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

Symptoms

An operation that needs equality (DISTINCT, GROUP BY, UNION, a hash/sort step) was applied to a type that has no equality operator. PostgreSQL raises SQLSTATE 42883 (undefined_function).

• The type lacks a defined = operator.
• Common with json, xml, or custom types.
• Triggered by DISTINCT/GROUP BY/UNION on such a type.

What the server log shows

ERROR:  could not identify an equality operator for type json

Why PostgreSQL raises this — what the manual says

As Section 8.14 JSON Types explains:

The operation (DISTINCT, GROUP BY, UNION, or similar) needed an equality operator, but the json type does not provide one; cast the values to jsonb, which supports equality and the comparison operators these operations require.

Set/grouping operations require an equality operator to compare values. For types that define none (like json), PostgreSQL cannot perform the comparison and reports 42883.

Common causes

• DISTINCT/GROUP BY/UNION on a json column.
• Comparing a custom type without an equality operator.
• Using such a type as a grouping or join key.

How to fix it

1. Use jsonb instead of json — it supports equality.
2. Cast to a comparable type (e.g. col::text or col::jsonb).
3. Define an equality operator/operator class for custom types.

Related & next steps

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

Reference: PostgreSQL 18 Section 8.14 “JSON Types”.