generated_always cannot insert a non-DEFAULT value into column “…” — 428C9

Symptoms

An INSERT supplied an explicit value for a column defined as GENERATED ALWAYS (a generated/identity column). Such columns compute their own value, so PostgreSQL raises SQLSTATE 428C9 (generated_always).

• The column is GENERATED ALWAYS (identity or stored).
• A HINT suggests using OVERRIDING SYSTEM VALUE or DEFAULT.
• Common with identity primary keys.

What the server log shows

ERROR:  cannot insert a non-DEFAULT value into column "id"
DETAIL:  Column "id" is an identity column defined as GENERATED ALWAYS.
HINT:  Use OVERRIDING SYSTEM VALUE to override.

Why PostgreSQL raises this — what the manual says

the CREATE TABLE reference (GENERATED … AS IDENTITY):

“The clauses ALWAYS and BY DEFAULT determine how explicitly user-specified values are handled in INSERT and UPDATE commands.”

A GENERATED ALWAYS column is owned by the system (identity sequence or a computed expression). Supplying an explicit value conflicts with that ownership, so PostgreSQL rejects it with 428C9 unless explicitly overridden.

Common causes

• Inserting an explicit value into an identity/generated column.
• An ORM including the identity column in the insert list.
• Copying rows including a GENERATED ALWAYS column.

How to fix it

1. Omit the column so it is generated, or set it to DEFAULT.
2. Use INSERT … OVERRIDING SYSTEM VALUE when you must supply a value (e.g. data migration).
3. Configure the ORM to treat the column as database-generated.

Related & next steps

• SQLSTATE 428C9 — Generated Always (full reference)
• Message: column can only be updated to DEFAULT
• Glossary: Identity column

Reference: PostgreSQL 18 — CREATE TABLE.