2026-09-21 · 6 min read · Rahul Chitturi
- sql
- postgresql
- transactions
A deferrable constraint can postpone selected checks until later in a transaction, allowing temporary states that would fail an immediate check. It does not disable the rule, and a transaction can still fail when constraints are forced or at commit.
Debugging becomes confusing when schema declarations, per-transaction modes, connection pooling, and ORM flush behavior do not match the application's mental model.
Confirm the deployed constraint
Inspect the database catalog and table definition for the constraint type, DEFERRABLE setting, initial mode, columns, referenced key, and validation state. Similar-looking indexes and constraints do not necessarily support the same timing controls.
- Name constraints explicitly in migrations.
- Set transaction mode on the same checked connection.
- Record when SET CONSTRAINTS is executed.
- Do not confuse deferral with NOT VALID.
Follow transaction boundaries
Trace BEGIN, writes, savepoints, mode changes, forced checks, and COMMIT. Switching a constraint to immediate can surface an accumulated violation before commit, while an ORM may flush SQL earlier than application code suggests.
A commit-time error means the transaction did not commit. Handle it as a transaction failure, preserve the database error category, and retry only when the complete operation is safe to repeat.
Build a minimal concurrent test
Use Flashman's SQL formatter for DDL and statements, diff for catalog snapshots, JSON formatter for synthetic rows, and timestamp converter for concurrent timelines.
Test reordered unique values, circular references, savepoints, forced checks, multiple constraints, pooled connections, concurrent writers, deadlocks, serialization failures, commit errors, retries, and rollback.