Data Quality Constraints in Unity Catalog

• NOT NULL and CHECK constraints reject invalid writes
• Use CHECK for range/cardinality; PK and UNIQUE are informational
• The column schema enforces data-type validity
• Delta schema enforcement rejects mismatched writes
• Handle additive schema drift with schema evolution
• Auto Loader preserves unmatched data in _rescued_data
• Expectations offer warn, drop, and fail actions
• expect_or_fail stops the update and rolls back
• Dropped and failed record counts are tracked in the event log
• expect_all applies multiple named constraints together
• NOT ENFORCED and ENABLE NOVALIDATE attach to key constraints only, never to CHECK
• A CHECK constraint is a per-row predicate and cannot look at other rows or tables
• Dropping a check constraint does not undo the writer protocol upgrade it caused
• Declaring an unverified key constraint RELY can produce incorrect query results
• Automatic type widening needs schema evolution on the write AND the feature on the target
• Setting enableTypeWidening to false removes neither the feature nor the applied changes
• Widening is a closed list of numeric and date promotions; int to string is not one
• Type widening raises the reader protocol and can break Iceberg reads
• Anomaly detection is enabled on a schema and learns each table's own commit history to flag stale freshness and shortfalls in completeness, with no per-table configuration
• A data profiling monitor is configured per table and is the only instrument that produces profile and drift metric tables
• Data quality monitoring reports only after data has landed and never rejects a write or modifies the monitored table
• Violating rows must be routed to a quarantine dataset because nothing in the pipeline retains a record it dropped
• One non-discarding pass that flags validity on an intermediate streaming table can feed both the clean dataset and the quarantine dataset
• Quarantining trades storing and governing bad data for keeping the correction-and-replay loop inside the pipeline

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