Shaun Thomas: Enforcing Constraints Across Postgres Partitions

Why It Matters

Ensuring cross‑partition uniqueness is critical for data integrity in high‑volume systems; the dedup table technique offers a practical, performance‑predictable path until PostgreSQL adds native global indexes.

Key Takeaways

• Global unique indexes not supported in PostgreSQL partitions
• Trigger scanning all partitions adds >3x insert latency
• Deduplication table provides constant‑time uniqueness with ~2x overhead
• Hash‑partitioned dedup table scales without affecting insert cost

Pulse Analysis

PostgreSQL’s declarative partitioning is a powerful tool for scaling large tables, but it comes with a hard limitation: unique constraints must include the partition key. Without a global index, duplicate detection across partitions must be handled at the application or database layer. This gap forces engineers to choose between cumbersome workarounds or accepting potential data anomalies, especially in event‑driven pipelines where deduplication is essential.

Two common strategies emerge. The first uses a trigger that scans every partition’s index on each insert, guaranteeing uniqueness but incurring a steep performance penalty that grows with the number of partitions. In a test with three partitions, insert time jumped from 7 seconds to 25 seconds, a more‑than‑three‑fold slowdown. The second strategy introduces a dedicated deduplication table—either unpartitioned or hash‑partitioned—that stores the unique key and relies on its primary‑key index for enforcement. This approach adds a fixed overhead of roughly 2× the baseline insert time, regardless of how many partitions exist, offering predictable scaling for systems that may eventually host hundreds of partitions.

While the dedup table method solves the performance problem, it introduces new considerations. Triggers add database‑level business logic, which some teams prefer to keep in application code. Concurrency must be managed carefully; the dedup table’s primary‑key lock naturally serializes conflicting inserts, whereas a scan‑based trigger may need advisory locks to avoid race conditions. Maintenance of the dedup table itself—especially its size—can be mitigated by hash partitioning, keeping index lookups fast. Until PostgreSQL implements native global indexes, the dedup table pattern provides the most reliable, scalable path for enforcing cross‑partition uniqueness.