MySQL raises ER_WRONG_SUB_KEY when a prefix length on an index column is invalid, longer than the column itself, or unsupported by the storage engine.
MySQL Error 1089: ER_WRONG_SUB_KEY appears when a prefix length on an indexed column is longer than the column, not a string, or unsupported by the engine. Shorten or remove the prefix, use a full-length key, or switch to a compatible engine to resolve the issue.
Incorrect prefix key; the used key part isn't a string,
MySQL returns ER_WRONG_SUB_KEY when you attempt to create an index that specifies a prefix length larger than the declared size of the column or when the storage engine cannot apply prefix indexing on the chosen data type.
The error stops the CREATE TABLE or CREATE INDEX statement.
The message usually reads: "Incorrect prefix key; the used key part isn't a string, or the used length is longer than the key part, or the storage engine doesn't support unique prefix keys." Understanding the exact trigger is critical to restoring normal DDL operations.
Invalid prefix lengths generate the error most often.
Declaring a VARCHAR(50) column but indexing it with a prefix length of 100 exceeds the column limit and fails instantly. Numbers and BLOB/TEXT columns without the right engine support also break.
Engine limitations matter. InnoDB supports prefix indexes on VARCHAR, VARBINARY, and BLOB/TEXT, but only up to 3072 bytes in total index key length. MyISAM has different limits. Choosing a non-supporting engine or exceeding length thresholds triggers ER_WRONG_SUB_KEY.
First, confirm column definitions.
Ensure the prefix length is not larger than the column length. For multi-byte charsets, convert length to bytes before comparing. Second, remove the prefix or use the full column length if the engine supports it. Third, switch to a storage engine such as InnoDB when indexing BLOB/TEXT columns.
Galaxy users can detect the issue quickly with inline linting.
The editor warns when a prefix length exceeds column size, letting engineers adjust SQL before execution.
Attempting to create a unique index on VARCHAR(10) with prefix 20 fails. Adjust to VARCHAR(10) prefix 10 or expand the column. Indexing a JSON column with a prefix in MySQL 5.7 fails because JSON was treated as BLOB. Upgrade to MySQL 8.0 or remove the prefix.
BLOB/TEXT columns need explicit prefix lengths in InnoDB.
If the sum of all indexed prefixes exceeds 3072 bytes, reduce individual lengths or convert the column to VARCHAR with a smaller size.
Always size columns accurately and align prefix lengths accordingly. Validate charset byte requirements before setting prefixes. Prefer full-length indexes on smaller columns to remove complexity.
Leverage Galaxy's AI copilot to suggest legal prefix lengths based on schema introspection.
Automated reviews catch oversize prefixes in pull requests, preventing ER_WRONG_SUB_KEY from reaching production.
Error 1071 (ER_TOO_LONG_KEY) occurs when the combined index key length exceeds engine limits. Reduce prefix sizes or column lengths.
Error 1406 (ER_DATA_TOO_LONG) surfaces when inserted data exceeds column size. Adjust column length or truncate data during inserts.
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Yes. InnoDB supports prefix indexes on TEXT or BLOB columns, but you must specify a prefix length and keep the combined length of all indexed columns within 3072 bytes.
Absolutely. Prefix lengths are defined in characters, but engine limits apply to bytes. Multi-byte charsets like utf8mb4 multiply the length. Calculate bytes before indexing.
Run SHOW INDEX FROM table_name to list prefix lengths. Any length exceeding the associated column size signals a problem.
Galaxy's schema-aware linting flags invalid prefix lengths while you type. The AI copilot suggests corrected DDL statements, preventing runtime errors.
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