<p>MySQL raises ER_KEY_PART_0 when you try to create or alter an index whose column length is declared as 0.</p>
<p>MySQL Error 1391: ER_KEY_PART_0 appears when an index column length is set to 0. Remove the zero length or specify a positive prefix length to create the index successfully.</p>
Key part '%s' length cannot be 0
MySQL throws ER_KEY_PART_0 with message "Key part 'column' length cannot be 0" when an index definition specifies a column prefix length of zero. The server cannot build an index on a key part that indexes no characters.
The error typically appears during CREATE TABLE, ALTER TABLE, or CREATE INDEX statements where you manually set a prefix length for VARCHAR, TEXT, or BLOB columns.
ER_KEY_PART_0 occurs at parse time before the statement is executed. MySQL validates each key part and immediately stops if it detects a zero-length prefix, preventing table or index creation.
It can surface in migrations generated by ORMs, hand-written DDL, or schema diff tools that mistakenly insert a 0 after the column name in an index definition.
The DDL statement fails, leaving the table or index unchanged. Deployment scripts halt, applications may fail to start, and automated migrations roll back, blocking new features.
Addressing the error keeps schema changes reproducible across environments and avoids stalled CI/CD pipelines.
Including a VARCHAR column as (column(0)) inside a composite index definition triggers the error.
ORMs or migration tools that calculate prefix lengths may output 0 when length metadata is missing, producing invalid DDL.
Developers adding an index by hand sometimes type column(0) instead of omitting the parentheses or choosing a positive value like column(10).
Some assume specifying 0 on TEXT or BLOB columns disables prefixing; MySQL instead rejects the statement with ER_KEY_PART_0.
Raised when the total length of all index columns exceeds MySQL's key length limit.
Occurs when you index a BLOB or TEXT column without specifying any prefix length.
Thrown when a column prefix length is too long for the column's data type.
No. MySQL requires a positive prefix length for partial indexing. Use a value like column(10) or omit the prefix.
No. The validation happens before the engine layer, so InnoDB and MyISAM behave the same.
Pick the shortest length that maintains sufficient selectivity. For email, 20 characters often works.
Yes. Galaxy's editor highlights invalid index definitions and its AI copilot suggests correct prefix lengths during code review.
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