could not send data to shared-memory queue

Symptoms

A statement failed with SQLSTATE 55000 (object_not_in_prerequisite_state), reported at severity ERROR. This is a Object Not In Prerequisite State condition: PostgreSQL emits the message could not send data to shared-memory queue.

• The client receives SQLSTATE 55000 (object not in prerequisite state).
• The operation is rejected at ERROR level; the statement does not complete.

What the server log shows

ERROR:  could not send data to shared-memory queue

Why PostgreSQL raises this

Class 55 (Object Not In Prerequisite State) is raised when an object exists but is not in a state that allows the requested operation right now.

As described in PostgreSQL’s Appendix A PostgreSQL Error Codes, SQLSTATE 55000 carries the condition name object_not_in_prerequisite_state in class Object Not In Prerequisite State. (Paraphrased — see the linked reference for the exact wording.)

Common causes

• The object is locked, in use, or being modified by another session.
• A required precondition (e.g. an index, a prepared state) is not met.
• The operation cannot run while the object is in its current state.

How to fix it

1. Wait for the conflicting session/operation to finish, then retry.
2. Establish the missing precondition before retrying.
3. Check for blocking locks with pg_locks.

Version applicability

This message text is present in PostgreSQL 16, 17, 18, 19. It was introduced around PostgreSQL 16; earlier releases do not emit this exact text.

Related & next steps

• SQLSTATE 55000 — Object Not In Prerequisite State (full reference)
• PostgreSQL error code index

Reference: PostgreSQL Appendix A PostgreSQL Error Codes.