3125
MariaDBERRORNotableDDLHIGH confidence
ALTER TABLE not supported: foreign key check requires table copy
Production Risk
Medium — ensure referential integrity is intact before adding FK constraints.
What this means
The foreign key constraints on the table must be validated during the ALTER, which requires a table copy and cannot proceed with ALGORITHM=INPLACE.
Why it happens
- 1Adding or modifying foreign key constraints requires referential integrity checks that prevent in-place DDL.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE child ADD CONSTRAINT fk1 FOREIGN KEY (pid) REFERENCES parent(id), ALGORITHM=INPLACE;
expected output
ERROR 3125 (HY000): ALGORITHM=INPLACE is not supported. Reason: Adding a foreign key requires table copy.
Fix
Use ALGORITHM=COPY
Use ALGORITHM=COPY
ALTER TABLE child ADD CONSTRAINT fk1 FOREIGN KEY (pid) REFERENCES parent(id), ALGORITHM=COPY;
Why this works
Allows full validation of referential integrity during rebuild.
What not to do
✕
Sources
Official documentation ↗
MySQL 8.0 — 3125 ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_FK_CHECK2
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