<p>MySQL raises Error 1709 when an indexed column or index prefix exceeds the storage engine's maximum byte length for a single index column.</p>
<p>MySQL Error 1709: ER_INDEX_COLUMN_TOO_LONG occurs when the combined length of an indexed column or prefix exceeds the storage engine limit (often 767 or 3072 bytes). Reduce the indexed column size or change row format or charset to fix the error.</p>
Index column size too large. The maximum column size is
MySQL throws error 1709 when you create or alter a table so that a single indexed column or prefix exceeds the maximum byte length allowed by the storage engine and row format. InnoDB with COMPACT or REDUNDANT row format allows 767 bytes per index column, while DYNAMIC and COMPRESSED allow 3072 bytes.
The error appears during CREATE INDEX, CREATE TABLE, or ALTER TABLE statements that define a key on an oversized VARCHAR, TEXT, or BLOB column. Understanding charset byte size is critical because multibyte charsets magnify stored length.
Storage engines allocate fixed space for index entries. Oversized keys bloat index pages, slow searches, and may break internal page structure. To protect integrity and performance, MySQL rejects index definitions that violate size limits.
Developers commonly hit the limit when indexing long VARCHAR columns with utf8mb4 (up to 4 bytes per character) or when using large prefix lengths on TEXT/BLOB columns. Legacy tables migrated from utf8 to utf8mb4 regularly trigger the error if indexes are unchanged.
Three primary fixes exist: reduce indexed column length, switch to a row format or MySQL version with higher limits, or convert to a more space-efficient character set. Each approach lowers the byte count per index entry.
Yes. Galaxy's SQL editor flags schema DDL issues in real time and suggests safe index lengths. Its AI copilot automatically adjusts prefix lengths based on active charset and row format, preventing Error 1709 before execution.
A VARCHAR(255) column under utf8mb4 can consume up to 1020 bytes, breaching the 767-byte limit for older InnoDB row formats.
Creating an index on a TEXT or BLOB column without specifying a prefix defaults to full length, instantly exceeding limits.
Even with a prefix, specifying too many characters (e.g., 2000) can surpass 3072-byte limit in modern formats.
Charset upgrade multiplies byte usage per character, causing previously valid indexes to break.
Tables using COMPACT or REDUNDANT formats retain the 767-byte ceiling regardless of MySQL 5.7+ global settings.
Raised when you define an index on a BLOB/TEXT column without specifying a prefix.
Occurs when spatial data exceeds allowed limits, similar to oversized index entries for spatial indexes.
Constraint violations during INSERT/UPDATE operations, often confused with index errors but unrelated to size.
No. MySQL still stores column prefixes for hash calculations, so byte limits apply.
With DYNAMIC or COMPRESSED row formats, the per-column limit rises to 3072 bytes, but older formats remain at 767 bytes.
The byte limit applies per column, then a separate overall index size limit (3072 or 767 bytes multiplied by columns) also exists.
Collation does not change byte length; only the character set determines bytes per character.
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