Fixing the MySQL Error “Specified key was too long”

1. Overview

MySQL is a widely used open-source relational database management system known for its flexibility and performance. When working with MySQL, we may sometimes encounter the “Specified key was too long” error.

In this tutorial, we’ll look at the various causes that trigger this error and discuss multiple solutions that we can employ to resolve it.

2. Understanding the Error

The “Specified key was too long” error typically occurs when we attempt to create an index or define a column with a key length that exceeds the maximum allowed length in MySQL. The maximum index length depends on the storage engine, and character set used in MySQL configuration.

This error is designed to prevent us from creating indexes or columns that could cause performance issues or exceed storage limitations. Therefore, it’s very important for us to understand the causes behind this error to effectively resolve it.

3. Using Different Character Set

When storing character data in MySQL, different character sets have different storage requirements. Some character sets, such as UTF8, employ a variable number of bytes per character to accommodate a larger number of characters.

If we encounter the “Specified key was too long” error due to character set limitations, we can choose a different character set that requires fewer bytes per character. Switching from UTF8 to the latin1 character set can significantly increase the available key length.

Let’s look at the command to update the character set of a column:

$ ALTER TABLE my_table MODIFY COLUMN title VARCHAR(255) CHARACTER SET latin1;

In the above command, we modified the column “title” to use the latin1 character set instead of UTF8. By making this change, we can effectively reduce the storage requirement per character in our MySQL database. Importantly, latin1 uses a fixed-length character encoding, whereas UTF8 uses a variable-length encoding, so read/write speeds will be slower and disk usage will be higher.

4. Modifying Index Length

MySQL storage engines impose limitations on the maximum length of indexes. Among MySQL’s common storage engines, InnoDB has a maximum index length of 767 bytes by default. If we encounter the “Specified key was too long” error due to index length limitations, we can simply modify the MySQL configuration to increase the maximum index length.

To illustrate, let’s look at the configuration to update it in the “my.cnf” or “my.ini” config file:

innodb_large_prefix=true
innodb_file_format=barracuda
innodb_file_per_table=true

By making the above configuration changes, we can enhance InnoDB’s capability to handle larger index lengths.

It’s essential for us to consider that increasing the index length can potentially impact performance, as larger indexes demand more memory and storage resources.

5. Adjusting the Column or Index Definition

Another common way to solve this issue is by adjusting the column or index definitions to make sure they fit comfortably within the available index length limit. By doing so, we can tackle the issue effectively and ensure smooth database operations without encountering errors. This can be done in two different ways.

Let’s first reduce the column size that is causing the issue:

$ ALTER TABLE my_table MODIFY COLUMN title VARCHAR(191);

In the above command, we simply reduced the column size from 256 to 191. However, reducing the column size too much may lead to data truncation or loss if the new size is insufficient to store the data accurately. If the column size needs to remain the same, we can specify a prefix length for the index.

The prefix length limits the number of characters or bytes included in the index. Let’s check out the command to create an index with a prefix length of 191:

$ CREATE INDEX idx_title ON my_table(title(191));

By adjusting the column size or specifying a prefix length, we ensure that the column or index fits within the available index length limit, avoiding the “Specified key was too long” error. Furthermore, specifying a prefix length for the index may impact its precision and lead to more duplicate entries.

6. Conclusion

In this article, we explored three different ways to solve the “Specified key was too long” error in MySQL. First, we explored the basic details of the error. Afterward, we changed the character set, modified the index length limit, and adjusted the column or index definition to resolve the error.