<p>The replica aborts because a column in the target table cannot be converted to the new data type during replication or ALTER TABLE.</p>
<p>MySQL Error 1677: ER_SLAVE_CONVERSION_FAILED occurs when a replica (or ALTER TABLE) cannot convert a column to the requested data type. Check incompatible type changes, adjust the schema on master and replica, then restart replication to resolve the issue.</p>
Column %d of table '%s.%s' cannot be converted from type
Error 1677 fires when a replication SQL thread or an in-place ALTER TABLE tries to modify a column but MySQL cannot safely convert existing data from the old type to the new type.
The error stops the replica at the failing event and returns the message: Column N of table 'db.tbl' cannot be converted from type 'old_type' to type 'new_type'.
It appears during statement-based or row-based replication, during mysqlbinlog replay, or on the primary if ALGORITHM=INPLACE conversion fails.
Because replication halts, transactions on the replica lag indefinitely until the schema mismatch is fixed.
Replication lag risks data divergence and breaks read scaling. Fixing the schema promptly restores high availability and data freshness.
Converting VARCHAR to INT or reducing numeric precision can reject existing values.
Switching from INT UNSIGNED to INT may overflow negative conversion limits.
Changing from utf8mb4 to latin1 can truncate multi-byte characters.
Changing a nullable column to NOT NULL fails if rows contain NULLs.
The replica already ran additional ALTER statements, so its type differs from the master.
Replication stops because a referenced table does not exist on the replica.
Indicates improper slave thread state when issuing START SLAVE.
Replica has missing binary log events after a purge on the primary.
Deadlock detected in replicated statement, halting the slave.
Yes, if the data type change is non-critical and you accept the replica diverging for that column. Always document the skip.
No, it only stops the replica or the client session running ALTER TABLE.
Use pt-osc or gh-ost for online schema changes and test on a replica first.
Galaxy surfaces full DDL history and lets teams review ALTER statements collaboratively, reducing untested schema changes that trigger Error 1677.
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