Polymorphic associations — where a base table LEFT JOINs multiple subtype tables via a (type, id) discriminator pair — are auto-generated by ORMs like Rails, Django, and Hibernate, and cause serious PostgreSQL performance regressions at scale. The core issue is O(M×N) join probes where only O(M) are useful. Three patches under discussion on pgsql-hackers address distinct pain points: (1) Result Filter / one-sided gating skips inner-side scans when the discriminator condition is already false, eliminating fruitless buffer touches; (2) Sort Pushdown inserts a pre-sort above the base table scan so LIMIT can short-circuit the join tree without a full scan; (3) SubLink relocation pushes EXISTS-to-SEMI-JOIN transformations down to the smallest relevant subtree rather than floating them to the top of the join tree. A fourth direction — join selectivity statistics via CREATE STATISTICS — is also in prototype. None of the patches are committed yet, but each targets a specific, measurable regression source in the polymorphic query pattern.
