MySQL throws ER_TOO_MANY_KEY_PARTS when a primary key or index lists more columns than the server’s maximum (16).
MySQL Error 1070: ER_TOO_MANY_KEY_PARTS occurs when a PRIMARY KEY or INDEX contains more columns than the allowed limit (16 parts in MySQL 8.0). Reduce the number of indexed columns or split the index to fix the problem.
Too many key parts specified; max %d parts allowed
MySQL raises error 1070 with the message “Too many key parts specified; max 16 parts allowed” when a PRIMARY KEY or secondary INDEX contains more columns than the server permits. Each column in a composite index is called a key part, and MySQL 8.0 allows 16 parts per index.
The error blocks table creation or ALTER TABLE because oversized indexes hurt performance and exceed internal storage limits.
Understanding why the limit exists helps you redesign indexes efficiently and keep schema migrations smooth.
Error 1070 occurs during CREATE TABLE, ALTER TABLE ADD INDEX, or CREATE INDEX statements. It can also fire during SQL import or ORM migrations that implicitly generate composite keys exceeding the part limit.
Leaving the error unresolved prevents the table definition from executing, halting deployments, backups, or CI pipelines.
Large composite indexes also degrade write performance and waste storage, so fixing them improves scalability.
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Developers sometimes treat a primary key as a uniqueness guarantee for every descriptive column, ending up with more than 16 columns in the key.
Several ORMs build composite indexes from model attributes automatically.
If the model has many unique columns, the generated SQL may breach the key part limit.
Dump files from other database systems might include indexes with dozens of columns, which MySQL rejects with ER_TOO_MANY_KEY_PARTS.
Counting varchar prefix lengths incorrectly can lead to adding extra columns instead of defining a length on the same column.
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MySQL 8.0 allows up to 16 columns per index, regardless of data type.
No. The 16-column limit counts parts, not byte length. Prefix indexes still count as one part.
innodb_large_prefix expands index key size in bytes but does not change the 16-part limit enforced by error 1070.
Yes. Galaxy’s schema linter flags any CREATE/ALTER statements that exceed MySQL’s index part limit, saving debugging time.
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