<p>MySQL raises this error when an ALTER TABLE statement specifies an ALGORITHM value the server does not support or recognize.</p>
<p>MySQL Error 1800: ER_UNKNOWN_ALTER_ALGORITHM means the server does not recognize the ALGORITHM specified in an ALTER TABLE statement. Verify spelling, use a supported value like INPLACE or COPY, or upgrade MySQL to a version that supports the requested algorithm.</p>
Unknown ALGORITHM '%s'
The exact message "Unknown ALGORITHM '%s'" appears when MySQL parses an ALTER TABLE command containing the ALGORITHM clause and finds a value it cannot interpret. The parser aborts before any metadata or data changes occur, preventing the statement from executing.
The error is version-dependent: newer algorithms such as INSTANT exist only in MySQL 8.0.12+. Attempting to use them on 5.7 or earlier immediately triggers error 1800. Understanding supported algorithms for your MySQL build is crucial to avoid this failure.
An aborted ALTER TABLE can block migrations, CI pipelines, or application deployments. In production, repeated failures may leave tables locked, disrupt replication, or stall schema evolution workflows. Fixing the problem quickly restores predictable database operations.
Most incidents trace back to typos, unsupported keywords, or mismatched client-server versions. Storage-engine limitations can also reject a valid algorithm if the engine cannot perform the requested in-place or instant change.
Identify the unsupported algorithm, replace it with a supported one, or remove the ALGORITHM clause entirely so MySQL can pick the default. Upgrading the server or switching storage engines resolves cases where new functionality is required.
CI migrations often specify ALGORITHM=INSTANT for zero-downtime changes. Running those scripts on staging servers still on 5.7 produces the error. Aligning environments or adding version guards in migration tools eliminates the issue.
Maintain a compatibility matrix, lint migration scripts, and rely on tooling such as Galaxy's context-aware AI copilot to suggest only algorithms supported by the connected MySQL instance. Continuous integration should test against the lowest supported version to catch problems early.
Similar DDL failures include ER_ALTER_OPERATION_NOT_SUPPORTED, ER_NOT_SUPPORTED_YET, and ER_ALTER_FAILED. These often point to feature or engine limitations and follow similar troubleshooting steps: verify version, engine, and syntax.
The specified algorithm is not implemented in the running MySQL version, for example INSTANT on MySQL 5.7.
Misspellings such as INPLACEE or COPPY cause MySQL to treat the word as unknown.
Client libraries or migration tools generate statements for newer MySQL versions than the server, leading to incompatibility.
Certain engines, notably older InnoDB variants or third-party engines, lack support for INSTANT or INPLACE algorithms even if MySQL core does.
Occurs when the requested ALTER TABLE operation is impossible with the chosen ALGORITHM.
Signals that a feature exists conceptually but is not yet available in the current version.
Generic ALTER TABLE failure often tied to constraints or engine limitations.
Yes. When the clause is absent, MySQL selects the safest algorithm. This is often the quickest fix if zero-downtime guarantees are not critical.
Supported values include INSTANT, INPLACE, and COPY. INSTANT offers metadata-only changes for certain operations.
MariaDB supports INSTANT from 10.3 but implementation details differ. Always consult the specific documentation.
Galaxy's context-aware AI references the connected server version and warns when you type an unsupported ALGORITHM, preventing the migration from reaching production with errors.
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