EC
1784
MySQLERRORNotableDDLHIGH confidence

INPLACE ALTER not supported with foreign key check

Production Risk

Low — DDL rejected.

What this means

The in-place ALTER TABLE operation cannot be performed because foreign key constraint validation is required and is incompatible with INPLACE.

Why it happens
  1. 1Adding a foreign key or changing the referenced column requires full FK validation, which needs COPY.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE child ALGORITHM=INPLACE, ADD CONSTRAINT fk1 FOREIGN KEY (pid) REFERENCES parent(id);

expected output

ERROR 1784 (0A000): ALGORITHM=INPLACE is not supported. Reason: Cannot change column type: FOREIGN KEY CHECK.

Fix

Add the foreign key using ALGORITHM=COPY

Add the foreign key using ALGORITHM=COPY
ALTER TABLE child ALGORITHM=COPY, ADD CONSTRAINT fk1 FOREIGN KEY (pid) REFERENCES parent(id);

Why this works

COPY performs the full FK validation during the table rebuild.

What not to do

✕

Sources
Official documentation ↗

MySQL 8.0 — 1784 ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_FK_CHECK

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