Informix Error -518: Child constraint constraint-name not found.

Reasons / Common Causes

-518, per the official text, is documented as an internal error with no stated application-level trigger — the guidance is simply to capture circumstances and escalate if it recurs, the same framing as other engine-internal catalog-consistency errors in this range.

1. An internal catalog inconsistency between a referential (foreign-key) constraint's parent and its expected child-side constraint entry — the general category of condition this error's name and sparse official guidance implies.
2. A referential constraint left in a partially-modified state by an interrupted or failed DDL operation (an ALTER TABLE involving a foreign key that didn't complete cleanly).
3. Concurrent DDL against the same referential relationship — one session altering a foreign key's constraints while another session's operation still expects the prior catalog state.

Solutions / Resolution

1. If this recurs, note all circumstances and contact IBM Informix Technical Support, per the official guidance — there is no documented general application-level fix.
2. Capture the exact statement in progress, the table/constraint names involved, and any preceding DDL against the same referential relationship before escalating.
3. Check for a preceding failed or interrupted ALTER TABLE involving the foreign key, since a partially-applied constraint change is a plausible non-engine-internal contributor worth ruling out before treating this as purely internal.

Examples

Capturing circumstances for escalation

-- Record the exact statement and constraint names involved:
ALTER TABLE order_items DROP CONSTRAINT fk_order_items_orders;
-- -518: Child constraint fk_order_items_orders not found.

along with whether any prior ALTER TABLE against order_items or orders involving this foreign key failed or was interrupted.

Diagnostic Checks

1. Check for a recent failed or interrupted ALTER TABLE touching the foreign key relationship named in the error.
2. Query sysconstraints/sysreferences for the constraint's current catalog state to see whether the parent-side constraint exists without a matching child-side entry.
3. Capture full circumstances for escalation if no preceding failed DDL explains the state.

Related Errors / Related Topics

No closely related error codes are cross-referenced for -518 in this set yet.

Sparse official guidance, framed as internal — check for a preceding failed or interrupted DDL operation against the same foreign key before treating it as purely engine-internal.