement: Delta Lake schema enforcement (or schema evolution) is designed to ensure data type and column name consistency. It does not validate data uniqueness or enforce primary key constraints.
• VACUUM: The VACUUM command is used for data retention and storage optimization by deleting old data files that are no longer in the current table state. It has no functionality related to data deduplication.
• Full Outer Join + Overwrite: While a join can identify duplicates, performing a full outer join and overwriting the table on every batch is computationally expensive and non-performant for large production datasets, especially compared to the incremental nature of a MERGE operation.