<p>MySQL returns error 1852 when a statement tries to use the deprecated IGNORE algorithm to create a unique index without allowing a full table copy.</p>
<p>MySQL Error 1852 ER_ALTER_OPERATION_NOT_SUPPORTED_REASON_IGNORE occurs when you run ALTER TABLE or CREATE INDEX with the IGNORE keyword. The IGNORE algorithm was removed after MySQL 5.7.3, so the server blocks the statement. Drop IGNORE, clean duplicates, or run the operation with ALGORITHM=COPY to resolve the issue.</p>
Creating unique indexes with IGNORE requires COPY
Error 1852 signals that MySQL cannot execute an ALTER TABLE or CREATE INDEX statement that includes the IGNORE clause. The IGNORE algorithm once let you build a unique index while silently dropping duplicate rows, but it was removed after MySQL 5.7.3 for data-integrity reasons.
The server now requires a full table copy to handle duplicates. If the statement requests ALGORITHM=INPLACE or omits ALGORITHM entirely, the operation is rejected and error 1852 is returned.
You will see the error on MySQL 5.7.3 and later whenever you attempt to add a unique constraint using the IGNORE modifier. The same response is triggered by any ALTER TABLE operation that relies on IGNORE to skip conflicting rows.
Ignoring the error leaves your schema without the intended uniqueness guarantee. Duplicate data can accumulate, causing inconsistent query results and application bugs. Addressing the problem ensures reliable constraints and stable application logic.
Developers migrate legacy scripts that still append the IGNORE keyword assuming MySQL will drop duplicates automatically.
Database migration frameworks generated before MySQL 5.7.3 may default to IGNORE when adding unique keys.
Combining ALGORITHM=INPLACE with IGNORE is blocked because removing duplicates needs a table copy.
Code copied from online examples written for older MySQL versions still contains the deprecated modifier.
Raised when renaming an indexed column involved in a foreign key while using ALGORITHM=INPLACE.
Occurs if you change a column type incompatible with INPLACE algorithm.
Duplicate entry error surfaces when unique indexes already exist and data conflicts.
Only MySQL 5.7.1 to 5.7.2 supported IGNORE for unique index creation. Later versions reject it.
Yes. The table is locked while the copy is created, which can impact write-heavy workloads.
Run a GROUP BY query on the target columns and filter groups with COUNT(*) > 1 to list duplicates.
Galaxy's editor surfaces MySQL deprecation warnings in real time and offers one-click fixes to strip IGNORE or switch to ALGORITHM=COPY.
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