undefined_object constraint “…” of relation “…” does not exist — 42704

Symptoms

A statement referenced a constraint that does not exist on the given relation. PostgreSQL raises SQLSTATE 42704 (undefined_object).

• The named constraint is not defined on that table.
• Common in ALTER TABLE … DROP CONSTRAINT.
• Constraint names are case-sensitive when quoted.

What the server log shows

ERROR:  constraint "orders_amount_check" of relation "orders" does not exist

Why PostgreSQL raises this — what the manual says

the ALTER TABLE reference (DROP CONSTRAINT):

“This form drops the specified constraint on a table, along with any index underlying the constraint.”

Constraints are named objects attached to a relation. Referencing a name that isn’t present on that table (typo, already dropped, or wrong table) cannot be resolved, so PostgreSQL reports 42704.

Common causes

• A typo or wrong case in the constraint name.
• The constraint was already dropped or never existed.
• Referencing the wrong relation.

How to fix it

1. List constraints: \d tablename in psql, or query pg_constraint.
2. Use the exact constraint name (mind quoting/case).
3. Use DROP CONSTRAINT IF EXISTS to make the drop idempotent.

Related & next steps

• SQLSTATE 42704 — Undefined Object (full reference)
• Message: trigger for relation does not exist
• Glossary: Constraint

Reference: PostgreSQL 18 — ALTER TABLE.