How Does MySQL Handle VARCHAR Index Length for UTF-8 Columns?

Understanding MySQL varchar Index Length for UTF-8 Columns

In MySQL, the index length for a varchar(n) column is specified in characters, not bytes. However, MySQL assumes that each UTF-8 character occupies 3 bytes. This means that for a varchar column with a specified index length of n, the actual maximum number of bytes that can be indexed is 3n.

To break it down:

• UTF-8 Character Size: UTF-8 characters can have a variable length, ranging from 1 to 4 bytes.
• MySQL's Assumption: MySQL assumes that UTF-8 characters have a maximum size of 3 bytes.
• Index Length Conversion: For varchar columns, MySQL converts the specified index length (in characters) into a maximum index size (in bytes) using the formula 3n, where n is the index length in characters.

Impact on Index Size for UTF-8 Columns

Based on the assumption of 3 bytes per character, MySQL imposes a practical limit on the index size for varchar columns with the UTF-8 character set.

• Maximum Index Size: The maximum index size for a single varchar column is 255 characters, which translates to a maximum of 767 bytes (255 x 3).
• Composite Indexes: For composite indexes involving varchar columns, the total index size must not exceed 767 bytes.

Consideration for Dynamic and Compressed Row Formats

In MySQL versions 8.0 and above, you can potentially increase the maximum index size for varchar columns in InnoDB tables using the DYNAMIC or COMPRESSED row formats. These row formats allow for longer prefix lengths, effectively increasing the index size limit. However, this optimization is not supported for earlier MySQL versions, including 5.5.27.

Recommendation for Index Design

When designing indexes for varchar columns with the UTF-8 character set, it's recommended to:

• Keep the index length as small as possible to minimize the impact on storage space and performance.
• Consider using shorter character lengths (e.g., varchar(128)) instead of the maximum length (e.g., varchar(512)) to increase the likelihood of using the index effectively.
• If you anticipate storing mostly 1-byte characters, you may experiment with slightly larger index lengths, but keep in mind the potential byte limit.
• Monitor index usage to ensure that the indexes are effectively used and not causing performance issues.

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