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Description

Error 1709, 'Index column size too large,' indicates that an attempt to create an index has failed because the total size of the indexed column(s) exceeds MySQL's internal limits. This often occurs when indexing very long text or binary data types, or when a composite index combines multiple lengthy columns.

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Error Message

Index column size too large. The maximum column size is %lu bytes.

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Known Causes

3 known causes

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Indexing Long Text/Binary Columns

Directly indexing `TEXT`, `BLOB`, or very long `VARCHAR` columns without specifying a prefix length can exceed the maximum index key size.

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Excessive Composite Index Length

When creating a composite index on multiple columns, their combined byte length might exceed the allowed maximum key length for the storage engine.

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Character Set Byte Consumption

The character set encoding used for a column (e.g., `utf8mb4`) can significantly increase its byte size, causing an index to exceed the limit even with fewer characters.

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Solutions

4 solutions available

1. Reduce Index Column Size medium

Shrink the size of the column(s) being indexed to comply with MySQL's limits.

1

Identify the problematic table and column(s) causing the error. This often involves the error message pointing to a specific table or index definition. If not, examine `SHOW CREATE TABLE your_table_name;` for large `VARCHAR`, `TEXT`, or `BLOB` columns that are indexed.

SHOW CREATE TABLE your_table_name;

2

Determine the maximum allowed index prefix size. This limit varies by MySQL version and storage engine. For InnoDB, it's typically 767 bytes for older versions or 3072 bytes for newer versions with `innodb_large_prefix=ON` and `innodb_file_format=barracuda` or `innodb_file_per_table=ON`.

SHOW VARIABLES LIKE 'innodb_page_size';
SHOW VARIABLES LIKE 'innodb_large_prefix';
SHOW VARIABLES LIKE 'innodb_file_format';

3

If the column is a `VARCHAR` or `TEXT` type and its defined length exceeds the limit for indexing, reduce the length. For example, change `VARCHAR(255)` to `VARCHAR(191)` if that's sufficient for your data and the index limit is 767 bytes (191 * 3 bytes per character for utf8mb4).

ALTER TABLE your_table_name MODIFY COLUMN your_column_name VARCHAR(191);

4

If you need to index the entire column but it's too large, consider using a prefix index. This indexes only the first N characters of the column.

ALTER TABLE your_table_name ADD INDEX index_name (your_column_name(191));

5

After altering the table, re-create the index if it was dropped or if the modification implicitly affected it.

DROP INDEX index_name ON your_table_name;
CREATE INDEX index_name ON your_table_name (your_column_name(191));

2. Enable Large Prefix Support (InnoDB) medium

Configure InnoDB to allow larger index prefixes for `COMPACT` and `DYNAMIC` row formats.

1

Ensure your MySQL server is configured to use the `Barracuda` file format or `Antelope` with `innodb_file_per_table=ON` and `innodb_large_prefix=ON`. The `Barracuda` format is required for `DYNAMIC` or `COMPRESSED` row formats, which are necessary for `innodb_large_prefix` to work effectively.

SHOW VARIABLES LIKE 'innodb_file_format';
SHOW VARIABLES LIKE 'innodb_file_per_table';
SHOW VARIABLES LIKE 'innodb_large_prefix';

2

If `innodb_large_prefix` is OFF, you need to enable it in your MySQL configuration file (e.g., `my.cnf` or `my.ini`). You might also need to set `innodb_file_format` to `barracuda` and `innodb_file_per_table` to `ON`.

[mysqld]
innodb_file_format = barracuda
innodb_file_per_table = ON
innodb_large_prefix = ON

3

Restart your MySQL server for these configuration changes to take effect.

sudo systemctl restart mysql

4

After restarting, you may need to alter the table to use the `DYNAMIC` or `COMPRESSED` row format to fully benefit from `innodb_large_prefix`.

ALTER TABLE your_table_name ROW_FORMAT=DYNAMIC;

5

Once enabled and the table is reconfigured, you can create or alter indexes on columns that were previously too large.

CREATE INDEX index_name ON your_table_name (large_column_name);

3. Use a Hash-Based Index (Full-Text Search) medium

For large text fields where exact matches are infrequent, consider using full-text indexing for better performance and to avoid index size issues.

1

Identify if the large column is intended for searching within its content, rather than exact matches on the entire value.

text

2

If the column is suitable for full-text searching, convert it to a `FULLTEXT` index. This is particularly useful for `TEXT` and `VARCHAR` columns.

ALTER TABLE your_table_name ADD FULLTEXT INDEX ft_index_name (your_large_text_column);

3

You can then use `MATCH()` and `AGAINST()` clauses for searching.

SELECT * FROM your_table_name WHERE MATCH(your_large_text_column) AGAINST ('search term');

4

Note that `FULLTEXT` indexes have their own limitations and are optimized for different types of queries than standard B-tree indexes.

text

4. Normalize Data or Use Separate Tables advanced

Decompose large data into smaller, more manageable parts or move them to separate tables.

1

Analyze if the large column contains data that could be logically separated. For example, a single `VARCHAR` storing multiple comma-separated values might be better as a separate table with a one-to-many relationship.

text

2

If the large column stores data that is frequently accessed independently or needs its own indexing, create a new table for this data and link it back to the original table using a foreign key.

CREATE TABLE your_table_name_extra_data (
    id INT AUTO_INCREMENT PRIMARY KEY,
    your_table_name_id INT,
    large_data_part VARCHAR(255), -- or appropriate type
    FOREIGN KEY (your_table_name_id) REFERENCES your_table_name(id)
);

3

Modify your application to store and retrieve data from these new, normalized tables.

text

4

After data migration, remove the problematic large column from the original table if it's no longer needed there.

ALTER TABLE your_table_name DROP COLUMN large_column_name;

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Related Errors

5 related errors

MySQL Error 1709: Index Column Too Large