undefined_function could not identify an ordering operator for type … — 42883

Symptoms

An operation requiring ordering (ORDER BY, </>, a sort/merge step) was applied to a type that has no ordering operator. PostgreSQL raises SQLSTATE 42883 (undefined_function).

• The type has no defined less-than/greater-than ordering.
• Common with json, point, or custom types.
• Triggered by ORDER BY or range comparisons.

What the server log shows

ERROR:  could not identify an ordering operator for type point
HINT:  Use an explicit ordering operator or modify the query.

Why PostgreSQL raises this — what the manual says

As Section 11.2 Index Types explains:

An operation requiring ordering (ORDER BY, DISTINCT, a merge join, or a B-tree index) needs a less-than operator from a default B-tree operator class, but the data type provides none; supply an explicit operator class or use a type that has one.

Ordering operations need a comparison (less-than) operator. Types without a natural ordering (such as geometric or json types) cannot be sorted, so PostgreSQL reports 42883.

Common causes

• ORDER BY on a type with no ordering operator.
• Range/inequality comparisons on such a type.
• Sorting a custom type lacking a B-tree operator class.

How to fix it

1. Order by a derived comparable value (e.g. an expression or cast).
2. Use a type that supports ordering, or extract an orderable field.
3. Define a B-tree operator class for custom types if ordering is needed.

Related & next steps

• SQLSTATE 42883 — Undefined Function (full reference)
• Message: could not identify an equality operator for type
• Glossary: Operator class

Reference: PostgreSQL 18 Section 11.2 “Index Types”.