EC
3101
MySQLERRORNotableSchemaHIGH confidence

ALTER TABLE with FK on generated column not supported in-place

Production Risk

Medium — requires a table copy which locks the table for longer.

What this means

The requested ALTER TABLE operation cannot be performed in-place because it involves a foreign key on a generated column.

Why it happens
  1. 1Using ALGORITHM=INPLACE for an ALTER TABLE that adds or modifies a foreign key involving a generated column.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE t ADD FOREIGN KEY (gen_col) REFERENCES other(id), ALGORITHM=INPLACE;

expected output

ERROR 3101 (HY000): ALGORITHM=INPLACE is not supported. Reason: Cannot add foreign key on a generated column. Try ALGORITHM=COPY.

Fix

Use ALGORITHM=COPY instead

Use ALGORITHM=COPY instead
ALTER TABLE t ADD FOREIGN KEY (gen_col) REFERENCES other(id), ALGORITHM=COPY;

Why this works

COPY algorithm rebuilds the table, supporting the full range of ALTER operations.

What not to do

✕

Sources
Official documentation ↗

MySQL 8.0 — 3101 ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_FK_ON_GENERATED

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