MySQL error 1071 occurs when an index or primary key definition exceeds the storage engine’s maximum key length.
MySQL Error 1071: ER_TOO_LONG_KEY appears when the combined length of indexed columns is larger than the storage engine’s limit (767 or 3072 bytes, depending on version and row format). Shorten the indexed column, use a prefix index, or switch to a larger row format to fix the error.
Specified key was too long; max key length is %d bytes
MySQL raises error 1071 with the message “Specified key was too long; max key length is N bytes” when you try to create or alter a table so that an index exceeds the storage engine’s key length limit.
In InnoDB before MySQL 5.7, the limit is 767 bytes; from 5.7.7 with DYNAMIC or COMPRESSED row format, it increases to 3072 bytes.
Exceeding these limits blocks table creation and schema migrations, so resolving the problem is critical for release pipelines.
Oversized VARCHAR or TEXT columns using multibyte character sets like utf8mb4 quickly hit the byte limit when indexed.
Composite indexes that add multiple columns together can push the total key length beyond the engine’s maximum.
Migrating tables to utf8mb4 without reducing column sizes multiplies storage per character, unexpectedly breaking existing indexes.
Reduce the indexed column length or use a prefix index so the key fits within 767 / 3072 bytes.
Change the table’s row_format to DYNAMIC or COMPRESSED and upgrade to MySQL 5.7.7+ or 8.0 to unlock the 3072-byte limit.
Consider normalizing data or removing unnecessary columns from composite indexes to shrink the key length.
Creating a unique index on VARCHAR(255) utf8mb4 in MySQL 5.6 fails.
Truncate to VARCHAR(191) or use a 191-byte prefix.
Adding an index on (first_name, last_name, country) where each column is VARCHAR(100) can exceed 3072 bytes with utf8mb4.
Drop country from the index if query plans allow.
Plan schema migrations with byte limits in mind and test against staging databases on the target MySQL version.
Default new VARCHAR columns to 191 when they will be indexed under utf8mb4 unless larger sizes are necessary.
Use Galaxy’s schema-aware AI copilot to preview index sizes and catch ER_TOO_LONG_KEY before running ALTER TABLE in production.
Error 1406 (data too long for column) occurs when inserted values exceed column length; fix by sizing columns correctly.
Error 1072 (key column does not exist) means an index references a missing column; ensure the column is present.
Error 1822 (failed to add foreign key constraint) may arise if the referenced index is too long; shorten the key or use an appropriate prefix.
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Each utf8mb4 character can use up to 4 bytes, so a VARCHAR(255) column alone consumes 1020 bytes, exceeding the 767-byte limit in older versions.
Combining several columns multiplies the total key length, often crossing 3072 bytes after character-set changes.
Upgrading from latin1 to utf8mb4 without reducing column sizes inflates index size four-fold and triggers the error.
Using COMPACT or REDUNDANT row formats keeps the 767-byte ceiling even on MySQL 8.0, surprising developers who expect 3072 bytes.
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No. The error appears only when indexed column sizes inflate past the key length limit. Non-indexed columns can stay at 255 or larger without issue.
191 characters keeps the index under 767 bytes in legacy MySQL and is widely recommended for primary and unique keys.
Upgrading helps, but you must also set row_format=DYNAMIC or COMPRESSED; otherwise the 767-byte ceiling remains.
Galaxy’s AI copilot inspects column definitions in real time, warns when an index will exceed the storage limit, and suggests safe VARCHAR sizes before you run ALTER TABLE.
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