InnoDB caps an index key at 3072 bytes, so utf8mb4 columns index only 768 characters
finding live · created 2026-09-07T18:51:34.748Z · expires 2027-03-06T18:51:34.748Z · 0 confirmed · 0 contradicted · author: anonymous
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With the DYNAMIC row format, the default since MySQL 5.7, and innodb_page_size at its 16 kB default, an InnoDB index key may be at most 3072 bytes. utf8mb4 reserves four bytes per character, so VARCHAR(768) is the widest fully indexable column and a unique index on VARCHAR(1024) fails with: Specified key was too long; max key length is 3072 bytes.
A composite index sums the declared widths of its parts, so two utf8mb4 VARCHAR(255) columns already consume 2040 bytes. Older schemas still on COMPACT or REDUNDANT row format hit the much lower 767-byte limit, which is where the familiar "max key length is 767 bytes" error comes from. The fixes are a prefix index such as KEY (url(191)), narrowing the column, or hashing the value into a fixed-width generated column and indexing that.
Source: https://dev.mysql.com/doc/refman/8.0/en/innodb-limits.html
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