MySQL string columns key length

After changing the default charset from utf8 to utf8mb4 to support emojis on Directus, we started to received errors that the key was too long.

How can be too long if we only change the charset? one may ask.

First these are the errors I'm talking about:

#1071 - Specified key was too long; max key length is 767 bytes
#1071 - Specified key was too long; max key length is 1000 bytes
#1071 - Specified key was too long; max key length is 3072 bytes

It can be any of those errors depending on whether the table's storage engine is: MySIAM, InnoDb or InnoDb with innodb_large_prefix enabled.


utf8 charset requires only 3 bytes per character, while utf8mb4 requires 4 bytes, which mean you have to use at most 191 characters in your string column

191 characters x 4 bytes = 764 bytes which is less than the maximum length of 767 bytes;

String storage

String storage size vary depends on if the column is fixed-length or variable-length, it also depends on the charset it takes more bytes to storage a japanese character than an ASCII/Latin letter.

As an example CHAR is a fixed-length while VARCHAR and TEXT are variable-length.

All fixed-length data types uses all the bytes declared it with, for example CHAR(16), no matter what's inside, there are not truncated as VARCHAR will be, but right padded with spaces to the specific length.

In the other hand using VARCHAR(16) MySQL internally will remove all trailing spaces from any variable-length column data type.

VARCHAR requires a prefix value of 1 byte to store the length of the string if the size is less than 256, otherwise it will uses 2 bytes.

One tip is not to use CHAR if you are not going to use all the characters almost all the time, because the size can pile up with empty strings column.

Character Set

The UTF8 character set uses a maximum of 3 bytes per character and only contains Basic Multilingual Plane (BMP) characters, which is the home of 65,536 characters (16 bits) from U+0000 to U+FFFF.

The UTF8mb4 character set uses a maximum of 4 bytes per character which include all of BMP characters and Supplementary Multilingual Plane (SMP) which include another possibility of 65,536 new characters from U+10000 to U+1FFFF.

Emojis (Unicode characters)

UTF8 can support emoji, but not all of them, all the new emoji are part of the SMP, so in other to support both basic and supplementary we must use UTF8mb4.

The sparkle emoji (✨ U+2728) is between U+0000 and U+FFFF then it can be used on utf8 but the Woman Health Worker (👩 U+1F469) which is not between U+0000 and U+FFFF, must use the utf8mb4 charset.

Index length

Now all the characters use 4 bytes instead of 3, so all columns that has more than 191 characters now uses more than 767 bytes, because 192 x 4 bytes is 768 bytes.

The options were removing the index, keep using utf8 or reduce the length. Removing the index wasn't an option neither keep using the utf8, but reducing the length was possible was the columns will probably never met the actual length which is 255 characters, reducing it to 191 was optimal and in no way will harm the system.


Changing all string columns that length are greater than 191 characters to 191.

If changing the length is not a possible or desired option, changing the column index to only a chunk of x characters.

CREATE INDEX index_name ON posts (title(191));